Display overlays signaling with alpha channel

Abstract

An apparatus includes at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus to perform at least: providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

Description

Display Overlays Signaling With Alpha ChannelTECHNICAL FIELD

[0001] The examples and non-limiting embodiments relate generally to displays overlays signaling with alpha channel.BACKGROUND

[0002] It is known to process and render video in a multimedia system.BRIEF DESCRIPTION OF THE DRAWINGS

[0003] The foregoing embodiments and other features are explained in the following description, taken in connection with the accompanying drawings, wherein:

[0004] FIG. 1 show multi-languages TV services with customized overlay.

[0005] FIG. 2 is an illustration of DOI SEI usage.

[0006] FIG. 3 is an illustration of overlay generation with alpha component enabled.

[0007] FIG. 4 depicts an example implementation of formation of a target display picture alpha when an alpha channel for the target picture is present.

[0008] FIG. 5 depicts an example implementation of formation of the target picture with both alpha and texture components, with proper bit depth management.

[0009] FIG. 6 depicts an example implementation of formation of the target display picture alpha, when the presence of an alpha channel for the target picture is signaled.

[0010] FIG. 7 shows an encoder according to an embodiment.

[0011] FIG. 8 shows a decoder according to an embodiment.

[0012] FIG. 9 is a block diagram illustrating a system in accordance with an example.

[0013] FIG. 10 is an example apparatus configured to implement the examples described herein.

[0014] FIG. 11 shows a representation of an example of non-volatile memory media used to store instructions that implement the examples described herein

[0015] FIG. 12 is an example method based on the examples described herein.

[0016] FIG. 13 is an example method based on the examples described herein.

[0017] FIG. 14 is an example method based on the examples described herein.

[0018] FIG. 15 is an example method based on the examples described herein.DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0019] A “display overlay” is a rectangular region of texture samples and optional corresponding alpha channel samples, referred to as the texture component and alpha component, respectively. For the purposes of an SEI message, even a background or “base” display layer is considered to be a display overlay. Each display overlay component can be coded as a picture, a subpicture, or a constituent rectangle in a single layer or multi-layer CVS. SEI message syntax elements are used to identify the location of the display overlaycomponent in the coded video and its intended display order and display location in a target display picture. Resampling of individual display overlays is supported.

[0020] The examples described herein apply to the technologies under consideration for a future edition of VSEI (H.274), currently specified in the VSEI TuC (JVET-AJ2032). More specifically, referring to FIG. 1 which depicts multi-languages TV services with customized overlay, the examples described herein apply to the Display Overlays Information (DOI) Supplemental Enhancement Information (SEI) message, for example of one or more SEI parameters 102, enabling to indicate how a receiver (including English receiver 108 and French receiver 110) should assemble different layers (including overlays 104) from a video bitstream 106 together to generate a target picture (such as target picture 112 produced by the English receiver 108 and target picture 114 produced by the French receiver 110), as illustrated in FIG. 1. Encoding packaging 107 encodes the one or more SEI parameters 102, overlays 104, and video 106.

[0021] As further shown in FIG. 1, there are five overlays corresponding to a banner in a respective language, namely overlay 103 for an English target picture, overlay 105, overlay 109 fora French target picture, overlay 111, and overlay 113, which are each input to overlays generator 115 which provides information about the five overlays to production 119. One or more cameras 117 capture information about a scene 101 which is provided to production 119. Production 119 produces one or more SEI parameters 102, overlays 104 and video 106 and provides these to encoding packaging 107 which encodes the one or more SEI parameters 102, overlays 104 and video 106 into or along a bitstream that is input to distribution 120. Distribution 120 includes distribution 122 for the English receiver 108 and distribution 124 for the French receiver. The English receiver 108 generates the target picture 112 so that the English overlay banner 103 is included within the target picture 112, and the French receiver 110 generates the target picture 14 so that the French overlay banner 109 is included within the target picture 114.

[0022] The examples described herein address a gap identified in the current definition of the SEI message, where it is currently impossible to generate a target picture with an alpha channel in the DOI SEI message system. This gap limits the area of cases that the DOI SEI can currently address, limiting it to only few cases.

[0023] DOI SEI in VSEI TuC (JVET-AJ2032):

[0024] The VSEI specification (H.274) includes syntax and semantics of the SEI messages, enabling new use-cases for VVC (H.266). For example, SEI can carry information about neural network based post-filtering, film grain synthesis, and many other types of information.

[0025] The VSEI TuC is gathering all candidate technologies for inclusion in a future version of the VSEI specification. FIG. 2 is an illustration of DOI SEI usage. Described herein are methods for addressing the DOI SEI message, which is part of the VSEI TuC and which enable signaling of how multiple layers should be combined together to assemble a target display picture, as illustrated in FIG. 2. As shown in FIG. 2, coded layer 0 picture 204 and coded layer 1 picture 206 are combined to assemble target display picture 202.

[0026] DOI SEI in VSEI TUC has been further refined with adoption of various other aspects.

[0027] In the current design of DOI SEI, multiple overlays are successively applied to a target picture, to progressively build the final version of the target picture. The target picture is only composed by texture information, that can be initialized with a fixed pixel samples value. The overlays that are applied to the target pictures are delimited either by picture, subpicture, constituent rectangles (OR) or explicitly signaled rectangle areas. The applied overlays can include texture and optional alpha information to manage transparency.

[0028] Problem: As indicated above, the current DOI SEI design does not support target picture to include an alpha component. Thus, it is impossible to use the DOI SEI to represent an overlay with a transparent background, for example to be applied on a picture in a later stage. Additionally, the current design prevents DOI SEI to be used in a processing chain where the target picture would be applied on top of a previously generated image.

[0029] The distinguishing features of the examples described herein include the following:

[0030] The DOI signaling includes a flag indicating whether the target picture has an alpha channel or not.

[0031] If the target picture includes an alpha channel, a mode can be signaled to indicate how the alpha channel shall be initialized. A default mode would be to initialize it as fully transparent. Fully transparent can have an initialization value of 0, or the transparent value indicated in an alpha channel information SEI message. Another mode would be to initialize it with a constant value, that would be then signaled explicitly. Another mode would be to initialize it with an alpha channel carried in the bitstream, e.g. with a picture, subpicture, constituent rectangle or explicit signaling of the area.

[0032] If the target picture is initialized with fully transparent background, the texture initialization signaling is skipped.

[0033] FIG. 3 is an illustration of overlay generation with an alpha component enabled. The formation of a standalone overlay using the method described herein is illustrated in FIG. 3. The coded picture 302 comprises both the texture information and the alpha information, and the overlay signaling enables a target to be initialized with a fully transparent alpha component 306 (value 0, represented in black), resulting in initialized target picture 308. The final overlayed picture 310 is fully transparent, except in the locations where the banner 312 is inserted. FIG. 3 also shows YUV texture component 304 and banner 314 of the final target picture 310.

[0034] Having the alpha component enables the standalone overlay to be applied to a picture carried in another bitstream.

[0035] Signaling of the presence of an alpha channel

[0036] In one embodiment, the presence of an alpha channel for the target picture is signaled (e.g. with doi_target_pic_alpha_flag).

[0037] If doi_target_pic_alpha_flag equals to 1, the target display picture alpha should be formed as follows (and as shown in FIG. 4), with picture alpha array, TargetPictureAlpha[ x][y ], with x = O.. TargetPicWidth, y = O. JargetPicHeight.for( y = 0; y < TargetPicHeight; y++ )for( x = 0; x < TargetPicWidth; x++ )TargetPictureAlpha [ x ][ y ] = 0for( i = 0; i < doi_num_display_overlays_minus1 + 1; i++ ) {for( h = 0, y = doi op _left_y[ i ]; y < DisplayOverlayHeight[ i ]; h++, y++ )for( w = 0, x = doi_top_left_x[ i ]; x < DisplayOverlay Width[ i ]; w++, x++ )if( doi_alpha_present_flag [ i ] )TargetPictureAlpha[ x ][ y ] + = ( OverlayAlpha[ i ][ w ][ h ] * ( (1 «BitDepth) - TargetPictureAlpha[ x ][ y ] ) ) >>BitDepth

[0038] Signaling of alpha channel dimensions and bitdepth

[0039] Resolution

[0040] In one embodiment, the alpha channel can be represented at a different resolution than the target picture texture resolution.

