METHOD AND DEVICE FOR COLOR GAMMA MAPPING

Abstract

These principles relate to a method and device for gamut mapping from a first color gamut to a second color gamut. The method comprises obtaining a preserved key color (prsv_color k0) and a hue alignment angle (ok0) of the key color (color k0) for each key color (color k0) of at least 3 key colors to define a preserved gamut and a rotated gamut; and performing a hue mapping of a current color (color) from the first color gamut to the second color gamut, wherein, if the current color (color) is in a preserved gamut defined by the preserved key color (prsv_color k0), the hue-mapped current color remains unchanged, and if the current color is outside the preserved gamut, the hue-mapped current color is calculated from the hue alignment angle (ok0) of two adjacent key colors of the current color (rotated gamut) and from the preserved area.

Description

1 / 36 “METHOD AND DEVICE FOR COLOR GAMMA MAPPING” FIELD

[001] These principles generally relate to image / video encoding / decoding. Specifically, but not exclusively, the technical field of these principles relates to color gamut mapping of an image whose pixel values ​​belong to a high dynamic range and reverse gamut mapping of an image whose pixel values ​​belong to a low dynamic range. FOUNDATION

[002] This section is intended to introduce the reader to various aspects of the technique, which may be related to various aspects of the present principles that are described and / or claimed below. It is believed that this discussion will be useful in providing the reader with background information to facilitate a better understanding of the various aspects of the present principles. Therefore, it should be understood that these statements should be read in this light, and not as an admission of prior art.

[003] In what follows, an image contains one or more sample matrices (pixel values) in a specific image / video format that specifies all the information relating to the pixel values ​​of an image (or video) and all the information that can be used by a display and / or any other device to view and / or decode an image (or video), for example. An image comprises at least one component, in the form of a first sample matrix, usually a luma (or luminance) component, and possibly at least one other component, in the form of at least one other sample matrix, usually a chroma component. Or, equivalently, the same information can also be represented by a set of color sample matrices, such as the traditional trichromatic RGB representation. Petition 870260047576, dated 05 / 19 / 2026, page 8 / 89 2 / 36

[004] A pixel value is represented by a vector of C values, where C is the number of components. Each value in a vector is represented with a number of bits that defines a maximum dynamic range of pixel values.

[005] Low dynamic range images (LDR images) are images whose luminance values ​​are represented with a limited number of bits (usually 8 or 10). This limited representation does not allow for the correct rendering of small signal variations, particularly in dark and bright luminance ranges. In high dynamic range images (HDR images), the signal representation is extended in order to maintain high signal accuracy throughout its range. In HDR images, the pixel values ​​representing luminance levels are usually represented in floating-point format (32 bits or 16 bits for each component, namely single precision (float) or half-precision (half-float)), the most popular format being the half-precision openEXR format (16 bits per RGB component, i.e., 48 bits per pixel) or in integers with a long representation, typically at least 16 bits.

[006] The arrival of the High Efficiency Video Coding (HEVC) standard (ITU-T H.265 Telecommunication standardization sector of ITU (10 / 2014), series H: audiovisual and multimedia systems, infrastructure of audiovisual services - coding of moving video, High efficiency video coding, Recommendation ITU-T H.265) allows the deployment of new video services with enhanced viewing experience, such as Ultra HD broadcasting services. In addition to higher spatial resolution, Ultra HD can offer a wider color gamut (WCG) and a higher dynamic range (HDR) than the standard dynamic range HD-TV (SDR) currently deployed. Different solutions for HDR / WCG video representation and encoding have been proposed (SMPTE 2014, “High Dynamic Range Electro-Optical Transfer Function of Mastering Reference Displays”, or SMPTE ST 2084, 2014, or Diaz, R., Blinstein, S. and Qu, S.“Integrating HEVC Video Compression with a High Dynamic Range Video Pipeline”, SMPTE Motion Imaging Journal, Vol. 125, Issue 1. February, 2016, pp 14-21). Petition 870260047576, dated 05 / 19 / 2026, page 9 / 89 3 / 36

[007] Backward compatibility of SDR with decoding and rendering devices is an important feature in some video distribution systems, such as broadcast or multi-broadcast systems.

[008] Double-layer encoding is one solution to support this feature. However, due to its multi-layered design, this solution is not suited to all distribution workflows.