[0041] In one embodiment, a syntax flag can indicate if the target picture alpha channel has a different or the same resolution as the target picture texture.

[0042] In one embodiment, the ratio between target picture texture dimensions and alpha channel dimensions is signaled as a single, or multiple scaling factors (either a single for both dimensions, or two different respectively for width and height).

[0043] In one embodiment, the dimension of the alpha channel is explicitly signaled (width and height).

[0044] In one embodiment, a dimension initialization mode is signaled to indicate which of the method should be used to derive alpha channel dimensions.

[0045] In one embodiment, the dimension signaling is shared with texture channels.

[0046] Bit-depth of alpha channel

[0047] In one embodiment, the alpha channel can be represented at a different bit-depth than the target picture texture bit-depth.

[0048] In one embodiment, the alpha channel bit-depth is initialized by default to a value, for example 8.

[0049] In one embodiment, the alpha channel bit-depth is explicitly signalled with a dedicated syntax element.

[0050] In one embodiment, the bit-depth signalling is shared with texture channels.

[0051] The formation of the target picture with both alpha and texture components, with proper bit depth management is provided below and in FIG. 5 (FIG. 5 highlights items 502, 504, 506, 508, 510, and 512 which provide bit depth management):for( i = 0; i < doi_num_display_overlays_minus1 + 1; i++) {Bitshift = targetBitDepth - BitDepth[doi_layer_idx[ i ]]for( h = 0, y = doijop _left_y[ i ]; y < DisplayOverlayHeight[ i ]; h++, y++ )for( w = 0, x = doi _top_left_x[ i ]; x < DisplayOverlay Width[ i ]; w++, x++ )if(!doi_alpha_present_flag[ i ] )p = OverlayTexture[ i ]

[0000] [ w ][ h ]TargetPicture

[0000] [ x ][ y ] = ( Bitshift < 0? p » - Bitshift: p « BitShift)elseTargetPicture

[0000] [ x ][ y ] = OverlayWithAlpha( TargetPicture

[0000] [ x ][ y ], OverlayTexture

[0000] [ i ][ w ][ h ], OverlayAlpha[ i ][ w ][ h ] )for( ( cldx = 1; cldx < ChromaFormatldc = = 0 )? 1: 3; cldx++ ++ ) {for( h = 0, y = doi_top_left_y[ i ] / SubHeightC; y < DisplayOverlay Height[ i ] / SubHeightC; h++, y++ ) for( w = 0, x = doi_top_left_x[ i ] / SubWidthC; x < DisplayOverlay Width [ i ] / SubWidthC; w++, x++)if(!doi_alpha_present_flag[ i ] )p = OverlayTexture[ i ][ cldx ][ w ][ h ]TargetPicture[ cldx ][ x ][ y ] = ( Bitshift < 0? p » - Bitshift: p « BitShift) elseTargetPicture[ cldx ][ x ][ y ] = OverlayWithAlpha( TargetPicture[ cldx ][ x ][ y ], OverlayTexture[ cldx ][ i ][ w ][ h ], OverlayAlpha[ i ][ w ][ h ] )

[0052] If doi_target_pic_alpha_flag equals to 1, the target display picture alpha should be formed as follows (and as also shown in FIG. 6), with picture alpha array, TargetPictureAlpha[ x ][ y ], with x = 0..TargetPicWidth, y = 0..TargetPicHeight.for( y = 0; y < TargetPicHeight; y++ )for( x = 0; x < TargetPicWidth; x++ )TargetPictureAlpha [ x ][ y ] = 0for( i = 0; i < doi_num_display_overlays_minus1 + 1; i++ ) {Bitshift = targetBitDepth - BitDepth [doi_alpha_layer_idx[ i ]]for( h = 0, y = doi_top_left_y[ i ]; y < DisplayOverlay Height[ i ]; h++, y++ )for( w = 0, x = doi_top_left_x[ i ]; x < DisplayOverlay Width[ i ]; w++, x++ )if( doi_alpha_present_flag[ i ] )p = ( Bitshift < 0? OverlayAlpha[ i ][ w ][ h ] » - Bitshift:OverlayAlpha[ i ][ w ][ h ] « BitShift) TargetPictureAlpha[x][ Y ]+ =( P * ( U «TargetBitDepth )- TargetPictureAlpha[ x ][ y ] ) ) >> TargetBitDepth

[0053] Signaling of alpha channel initialization

[0054] In one embodiment, the alpha channel is by default initialized with full transparency when its presence is signaled without explicit signaling.

[0055] In one embodiment, the presence of alpha channel flag deactivates texture specific initialization signaling.

[0056] In another embodiment, an alpha channel initialization mode is explicitly signaled offering multiple possibilities to initialize the alpha channel of the target picture.

[0057] In one embodiment, a specific initialization mode value indicates that a default value is used to initialize the alpha channel, for example 0. Another value can be used for default initialization and defined in the semantics.

[0058] In one embodiment, a specific initialization mode value indicates that an explicitly signaled value is used to initialize the alpha channel.

[0059] In one embodiment, a specific initialization mode value indicates that the alpha channel initialization is based a specific picture, subpicture, or constituent rectangles, the identifier being signaled.

[0060] Processing of alpha channel

[0061] In one embodiment, the alpha channel is aggregated in front of the one or multiple texture components (aYUV).

[0062] In one embodiment, the alpha channel values are computed separately, or jointly with the texture loop.

[0063] Processing of texture and alpha channel with different bit depth than coded picture

[0064] In one embodiment, the target picture bit depth differs from the bit depth of the DOI component in the coded picture, requiring signaling of the target picture bit depth, determining bit depth from the bit depth of the coded picture containing the DOI component, computing a bit shift value, and applying a bit shift operation on the overlayed coded picture samples based on the computed bit shift value. Depending on the sign of the bit shift value, a left shift or right shift is applied.

[0065] In one embodiment, the bit depth differs for both texture and alpha components.

[0066] In one embodiment, the bit depth differs only for the texture component, and not for the alpha.

[0067] In one embodiment, the bit depth differs only for the alpha component, not for the texture.

[0068] FIG. 7 shows an encoder 700 according to an embodiment. FIG. 7 illustrates an image to be encoded (|n), a predicted representation of an image block (P’n), a prediction error signal (Dn), a reconstructed prediction error signal (D’n);a preliminary reconstructed image (i'n), a final reconstructed image (R'n). a transform (I) and inverse transform (T"1). a quantization (Q) and inverse quantization (CT1), entropy encoding (E), a reference frame memory (RFM), inter prediction (Pintw), intra prediction [R|ltra), mode selection (MS) and filtering (F).

[0069] Display overlays signaling with alpha channel 702 implements the examples described herein related to display overlays with alpha channel signaling. Bit-depth signaling 704 implements the examplesdescribed herein related to bit depth signaling, such as signaling a bit depth of a target picture.

[0070] FIG. 8 shows a decoder 800 according to an embodiment. FIG. 8 illustrates a predicted representation of an image block (p'n), a reconstructed prediction error signal (D’n). a preliminary reconstructed image (I'n), a final reconstructed image (R‘r.), an inverse transform (T-1), an inverse quantization (Q-1), an entropy decoding (E⁻¹), a reference frame memory (RFM), a prediction (either inter or intra) (P), and filtering (F).

[0071] Display overlays signaling with alpha channel 802 implements the examples described herein related to display overlays with alpha channel signaling. Bit-depth signaling 804 implements the examples described herein related to bit depth signaling, such as receiving signaling of a bit depth of a target picture.

[0072] Referring to FIG. 7, the output of E is the low level part of the bitstream, ending encapsulated into a NAL Unit 701. The different HLS structures are also encoded (not with entropy coding) and encapsulated in their own NAL units including NAL unit with HLS structure 710 and NAL unit with HLS structure 712. Then, display overlay signaling 702 is placed into a separated NAL Unit 703 for the SEI message. All the NAL units (including NAL unit 701, NAL unit 703, NAL unit with HLS structure 710, and NAL unit with HLS structure 712) are multiplexed together with multiplexer 718 to form the final bitstream 720, respecting a proper order. In the example shown in FIG. 7, the bit-depth signaling 704 is part of the display overlays signaling with alpha channel 702. In other examples the bit-depth signaling 704 is separate from an not part of the display overlays signaling with alpha channel 702.

[0073] Referring to FIG. 8, R'nis the one or more reconstructed frame(s), and R'nand display overlays signaling with alpha channel 802 are then input of a SEI processing unit 810 that outputs a target picture 812. In general SEI is processed in a separate processing block than the decoder 800 in silicon. In the example shown in FIG. 8, the bit-depth signaling 804 is part of the display overlays signaling with alpha channel 802. In other examples the bit-depth signaling 804 is separate from an not part of the display overlays signaling with alpha channel 802.

[0074] A video encoder transforms the input video into a compressed representation suited for storage / transmission and a video decoder decompresses the compressed video representation back into a viewable form. Typically, an encoder discards some information in the original video sequence in order to represent the video in a more compact form (that is, at lower bitrate).

[0075] A video encoder may encode the video information in two phases. Firstly, pixel values in a certain picture area (or “block”) are predicted for example by motion compensation means (finding and indicating an area in one of the previously coded video frames that corresponds closely to the block being coded) or by spatial means (using the pixel values around the block to be coded in a specified manner). Secondly the prediction error, e.g., the difference between the predicted block of pixels and the original block of pixels, is coded. This is typically done by transforming the difference in pixel values using a specified transform (e.g., Discrete Cosine T ransform (DCT) or a variant of it), quantizing the coefficients and entropy coding the quantized coefficients. By varying the fidelity of the quantization process, the encoder can control the balance between the accuracy of the pixel representation (picture quality) and size of the resulting coded video representation(file size or transmission bitrate).

[0076] Inter prediction, which may also be referred to as temporal prediction, motion compensation, or motion-compensated prediction, exploits temporal redundancy. In inter prediction the sources of prediction are previously decoded pictures (a.k.a. reference pictures).

[0077] Intra prediction utilizes the fact that adjacent pixels within the same picture are likely to be correlated. Intra prediction can be performed in spatial or transform domain, e.g., either sample values or transform coefficients can be predicted. Intra prediction is typically exploited in intra coding, where no inter prediction is applied.

[0078] An intra picture may be defined as a coded picture that is decoded using intra prediction only, or in other words, does not make use of inter prediction in decoding. An intra picture may be interchangeably called an intra frame.

[0079] An inter picture may be defined as a coded picture whose decoding may include intra prediction and inter prediction. An inter picture may be interchangeably called an inter frame.

[0080] FIG. 9 is a block diagram illustrating a system 900 in accordance with several examples. In an example, the encoder 930 is used to encode an image or video from the scene 915, which scene 915 may be captured with one or more cameras, and the encoder 930 is implemented in a transmitting apparatus 980. The encoder 930 produces a bitstream 910 comprising signaling that is received by the receiving apparatus 982, which implements a decoder 940. The encoder 930 sends the bitstream 910 that comprises the herein described signaling. The decoder 940 forms the image or video for the scene 915-1, and the receiving apparatus 982 would present this to the user, e.g., via a smartphone, television, or projector among many other options.

[0081] In some examples, the transmitting apparatus 980 and the receiving apparatus 982 are at least partially within a common apparatus, and for example are located within a common housing 950. In other examples the transmitting apparatus 980 and the receiving apparatus 982 are at least partially not within a common apparatus and have at least partially different housings. Therefore in some examples, the encoder 930 and the decoder 940 are at least partially within a common apparatus, and for example are located within a common housing 950. For example the common apparatus comprising the encoder 930 and decoder 940 implements a codec. In other examples the encoder 930 and the decoder 940 are at least partially not within a common apparatus and have at least partially different housings, but when together still implement a codec.

[0082] In some examples, 3D media from the capture (e.g., volumetric capture) at a viewpoint 912 of the scene 915, which includes a person 913) is converted via projection to a series of 2D representations with occupancy, geometry, attributes and / or displacements. Additional atlas information is also included in the bitstream to enable inverse reconstruction. For decoding, the received bitstream 910 is separated into its components with atlas information; occupancy, geometry, displacement, and attribute 2D representations. A 3D reconstruction is performed to reconstruct the scene 915-1 created looking at the viewpoint 912-1 with a “reconstructed” person 913-1. The “-1” are used to indicate that these are reconstructions of the original.

[0083] As indicated at 920, the decoder 940 performs an action or actions based on the received display overlays with alpha channel signaling and bit depth signaling.

[0084] Encoding 990 performs the examples described herein related to encoding of display overlays with alpha channel signaling. Decoding 992 performs the examples described herein related to decoding display overlays with alpha channel signaling to generate reconstructed scene 915-1.

[0085] FIG. 10 is an example apparatus 1000, which may be implemented in hardware, configured to implement the examples described herein. The apparatus 1000 comprises at least one processor 1002 (e.g., an FPGA and / or CPU and / or GPU), one or more memories 1004 including computer program code 1005, the computer program code 1005 having instructions to carry out the methods described herein, wherein the at least one memory 1004 and the computer program code 1005 are configured to, with the at least one processor 1002, cause the apparatus 1000 to implement circuitry, a process, component, module, or function (implemented with control module 1006) to implement the examples described herein.

[0086] Apparatus 1000 may be a smartphone, personal digital device or assistant, smart television, laptop, pad, tablet, head-mounted display (HMD), or other user device or terminal device. The memory 1004 may be a non-transitory memory, a transitory memory, a volatile memory (e.g. RAM), or a non-volatile memory (e.g., ROM).

[0087] Display overlays signaling with alpha channel 1030 implements the examples described herein related to display overlays with alpha channel signaling, including encoding and decoding of such signaling. Bit depth signaling 1040 implements the examples described herein related to bit depth signaling, such as signaling a bit depth of a target picture or receiving signaling of a bit depth of a target picture. In the example shown in FIG. 10, the bit depth signaling 1040 is depicted as being separate from the display overlays signaling with alpha channel 1030. In other examples, the bit depth signaling 1040 may be implemented as part of the display overlays signaling with alpha channel 1030.

[0088] The apparatus 1000 includes a display and / or I / O interface 1008, which includes user interface (Ul) circuitry and elements, that may be used to display features or a status of the methods described herein (e.g., as one of the methods is being performed or at a subsequent time), or to receive input from a user such as with using a keypad, camera, touchscreen, touch area, microphone, biometric recognition, one or more sensors, etc. The apparatus 1000 includes one or more communication e.g. network (N / W) interfaces (l / F(s)) 1010. The communication l / F(s) 1010 may be wired and / or wireless and communicate over the Internet / other network(s) via any communication technique including via one or more links 1024. The communication l / F(s) 1010 may comprise one or more transmitters or one or more receivers.

[0089] The transceiver 1016 comprises one or more transmitters 1018 and one or more receivers 1020. The transceiver 1016 and / or communication l / F(s) 1010 may comprise standard well-known components such as an amplifier, filter, frequency-converter, (de)modulator, and encoder / decoder circuitries and one or more antennas, such as antennas 1014 used for communication over wireless link 1026.

[0090] The control module 1006 of the apparatus 1000 comprises one of or both parts 1006-1 and / or 1006-2, which may be implemented in a number of ways. The control module 1006 may be implemented in hardware as control module 1006-1, such as being implemented as part of the one or more processors 1002. The control module 1006-1 may be implemented also as an integrated circuit or through other hardware such as a programmable gate array. In another example, the control module 1006 may be implemented as control module 1006-2, which is implemented as computer program code (having corresponding instructions) 1005 and is executed by the one or more processors 1002. For instance, the one or more memories 1004 store instructions that, when executed by the one or more processors 1002, cause the apparatus 1000 to perform one or more of the operations as described herein. Furthermore, the one or more processors 1002, one or more memories 1004, and example algorithms (e.g., as flowcharts and / or signaling diagrams), encoded as instructions, programs, or code, are means for causing performance of the operations described herein.

[0091] The apparatus 1000 to implement the functionality of control 1006 may correspond to any of the apparatuses depicted herein. Alternatively, apparatus 1000 and its elements may not correspond to any of the other apparatuses depicted herein, as apparatus 1000 may be part of a self-organizing / optimizing network (SON) node or other node, such as a node in a cloud.

[0092] The apparatus 1000 may also be distributed throughout the network including within and between apparatus 1000 and any network element (such as a base station and / or terminal device and / or user equipment).