[009] One alternative is a single-layer HDR distribution solution, as defined by the ETSI recommendation: ETSI TS 103 433. The reader may also consult the IBC 2016 paper (“A single-Layer HDR video coding framework with SDR compatibility”, E. François and L. Van de Kerkhof, IBC 2016) for further details. This single-layer distribution solution is SDR compatible and leverages existing SDR distribution networks and services. It enables high-quality HDR rendering on HDR-compatible CE (Consumer Electronics) devices, as well as providing high-quality SDR rendering on CE SDR devices.

[010] This single-layer distribution solution is based on a single-layer encoding / decoding process and is codec-independent (a 10-bit codec is recommended).

[011] This single-layer distribution solution uses side metadata (a few bytes per scene or video frame) that can be used in a post-processing stage to reconstruct the HDR signal from a decoded SDR signal.

[012] When the decoded SDR signal and the reconstructed HDR signal do not share the same color space, a reverse gamma mapping (being the dual function of a gamma mapping in a pre-processing stage in HDR encoding) can be used in such a post-processing stage. In practice, color gamuts can be defined by standards such as NTSC, ITU-R BT rec.709 (“rec. 709”), ITU-R BT rec. 2020 (“rec. 2020”), Adobe RGB, DCI-P3, or any other present or future standard for color reproduction or any other restriction(s) on the range of colors. Petition 870260047576, dated 05 / 19 / 2026, page 10 / 89 4 / 36 In the case of single-layer HDR / WCG distribution, “color gamut mapping” is the process of mapping or redistributing colors from the wider color gamut (“source colors”) to colors in a narrower color gamut (“target colors”). However, existing color gamut mapping methods are problematic because they result in a deterioration of color consistency (e.g., a distortion of a color's neighborhood in the resulting target color gamut) after color gamut mapping. Such problems arise from the compression or expansion of saturation and / or hue by existing methods and / or the luminance of colors relative to the boundaries of the source and target color gamuts.In color gamut mapping, and notably in contour color gamut mapping, non-uniform saturation modifications can occur when there is a significant mismatch between the primary colors defining the source color gamut and the primary colors defining the target color gamut (e.g., misaligned gamut boundaries). Thus, the saturation modification induced by color gamut mapping can be quite different for similar hues (e.g., for a hue angle of 75°, the saturation might be divided by a factor of 1.6, while for a hue angle of 85°, the saturation might be divided by a factor of 1.2). This results in the problem of degrading the consistency of mapped colors within a color neighborhood.

[013] Additional problems arise when a single point (e.g., a primary or secondary color) on the gamut boundary of the source color gamut and the corresponding single point on the gamut boundary of the target color gamut have different hues. Indeed, when mapping colors from a wide color gamut to a narrower one, saturation needs to be compressed; otherwise, the colors are clipped in the narrower color gamut, leading to visual artifacts (e.g., loss of detail). However, clipping is not reversible and induces loss of detail, which cannot be reversed by reverse gamut mapping during rendering. Petition 870260047576, dated 05 / 19 / 2026, page 11 / 89 5 / 36 in HDR. An alternative to this clipping is saturation compression, which means that the color is moved in the direction of the gray axis along a 'constant hue' line to alter the hue as little as possible ('constant hue' is specific to the working color space). However, in XYZ and even in so-called perceptual color spaces, such as CIE L*a*b* or IPT, perceived hues are not perfectly aligned on hue lines (perceived hue versus color space hue line). In this way, saturation compression leads to a change in perceived hue. For example, in the L*a*b* color space, a pure rec. 2020 red mapped to rec. 709 appears orange. Furthermore, shifting the hue lines so that the perceived hue of the most saturated colors (i.e., at the cusp) is preserved would lead to a hue shift for less saturated colors.

[014] An invertible gamma mapping method that preserves a perceived hue is therefore desirable to improve HDR signal rendering. SUMMARY

[015] The following is a simplified summary of these principles in order to provide a basic understanding of some aspects of these principles. This summary is not a comprehensive overview of these principles. It is not intended to identify key or critical elements of these principles. The following summary presents only some aspects of these principles in a simplified way, as a prelude to the more detailed description provided below.

[016] The present principles set forth to remedy at least one of the drawbacks of the previous technique with a method for invertible color gamut mapping of an image comprising a preservation area that maintains the hue unchanged for less saturated colors, and a geometric hue mapping based on the preservation area and a hue color correction required by primary and / or secondary colors for colors with higher saturation.