[0093] Interface 1012 enables data communication and signaling between the various items of apparatus 1000, as shown in FIG. 10. For example, the interface 1012 may be one or more buses such as address, data, or control buses, and may include any interconnection mechanism, such as a series of lines on a motherboard or integrated circuit, fiber optics or other optical communication equipment, and the like. Computer program code (e.g. instructions) 1005, including control 1006 may comprise object-oriented software configured to pass data or messages between objects within computer program code 1005. Computer program code (e.g. instructions) 1005, including control 1006 may comprise procedural, functional, or scripting code. The apparatus 1000 need not comprise each of the features mentioned, or may comprise other features as well. The various components of apparatus 1000 may at least partially reside in a common housing 1028, or a subset of the various components of apparatus 1000 may at least partially be located in different housings, which different housings may include housing 1028.

[0094] FIG. 11 shows a schematic representation of non-volatile memory media 1100a (e.g. computer / compact disc (CD) or digital versatile disc (DVD)) and 1100b (e.g. universal serial bus (USB) memory stick) and 1100c (e.g. cloud storage for downloading instructions and / or parameters 1102 or receiving emailed instructions and / or parameters 1102) storing instructions and / or parameters 1102 which when executed by a processor allows the processor to perform one or more of the operations of the methods described herein. Instructions and / or parameters 1102 may represent or correspond to a non-transitory computer readable medium.

[0095] FIG. 12 is an example method 1200 based on the examples described herein. At 1210, the methodincludes providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream. At 1220, the method includes wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order. At 1230, the method includes wherein the information message within the bitstream comprises an indication that the target picture has an alpha channel. Method 1200 may be performed with encoding packaging 107, encoder 700, transmitting apparatus 980 with encoder 930, or apparatus 1000.

[0096] FIG. 13 is an example method 1300 based on the examples described herein. At 1310, the method includes providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream. At 1320, the method includes wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order. At 1330, the method includes wherein the information message within the bitstream comprises an indication of a bit depth of the target picture. Method 1300 may be performed with encoding packaging 107, encoder 700, transmitting apparatus 980 with encoder 930, or apparatus 1000.

[0097] FIG. 14 is an example method 1400 based on the examples described herein. At 1410, the method includes receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream. At 1420, the method includes wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order. At 1430, the method includes wherein the information message within the bitstream comprises an indication that the target picture has an alpha channel. Method 1400 may be performed with receiver 108, receiver 110, decoder 800, receiving apparatus 982 with decoder 940, or apparatus 1000.

[0098] FIG. 15 is an example method 1500 based on the examples described herein. At 1510, the method includes receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream. At 1520, the method includes wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order. At 1530, the method includes wherein the information message within the bitstream comprises an indication of a bit depth of the target picture. Method 1500 may be performed with receiver 108, receiver 110, decoder 800, receiving apparatus 982 with decoder 940, or apparatus 1000.

[0099] The following examples are provided and described herein.

[0100] Example 1. An apparatus including: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus to perform at least: providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

[0101] Example 2. The apparatus of example 1, wherein the information message within the bitstream additionally comprises an indication of an initialization mode for the alpha channel.

[0102] Example 3. The apparatus of example 2, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized as fully transparent.

[0103] Example 4. The apparatus of any of examples 2 to 3, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized to the value zero.

[0104] Example 5. The apparatus of any of examples 2 to 4, wherein the indication of the initialization mode for the alpha channel comprises a mode that indicates that samples of the alpha channel should be initialized to a constant value, and wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: providing an indication of the constant value.

[0105] Example 6. The apparatus of any of examples 2 to 5, wherein the indication of the initialization mode for the alpha channel indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

[0106] Example 7. The apparatus of example 6, wherein the alpha channel carried in the bitstream comprises at least one of: a picture, or a subpicture, or a constituent rectangle.

[0107] Example 8. The apparatus of any of examples 1 to 7, wherein dimensions of the alpha channel of the target picture are derived to be the same as dimensions of the target picture.

[0108] Example 9. The apparatus of any of examples 1 to 8, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: signaling of dimensions of the alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

[0109] Example 10. The apparatus of any of examples 1 to 9, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: providing an indication of dimensions of the alpha channel of the target picture.

[0110] Example 11. The apparatus of any of examples 1 to 10, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: determining to not encode initialization signaling into or along the bitstream, when an indicator of an initialization mode for the alpha channel indicates that samples of the alpha channel should be initialized as fully transparent.

[0111] Example 12. The apparatus of any of examples 1 to 11, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: signaling of a bit depth of the alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

[0112] Example 13. The apparatus of any of examples 1 to 12, wherein the alpha channel is initialized with full transparency, when the indication of the information message within the bitstream that indicates that the target picture has an alpha channel does not explicitly indicate that the target picture has an alpha channel.

[0113] Example 14. An apparatus including: at least one processor; and at least one memory storinginstructions that, when executed by the at least one processor, cause the apparatus to perform at least: providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

[0114] Example 15. The apparatus of example 14, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: providing, in the bitstream, display overlays information comprising a flag indicating whether or not the target picture has an alpha channel.

[0115] Example 16. The apparatus of any of examples 14 to 15, wherein the information message within the bitstream comprising the indication of the bit depth of the target picture also comprises an indication of a bit depth of an alpha channel of the target picture.

[0116] Example 17. The apparatus of any of examples 14 to 16, wherein: the information message comprises information describing a first display overlay component, the bitstream comprises a first coded picture containing the first display overlay component, and the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: determining a bit depth of the first coded picture containing the first display overlay component; and computing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

[0117] Example 18. The apparatus of example 17, wherein when the bit shift value is non-zero, formation of the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

[0118] Example 19. The apparatus of any of examples 17 to 18, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

[0119] Example 20. The apparatus of example 19, wherein: the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, and the bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

[0120] Example 21. The apparatus of any of examples 17 to 20, wherein the bit depth of the first display overlay component in the first coded picture is different from the bit depth of the target picture.

[0121] Example 22. The apparatus of any of examples 14 to 21, wherein: the information message comprises an indication that the target picture has an alpha channel, the bitstream comprises a first coded picture containing a first display overlay component, and the first display overlay component is an alpha component, and the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: determining a bit depth of the alpha channel of the target picture; determining a bit depth of the first coded picture from a parameter set, and computing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

[0122] Example 23. The apparatus of example 22, wherein when the bit shift value is non-zero, formationof the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

[0123] Example 24. The apparatus of any of examples 22 to 23, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

[0124] Example 25. The apparatus of example 24, wherein: the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, and the bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

[0125] Example 26. The apparatus of any of examples 22 to 25, wherein the bit depth of the first display overlay component that is an alpha component in the first coded picture is different from the bit depth of the target picture.

[0126] Example 27. The apparatus of any of examples 14 to 26, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: signaling of dimensions of an alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

[0127] Example 28. The apparatus of any of examples 14 to 27, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: signaling of a bit depth of an alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

[0128] Example 29. An apparatus including: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus to perform at least: receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

[0129] Example 30. The apparatus of example 29, wherein the information message within the bitstream additionally comprises an indication of an initialization mode for the alpha channel.

[0130] Example 31. The apparatus of example 30, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized as fully transparent.

[0131] Example 32. The apparatus of example 31, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: initializing the samples of the alpha channel as fully transparent, based on the default mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alpha channel should be initialized as fully transparent.

[0132] Example 33. The apparatus of any of examples 30 to 32, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel shouldbe initialized to the value zero.

[0133] Example 34. The apparatus of example 33, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: initializing the samples of the alpha channel to the value zero, based on the default mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alpha channel should be initialized to the value zero.

[0134] Example 35. The apparatus of any of examples 30 to 34, wherein the indication of the initialization mode for the alpha channel comprises a mode that indicates that samples of the alpha channel should be initialized to a constant value, and wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: receiving an indication of the constant value.

[0135] Example 36. The apparatus of example 35, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: initializing the samples of the alpha channel to the constant value, based on the mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alpha channel should be initialized to the constant value and the received indication of the constant value.

[0136] Example 37. The apparatus of any of examples 30 to 36, wherein the indication of the initialization mode for the alpha channel indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

[0137] Example 38. The apparatus of example 37, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: initializing the alpha channel of the target picture with the alpha channel carried in the bitstream, based on the indication of the initialization mode for the alpha channel that indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

[0138] Example 39. The apparatus of any of examples 37 to 38, wherein the alpha channel carried in the bitstream comprises at least one of: a picture, or a subpicture, or a constituent rectangle.

[0139] Example 40. The apparatus of any of examples 29 to 39, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: derive dimensions of the alpha channel of the target picture to be the same as dimensions of the target picture.

[0140] Example 41. The apparatus of any of examples 29 to 40, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: receiving signaling of dimensions of the alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

[0141] Example 42. The apparatus of any of examples 29 to 41, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: receive an indication of dimensions of the alpha channel of the target picture.

[0142] Example 43. The apparatus of any of examples 29 to 42, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: receiving an indicator of an initialization mode for the alpha channel that indicates that samples of the alpha channel should be initialized as fully transparent; wherein initialization signaling is not decoded from or along the bitstream.

[0143] Example 44. The apparatus of any of examples 29 to 43, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: forming the target picture by overlaying the multiple ordered display overlays according to the specified order and the alpha channel.

[0144] Example 45. The apparatus of any of examples 29 to 44, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: receiving signaling of a bit depth of the alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

[0145] Example 46. The apparatus of any of examples 29 to 45, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: initialize the alpha channel with full transparency, when the indication of the information message within the bitstream that indicates that the target picture has an alpha channel does not explicitly indicate that the target picture has an alpha channel.

[0146] Example 47. An apparatus including: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus to perform at least: receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

[0147] Example 48. The apparatus of example 47, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: receiving, in the bitstream, display overlays information comprising a flag indicating whether or not the target picture has an alpha channel.

[0148] Example 49. The apparatus of any of examples 47 to 48, wherein the information message within the bitstream comprising the indication of the bit depth of the target picture also comprises an indication of a bit depth of an alpha channel of the target picture.

[0149] Example 50. The apparatus of any of examples 47 to 49, wherein: the information message comprises information describing a first display overlay component, the bitstream comprises a first coded picture containing the first display overlay component, and the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: determining a bit depth of the first coded picture containing the first display overlay component; and computing a bit shift valuebased on the bit depth of the target picture and the bit depth of the first coded picture.

[0150] Example 51. The apparatus of example 50, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: forming the target picture by applying a bit shift operation to samples of the first coded picture based on the computed bit shift value, when the bit shift value is non-zero.

[0151] Example 52. The apparatus of any of examples 50 to 51, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

[0152] Example 53. The apparatus of example 52, wherein: the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, and the bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

[0153] Example 54. The apparatus of any of examples 50 to 53, wherein the bit depth of the first display overlay component in the first coded picture is different from the bit depth of the target picture.

[0154] Example 55. The apparatus of any of examples 47 to 54, wherein: the information message comprises an indication that the target picture has an alpha channel, the bitstream comprises a first coded picture containing a first display overlay component, and the first display overlay component is an alpha component, and the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: determining a bit depth of the alpha channel of the target picture; determining a bit depth of the first coded picture from a parameter set, and computing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

[0155] Example 56. The apparatus of example 55, wherein when the bit shift value is non-zero, formation of the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

[0156] Example 57. The apparatus of any of examples 55 to 56, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

[0157] Example 58. The apparatus of example 57, wherein: the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, and the bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

[0158] Example 59. The apparatus of any of examples 55 to 58, wherein the bit depth of the first display overlay component that is an alpha component in the first coded picture is different from the bit depth of the target picture.

[0159] Example 60. The apparatus of any of examples 47 to 59, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least: receiving signaling of dimensions of an alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

[0160] Example 61. The apparatus of any of examples 47 to 60, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:receiving signaling of a bit depth of an alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

[0161] Example 62. An apparatus including: means for providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

[0162] Example 63. An apparatus including: means for providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

[0163] Example 64. An apparatus including: means for receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

[0164] Example 65. An apparatus including: means for receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

[0165] Example 66. A computer readable medium including instructions stored thereon for performing at least the following: providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

[0166] Example 67. A computer readable medium including instructions stored thereon for performing at least the following: providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

[0167] Example 68. A computer readable medium including instructions stored thereon for performing at least the following: receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, and wherein theinformation message within the bitstream comprises an indication that the target picture has an alpha channel.

[0168] Example 69. A computer readable medium including instructions stored thereon for performing at least the following: receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

[0169] Example 70. A method including: providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

[0170] Example 71. The method of example 70, wherein the information message within the bitstream additionally comprises an indication of an initialization mode for the alpha channel.

[0171] Example 72. The method of example 71, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized as fully transparent.

[0172] Example 73. The method of any of examples 71 to 72, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized to the value zero.

[0173] Example 74. The method of any of examples 71 to 73, wherein the indication of the initialization mode for the alpha channel comprises a mode that indicates that samples of the alpha channel should be initialized to a constant value, and wherein the method further comprises: providing an indication of the constant value.

[0174] Example 75. The method of any of examples 71 to 75, wherein the indication of the initialization mode for the alpha channel indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

[0175] Example 76. The method of example 75, wherein the alpha channel carried in the bitstream comprises at least one of: a picture, or a subpicture, or a constituent rectangle.

[0176] Example 77. The method of any of examples 70 to 76, wherein dimensions of the alpha channel of the target picture are derived to be the same as dimensions of the target picture.

[0177] Example 78. The method of any of examples 70 to 77, further including: signaling of dimensions of the alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

[0178] Example 79. The method of any of examples 70 to 78, further including: providing an indication of dimensions of the alpha channel of the target picture.

[0179] Example 80. The method of any of examples 70 to 79, further including: determining to not encodeinitialization signaling into or along the bitstream, when an indicator of an initialization mode for the alpha channel indicates that samples of the alpha channel should be initialized as fully transparent.

[0180] Example 81. The method of any of examples 70 to 80, further including: signaling of a bit depth of the alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

[0181] Example 82. The method of any of examples 70 to 81, wherein the alpha channel is initialized with full transparency, when the indication of the information message within the bitstream that indicates that the target picture has an alpha channel does not explicitly indicate that the target picture has an alpha channel.

[0182] Example 83. A method including: providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

[0183] Example 84. The method of example 83, further including: providing, in the bitstream, display overlays information comprising a flag indicating whether or not the target picture has an alpha channel.

[0184] Example 85. The method of any of examples 83 to 84, wherein the information message within the bitstream comprising the indication of the bit depth of the target picture also comprises an indication of a bit depth of an alpha channel of the target picture.

[0185] Example 86. The method of any of examples 83 to 85, wherein: the information message comprises information describing a first display overlay component, the bitstream comprises a first coded picture containing the first display overlay component, and the method further comprises: determining a bit depth of the first coded picture containing the first display overlay component; and computing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

[0186] Example 87. The method of example 86, wherein when the bit shift value is non-zero, formation of the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

[0187] Example 88. The method of any of examples 86 to 87, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

[0188] Example 89. The method of example 88, wherein: the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, and the bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

[0189] Example 90. The method of any of examples 86 to 89, wherein the bit depth of the first display overlay component in the first coded picture is different from the bit depth of the target picture.

[0190] Example 91. The method of any of examples 83 to 90, wherein: the information message comprises an indication that the target picture has an alpha channel, the bitstream comprises a first coded picture containing a first display overlay component, and the first display overlay component is an alpha component,and the method further comprises: determining a bit depth of the alpha channel of the target picture; determining a bit depth of the first coded picture from a parameter set, and computing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

[0191] Example 92. The method of example 91, wherein when the bit shift value is non-zero, formation of the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

[0192] Example 93. The method of any of examples 91 to 92, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

[0193] Example 94. The method of example 93, wherein: the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, and the bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

[0194] Example 95. The method of any of examples 91 to 94, wherein the bit depth of the first display overlay component that is an alpha component in the first coded picture is different from the bit depth of the target picture.

[0195] Example 96. The method of any of examples 83 to 95, further including: signaling of dimensions of an alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

[0196] Example 97. The method of any of examples 83 to 96, further including: signaling of a bit depth of an alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

[0197] Example 98. A method including: receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

[0198] Example 99. The method of example 98, wherein the information message within the bitstream additionally comprises an indication of an initialization mode for the alpha channel.

[0199] Example 100. The method of example 99, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized as fully transparent.

[0200] Example 101. The method of example 100, further including: initializing the samples of the alpha channel as fully transparent, based on the default mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alpha channel should be initialized as fully transparent.

[0201] Example 102. The method of any of examples 99 to 101, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized to the value zero.

[0202] Example 103. The method of example 102, further including: initializing the samples of the alpha channel to the value zero, based on the default mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alpha channel should be initialized to the value zero.

[0203] Example 104. The method of any of examples 99 to 103, wherein the indication of the initialization mode for the alpha channel comprises a mode that indicates that samples of the alpha channel should be initialized to a constant value, and the method further comprises: receiving an indication of the constant value.

[0204] Example 105. The method of example 104, further including: initializing the samples of the alpha channel to the constant value, based on the mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alpha channel should be initialized to the constant value and the received indication of the constant value.

[0205] Example 106. The method of any of examples 99 to 105, wherein the indication of the initialization mode for the alpha channel indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

[0206] Example 107. The method of example 106, further including: initializing the alpha channel of the target picture with the alpha channel carried in the bitstream, based on the indication of the initialization mode for the alpha channel that indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

[0207] Example 108. The method of any of examples 106 to 107, wherein the alpha channel carried in the bitstream comprises at least one of: a picture, or a subpicture, or a constituent rectangle.

[0208] Example 109. The method of any of examples 98 to 109, further including: derive dimensions of the alpha channel of the target picture to be the same as dimensions of the target picture.

[0209] Example 110. The method of any of examples 98 to 109, further including: receiving signaling of dimensions of the alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

[0210] Example 111. The method of any of examples 98 to 110, further including: receive an indication of dimensions of the alpha channel of the target picture.

[0211] Example 112. The method of any of examples 98 to 111, further including: receiving an indicator of an initialization mode for the alpha channel that indicates that samples of the alpha channel should be initialized as fully transparent; wherein initialization signaling is not decoded from or along the bitstream.

[0212] Example 113. The method of any of examples 98 to 113, further including: forming the target picture by overlaying the multiple ordered display overlays according to the specified order and the alpha channel.

[0213] Example 114. The method of any of examples 98 to 113, further including: receiving signaling of a bit depth of the alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

[0214] Example 115. The method of any of examples 98 to 114, further including: initialize the alpha channel with full transparency, when the indication of the information message within the bitstream thatindicates that the target picture has an alpha channel does not explicitly indicate that the target picture has an alpha channel.

[0215] Example 116. A method including: receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream, wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, and wherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

[0216] Example 117. The method of example 116, further including: receiving, in the bitstream, display overlays information comprising a flag indicating whether or not the target picture has an alpha channel.

[0217] Example 118. The method of any of examples 116 to 117, wherein the information message within the bitstream comprising the indication of the bit depth of the target picture also comprises an indication of a bit depth of an alpha channel of the target picture.

[0218] Example 119. The method of any of examples 116 to 118, wherein: the information message comprises information describing a first display overlay component, the bitstream comprises a first coded picture containing the first display overlay component, and the method further comprises: determining a bit depth of the first coded picture containing the first display overlay component; and computing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

[0219] Example 120. The method of example 119, further including: forming the target picture by applying a bit shift operation to samples of the first coded picture based on the computed bit shift value, when the bit shift value is non-zero.

[0220] Example 121. The method of any of examples 119 to 120, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

[0221] Example 122. The method of example 121, wherein: the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, and the bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

[0222] Example 123. The method of any of examples 119 to 122, wherein the bit depth of the first display overlay component in the first coded picture is different from the bit depth of the target picture.

[0223] Example 124. The method of any of examples 116 to 123, wherein: the information message comprises an indication that the target picture has an alpha channel, the bitstream comprises a first coded picture containing a first display overlay component, and the first display overlay component is an alpha component, and the method further comprises: determining a bit depth of the alpha channel of the target picture; determining a bit depth of the first coded picture from a parameter set, and computing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

[0224] Example 125. The method of example 124, wherein when the bit shift value is non-zero, formation of the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

[0225] Example 126. The method of any of examples 124 to 125, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

[0226] Example 127. The method of example 126, wherein: the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, and the bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

[0227] Example 128. The method of any of examples 124 to 127, wherein the bit depth of the first display overlay component that is an alpha component in the first coded picture is different from the bit depth of the target picture.

[0228] Example 129. The method of any of examples 116 to 128, further including: receiving signaling of dimensions of an alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

[0229] Example 130. The method of any of examples 116 to 129, further including: receiving signaling of a bit depth of an alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

[0230] Example 131. The apparatus of example 62, wherein the apparatus further comprises means for performing the methods as described in any of the examples 71 to 82.

[0231] Example 132. The apparatus of example 63, wherein the apparatus further comprises means for performing the methods as described in any of the examples 84 to 97.

[0232] Example 133. The apparatus of example 64, wherein the apparatus further comprises means for performing the methods as described in any of the examples 99 to 115.

[0233] Example 134. The apparatus of example 65, wherein the apparatus further comprises means for performing the methods as described in any of the examples 117 to 130.

[0234] Example 135. The computer readable medium of example 66, wherein the computer readable medium further comprises instructions stored thereon for performing the methods as described in any of the examples 71 to 82.

[0235] Example 136. The computer readable medium of example 66 or 135, wherein the computer readable medium comprises a non-transitory computer readable medium.

[0236] Example 137. A computer readable medium of example 67, wherein the computer readable medium further comprises instructions stored thereon for performing the methods as described in any of the examples 84 to 97.

[0237] Example 138. The computer readable medium of example 67 or 137, wherein the computer readable medium comprises a non-transitory computer readable medium.

[0238] Example 139. The computer readable medium of example 68, wherein the computer readable medium further comprises instructions stored thereon for performing the methods as described in any of the examples 99 to 115.

[0239] Example 140. The computer readable medium of example 68 or 139, wherein the computerreadable medium comprises a non-transitory computer readable medium.

[0240] Example 141. The computer readable medium of example 69, wherein the computer readable medium further comprises instructions stored thereon for performing the methods as described in any of the examples 117 to 130.

[0241] Example 142. The computer readable medium of example 69 or 141, wherein the computer readable medium comprises a non-transitory computer readable medium.

[0242] References to a ‘computer’, 'processor', etc. should be understood to encompass not only computers having different architectures such as single / multi-processor architectures and sequential / parallel architectures but also specialized circuits such as field-programmable gate arrays (FPGAs), application specific circuits (ASICs), signal processing devices and other processing circuitry. References to computer program, instructions, code etc. should be understood to encompass software for a programmable processor or firmware such as, for example, the programmable content of a hardware device such as instructions for a processor, or configuration settings for a fixed-function device, gate array or programmable logic device, etc.

[0243] The term “non-transitory,” as used herein, is a limitation of the medium itself (i.e., tangible, not a signal) as opposed to a limitation on data storage persistency (e.g., RAM vs. ROM).

[0244] As used herein, the term 'circuitry', 'circuit' and variants may refer to any of the following: (a) hardware circuit implementations, such as implementations in analog and / or digital circuitry, and (b) combinations of circuits and software (and / or firmware), such as (as applicable): (i) a combination of processor(s) or (ii) portions of processor(s) / software including digital signal processor(s), software, and one or more memories that work together to cause an apparatus to perform various functions, and (c) circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even when the software or firmware is not physically present. As a further example, as used herein, the term 'circuitry' would also cover an implementation of merely a processor (or multiple processors) or a portion of a processor and its (or their) accompanying software and / or firmware. The term 'circuitry' would also cover, for example and when applicable to the particular element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in a server, a cellular network device, or another network device. Circuitry or circuit may also be used to mean a function or a process used to execute a method.

[0245] It should be understood that the foregoing description is only illustrative. Various alternatives and modifications may be devised by those skilled in the art. For example, features recited in the various dependent claims could be combined with each other in any suitable combination(s). In addition, features from different embodiments described above could be selectively combined into a new embodiment. Accordingly, the description is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.

[0246] The following acronyms and abbreviations that may be found in the specification and / or the drawing figures are defined as follows (the abbreviations may be appended with each other or with other charactersusing e.g. a hyphen, dash (-), or number (or abbreviations having a character may be the same with a character removed), and may be case insensitive):2D two-dimensional3D three-dimensionalASIC application specific integrated circuitaYUV alpha channel (a) aggregated in front of texture components including luma component Y and two chroma components U and VCPU central processing unitCVS coded video sequenceDCT Discrete Cosine TransformDOI display overlays informationENG EnglishFPGA field programmable gate arrayFR FrenchGPU graphics processing unitH.2xx family of video coding standards (e.g. H.264, H.265, H.266, H.274)HEVC high efficiency video codingHMD head-mounted displayl / F interfaceI / O input / outputJVET joint video experts teamN / W networkRAM random access memoryRFM reference frame memoryROM read only memorySEI supplemental enhancement informationSON self-organizing / optimizing networkTuC technology under considerationTV televisionUI user interfaceUSB universal serial busVSEI versatile supplemental enhancement informationVVC versatile video codingYUV color model that includes a luma component Y and two chroma components U andV

Claims

CLAIMSWhat is claimed is:

1. An apparatus comprising:at least one processor; andat least one memory storing instructions that, when executed by the at least one processor, cause the apparatus to perform at least:providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

2. The apparatus of claim 1, wherein the information message within the bitstream additionally comprises an indication of an initialization mode for the alpha channel.

3. The apparatus of claim 2, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized as fully transparent.

4. The apparatus of claim 2, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized to the value zero.

5. The apparatus of claim 2, wherein the indication of the initialization mode for the alpha channel comprises a mode that indicates that samples of the alpha channel should be initialized to a constant value, and wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:providing an indication of the constant value.

6. The apparatus of claim 2, wherein the indication of the initialization mode for the alpha channel indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

7. The apparatus of claim 6, wherein the alpha channel carried in the bitstream comprises at least one of:a picture, ora subpicture, ora constituent rectangle.

8. The apparatus of claim 1, wherein dimensions of the alpha channel of the target picture are derived to be the same as dimensions of the target picture.

9. The apparatus of claim 1, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:signaling of dimensions of the alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

10. The apparatus of claim 1, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:providing an indication of dimensions of the alpha channel of the target picture.

11. The apparatus of claim 1, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:determining to not encode initialization signaling into or along the bitstream, when an indicator of an initialization mode for the alpha channel indicates that samples of the alpha channel should be initialized as fully transparent.

12. The apparatus of claim 1, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:signaling of a bit depth of the alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

13. The apparatus of claim 1, wherein the alpha channel is initialized with full transparency, when the indication of the information message within the bitstream that indicates that the target picture has an alpha channel does not explicitly indicate that the target picture has an alpha channel.

14. An apparatus comprising:at least one processor; andat least one memory storing instructions that, when executed by the at least one processor, cause the apparatus to perform at least:providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

15. The apparatus of claim 14, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:providing, in the bitstream, display overlays information comprising a flag indicating whether or not the target picture has an alpha channel.

16. The apparatus of claim 14, wherein the information message within the bitstream comprising the indication of the bit depth of the target picture also comprises an indication of a bit depth of an alpha channel of the target picture.

17. The apparatus of claim 14, wherein:the information message comprises information describing a first display overlay component, the bitstream comprises a first coded picture containing the first display overlay component, and the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:determining a bit depth of the first coded picture containing the first display overlay component; andcomputing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

18. The apparatus of claim 17, wherein when the bit shift value is non-zero, formation of the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

19. The apparatus of claim 17, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

20. The apparatus of claim 19, wherein:the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, andthe bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

21. The apparatus of claim 17, wherein the bit depth of the first display overlay component in the first coded picture is different from the bit depth of the target picture.

22. The apparatus of claim 14, wherein:the information message comprises an indication that the target picture has an alpha channel, the bitstream comprises a first coded picture containing a first display overlay component, and the first display overlay component is an alpha component,and the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:determining a bit depth of the alpha channel of the target picture;determining a bit depth of the first coded picture from a parameter set, and computing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

23. The apparatus of claim 22, wherein when the bit shift value is non-zero, formation of the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

24. The apparatus of claim 22, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

25. The apparatus of claim 24, wherein:the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, andthe bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

26. The apparatus of claim 22, wherein the bit depth of the first display overlay component that is an alpha component in the first coded picture is different from the bit depth of the target picture.

27. The apparatus of claim 14, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:signaling of dimensions of an alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

28. The apparatus of claim 14, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:signaling of a bit depth of an alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

29. An apparatus comprising:at least one processor; andat least one memory storing instructions that, when executed by the at least one processor, cause the apparatus to perform at least:receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

30. The apparatus of claim 29, wherein the information message within the bitstream additionally comprises an indication of an initialization mode for the alpha channel.

31. The apparatus of claim 30, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized as fully transparent.

32. The apparatus of claim 31, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:initializing the samples of the alpha channel as fully transparent, based on the default mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alpha channel should be initialized as fully transparent.

33. The apparatus of claim 30, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized to the value zero.

34. The apparatus of claim 33, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:initializing the samples of the alpha channel to the value zero, based on the default mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alpha channel should be initialized to the value zero.

35. The apparatus of claim 30, wherein the indication of the initialization mode for the alpha channel comprises a mode that indicates that samples of the alpha channel should be initialized to a constant value, and wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:receiving an indication of the constant value.

36. The apparatus of claim 35, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:initializing the samples of the alpha channel to the constant value, based on the mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alpha channel should be initialized to the constant value and the received indication of the constant value.

37. The apparatus of claim 30, wherein the indication of the initialization mode for the alpha channel indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

38. The apparatus of claim 37, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:initializing the alpha channel of the target picture with the alpha channel carried in the bitstream, based on the indication of the initialization mode for the alpha channel that indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

39. The apparatus of claim 37, wherein the alpha channel carried in the bitstream comprises at least one of:a picture, ora subpicture, ora constituent rectangle.

40. The apparatus of claim 29, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:derive dimensions of the alpha channel of the target picture to be the same as dimensions of the target picture.

41. The apparatus of claim 29, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:receiving signaling of dimensions of the alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

42. The apparatus of claim 29, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:receive an indication of dimensions of the alpha channel of the target picture.

43. The apparatus of claim 29, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:receiving an indicator of an initialization mode for the alpha channel that indicates that samples of the alpha channel should be initialized as fully transparent;wherein initialization signaling is not decoded from or along the bitstream.

44. The apparatus of claim 29, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:forming the target picture by overlaying the multiple ordered display overlays according to the specified order and the alpha channel.

45. The apparatus of claim 29, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:receiving signaling of a bit depth of the alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

46. The apparatus of claim 29, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:initialize the alpha channel with full transparency, when the indication of the information message within the bitstream that indicates that the target picture has an alpha channel does not explicitly indicate that the target picture has an alpha channel.

47. An apparatus comprising:at least one processor; andat least one memory storing instructions that, when executed by the at least one processor, cause the apparatus to perform at least:receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

48. The apparatus of claim 47, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:receiving, in the bitstream, display overlays information comprising a flag indicating whether or not the target picture has an alpha channel.

49. The apparatus of claim 47, wherein the information message within the bitstream comprising the indication of the bit depth of the target picture also comprises an indication of a bit depth of an alpha channel of the target picture.

50. The apparatus of claim 47, wherein:the information message comprises information describing a first display overlay component, the bitstream comprises a first coded picture containing the first display overlay component, and the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:determining a bit depth of the first coded picture containing the first display overlay component; andcomputing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

51. The apparatus of claim 50, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:forming the target picture by applying a bit shift operation to samples of the first coded picture based on the computed bit shift value, when the bit shift value is non-zero.

52. The apparatus of claim 50, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

53. The apparatus of claim 52, wherein:the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, andthe bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

54. The apparatus of claim 50, wherein the bit depth of the first display overlay component in the first coded picture is different from the bit depth of the target picture.

55. The apparatus of claim 47, wherein:the information message comprises an indication that the target picture has an alpha channel, the bitstream comprises a first coded picture containing a first display overlay component, and the first display overlay component is an alpha component,and the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:determining a bit depth of the alpha channel of the target picture;determining a bit depth of the first coded picture from a parameter set, and computing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

56. The apparatus of claim 55, wherein when the bit shift value is non-zero, formation of the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

57. The apparatus of claim 55, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

58. The apparatus of claim 57, wherein:the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, andthe bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

59. The apparatus of claim 55, wherein the bit depth of the first display overlay component that is an alpha component in the first coded picture is different from the bit depth of the target picture.

60. The apparatus of claim 47, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:receiving signaling of dimensions of an alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

61. The apparatus of claim 47, wherein the at least one memory stores instructions that, when executed by the at least one processor, cause the apparatus at to perform at least:receiving signaling of a bit depth of an alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

62. An apparatus comprising:means for providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

63. An apparatus comprising:means for providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

64. An apparatus comprising:means for receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

65. An apparatus comprising:means for receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

66. A computer readable medium comprising instructions stored thereon for performing at least the following:providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

67. A computer readable medium comprising instructions stored thereon for performing at least the following:providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

68. A computer readable medium comprising instructions stored thereon for performing at least the following:receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

69. A computer readable medium comprising instructions stored thereon for performing at least the following:receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

70. A method comprising:providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

71. The method of claim 70, wherein the information message within the bitstream additionally comprises an indication of an initialization mode for the alpha channel.

72. The method of claim 71, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized as fully transparent.

73. The method of claim 71, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized to the value zero.

74. The method of claim 71, wherein the indication of the initialization mode for the alpha channel comprises a mode that indicates that samples of the alpha channel should be initialized to a constant value, and wherein the method further comprises:providing an indication of the constant value.

75. The method of claim 71, wherein the indication of the initialization mode for the alpha channel indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

76. The method of claim 75, wherein the alpha channel carried in the bitstream comprises at least one of:a picture, ora subpicture, ora constituent rectangle.

77. The method of claim 70, wherein dimensions of the alpha channel of the target picture are derived to be the same as dimensions of the target picture.

78. The method of claim 70, further comprising:signaling of dimensions of the alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

79. The method of claim 70, further comprising:providing an indication of dimensions of the alpha channel of the target picture.

80. The method of claim 70, further comprising:determining to not encode initialization signaling into or along the bitstream, when an indicator of an initialization mode for the alpha channel indicates that samples of the alpha channel should be initialized as fully transparent.

81. The method of claim 70, further comprising:signaling of a bit depth of the alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

82. The method of claim 70, wherein the alpha channel is initialized with full transparency, when the indication of the information message within the bitstream that indicates that the target picture has an alpha channel does not explicitly indicate that the target picture has an alpha channel.

83. A method comprising:providing, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

84. The method of claim 83, further comprising:providing, in the bitstream, display overlays information comprising a flag indicating whether or not the target picture has an alpha channel.

85. The method of claim 83, wherein the information message within the bitstream comprising the indication of the bit depth of the target picture also comprises an indication of a bit depth of an alphachannel of the target picture.

86. The method of claim 83, wherein:the information message comprises information describing a first display overlay component, the bitstream comprises a first coded picture containing the first display overlay component, and the method further comprises:determining a bit depth of the first coded picture containing the first display overlay component; andcomputing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

87. The method of claim 86, wherein when the bit shift value is non-zero, formation of the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

88. The method of claim 86, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

89. The method of claim 88, wherein:the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, andthe bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

90. The method of claim 86, wherein the bit depth of the first display overlay component in the first coded picture is different from the bit depth of the target picture.

91. The method of claim 83, wherein:the information message comprises an indication that the target picture has an alpha channel, the bitstream comprises a first coded picture containing a first display overlay component, and the first display overlay component is an alpha component, andthe method further comprises:determining a bit depth of the alpha channel of the target picture;determining a bit depth of the first coded picture from a parameter set, and computing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

92. The method of claim 91, wherein when the bit shift value is non-zero, formation of the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

93. The method of claim 91, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

94. The method of claim 93, wherein:the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, andthe bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

95. The method of claim 91, wherein the bit depth of the first display overlay component that is an alpha component in the first coded picture is different from the bit depth of the target picture.

96. The method of claim 83, further comprising:signaling of dimensions of an alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

97. The method of claim 83, further comprising:signaling of a bit depth of an alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

98. A method comprising:receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays within the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication that the target picture has an alpha channel.

99. The method of claim 98, wherein the information message within the bitstream additionally comprises an indication of an initialization mode for the alpha channel.

100. The method of claim 99, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized as fully transparent.

101. The method of claim 100, further comprising:initializing the samples of the alpha channel as fully transparent, based on the default mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alpha channel should be initialized as fully transparent.

102. The method of claim 99, wherein the indication of the initialization mode for the alpha channel comprises a default mode that indicates that samples of the alpha channel should be initialized to the value zero.

103. The method of claim 102, further comprising:initializing the samples of the alpha channel to the value zero, based on the default mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alphachannel should be initialized to the value zero.

104. The method of claim 99, wherein the indication of the initialization mode for the alpha channel comprises a mode that indicates that samples of the alpha channel should be initialized to a constant value, and the method further comprises:receiving an indication of the constant value.

105. The method of claim 104, further comprising:initializing the samples of the alpha channel to the constant value, based on the mode of the indication of the initialization mode for the alpha channel that indicates that the samples of the alpha channel should be initialized to the constant value and the received indication of the constant value.

106. The method of claim 99, wherein the indication of the initialization mode for the alpha channel indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

107. The method of claim 106, further comprising:initializing the alpha channel of the target picture with the alpha channel carried in the bitstream, based on the indication of the initialization mode for the alpha channel that indicates that the alpha channel of the target picture is to be initialized with an alpha channel carried in the bitstream.

108. The method of claim 106, wherein the alpha channel carried in the bitstream comprises at least one of:a picture, ora subpicture, ora constituent rectangle.

109. The method of claim 98, further comprising:derive dimensions of the alpha channel of the target picture to be the same as dimensions of the target picture.

110. The method of claim 98, further comprising:receiving signaling of dimensions of the alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

111. The method of claim 98, further comprising:receive an indication of dimensions of the alpha channel of the target picture.

112. The method of claim 98, further comprising:receiving an indicator of an initialization mode for the alpha channel that indicates that samples of the alpha channel should be initialized as fully transparent;wherein initialization signaling is not decoded from or along the bitstream.

113. The method of claim 98, further comprising:forming the target picture by overlaying the multiple ordered display overlays according to thespecified order and the alpha channel.

114. The method of claim 98, further comprising:receiving signaling of a bit depth of the alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

115. The method of claim 98, further comprising:initialize the alpha channel with full transparency, when the indication of the information message within the bitstream that indicates that the target picture has an alpha channel does not explicitly indicate that the target picture has an alpha channel.

116. A method comprising:receiving, in a bitstream, an information message comprising metadata associated with multiple ordered display overlays within the bitstream,wherein a target picture is formable by overlaying the multiple ordered display overlays in the bitstream according to a specified order, andwherein the information message within the bitstream comprises an indication of a bit depth of the target picture.

117. The method of claim 116, further comprising:receiving, in the bitstream, display overlays information comprising a flag indicating whether or not the target picture has an alpha channel.

118. The method of claim 116, wherein the information message within the bitstream comprising the indication of the bit depth of the target picture also comprises an indication of a bit depth of an alpha channel of the target picture.

119. The method of claim 116, wherein:the information message comprises information describing a first display overlay component, the bitstream comprises a first coded picture containing the first display overlay component, and the method further comprises:determining a bit depth of the first coded picture containing the first display overlay component; andcomputing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

120. The method of claim 119, further comprising:forming the target picture by applying a bit shift operation to samples of the first coded picture based on the computed bit shift value, when the bit shift value is non-zero.

121. The method of claim 119, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

122. The method of claim 121, wherein:the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, andthe bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

123. The method of claim 119, wherein the bit depth of the first display overlay component in the first coded picture is different from the bit depth of the target picture.

124. The method of claim 116, wherein:the information message comprises an indication that the target picture has an alpha channel, the bitstream comprises a first coded picture containing a first display overlay component, and the first display overlay component is an alpha component, andthe method further comprises:determining a bit depth of the alpha channel of the target picture;determining a bit depth of the first coded picture from a parameter set, and computing a bit shift value based on the bit depth of the target picture and the bit depth of the first coded picture.

125. The method of claim 124, wherein when the bit shift value is non-zero, formation of the target picture comprises applying a bit shift operation to samples of the first coded picture based on the computed bit shift value.

126. The method of claim 124, wherein the bit shift value is computed by comparing the bit depth of the first coded picture to the bit depth of the target picture.

127. The method of claim 126, wherein:the bit shift is a right shift when the bit depth of the first coded picture is greater than the bit depth of the target picture, andthe bit shift is a left shift when the bit depth of the first coded picture less than the bit depth of the target picture.

128. The method of claim 124, wherein the bit depth of the first display overlay component that is an alpha component in the first coded picture is different from the bit depth of the target picture.

129. The method of claim 116, further comprising:receiving signaling of dimensions of an alpha channel of the target picture and signaling of dimensions of at least one texture channel of the target picture within a common syntax element.

130. The method of claim 116, further comprising:receiving signaling of a bit depth of an alpha channel of the target picture and signaling of a bit depth of at least one texture channel of the target picture within a common syntax element.

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