[017] According to a first aspect, a method for mapping Petition 870260047576, dated 05 / 19 / 2026, page 12 / 89 6 / 36 An invertible color gamut of an image is disclosed, which comprises, for each key color of at least 3 key colors, obtaining a preserved key color and a hue alignment angle of the key color; and performing a hue mapping of the current color from the first color gamut to the second color gamut, wherein: If the current color is within a preserved area defined by the preserved key color, the hue-mapped current color is the current color. If the current color is outside the preserved area, the hue-mapped current color is calculated from the hue alignment angle of two adjacent key colors of the current color and from the preserved area.

[018] According to a specific characteristic, an actual color is defined by a representative angle of a hue and by a representative absolute value of a saturation on a chroma plane.

[019] According to another specific characteristic, performing the hue mapping of the current color also includes: • Determine two adjacent key colors from among the at least three key colors of the current color; • For each adjacent key color, determine a rotated key color by projecting the key color onto a rotated hue line, with the hue line passing through the rotated key color at the hue alignment angle; • For each adjacent key color, determine a direct key hue line, being a line through the key color and the preserved key color; • For each adjacent key color, determine a reverse key hue line, being a line through the rotated key color and the preserved key color; • Determine a direct anchor at the intersection between the direct key hue line of the 2 adjacent key colors; • Determine a reverse anchor at the intersection between the reverse key hue lines of the two adjacent key colors; Petition 870260047576, dated 05 / 19 / 2026, page 13 / 89 7 / 36 • determine a preserved current color at the intersection of a defined direct hue line between the current color and the direct anchor and a preservation line defined between the preserved key color of the 2 adjacent key colors; and • determine the hue-mapped current color, where if the saturation of the current color is below the saturation of the preserved current color, then the hue-mapped current color is the current color; Otherwise, the current hue-mapped color is obtained by projecting the current color onto a reverse hue line defined by the preserved color and the reverse anchor.

[020] According to different variants of another specific feature, the preserved key color is located on one of a key hue line defined between a chroma plane origin and the key color, a rotated key hue line defined as the key hue line rotated by the hue rotation angle, a median key hue line defined as the key hue line rotated by half the hue rotation angle.

[021] According to different variants of another specific characteristic, a key color is selected from a group of primary colors and secondary colors.

[022] According to different variants of another specific characteristic, the key projection color is one of an oblique projection, an orthogonal projection, a rotation around the origin of the chroma plane, a rotation around the preserved key color.

[023] According to another specific feature, obtaining a preserved key color and a key color hue alignment angle involves receiving metadata relating to parameters used for invertible color gamut mapping.

[024] According to a second aspect, a device for gamut mapping from a first color gamut to a second color gamut is disclosed. The device comprises a processor configured to obtain a preserved key color and a hue alignment angle for each key color of at least 3 Petition 870260047576, dated 05 / 19 / 2026, page 14 / 89 8 / 36 key colors; and to perform a hue mapping of the current color from the first color gamut to the second color gamut, where: If the current color is within a preserved area defined by the preserved key color, the hue-mapped current color is the current color. If the current color is outside the preserved area, the hue-mapped current color is calculated from the hue alignment angle of two adjacent key colors of the current color and from the preserved area.

[025] In one variant, the device for color gamut mapping from a first color gamut to a second color gamut comprises means for obtaining a preserved key color and a hue alignment angle for each key color of at least 3 key colors; and means for performing a hue mapping of the current color from the first color gamut to the second color gamut, wherein in the case of the current color being in a preserved area defined by said preserved key color, the hue-mapped current color is the current color, and in the case of the current color being outside said preserved area, the hue-mapped current color is calculated from the hue alignment angle of two adjacent key colors of the current color and from the preserved area.

[026] According to a third aspect, a signal having an SDR video and parameter values ​​used for invertible color gamut mapping is disclosed. The signal is further formatted to comprise HueAdjmode, HueGlobalPreservationRatio, huePreservationratio[c], hueAlignCorrectionPresentflag and hueAlignCorrection[c].

[027] According to a fourth aspect, a processor-readable nontransient medium whose contents store an SDR video and metadata relating to parameters used for invertible color gamut mapping, the processor-readable nontransient medium further comprises HueAdjmode, HueGlobalPreservationRatio, huePreservationratio[c], hueAlignCorrectionPresentflag and hueAlignCorrection[c]. Petition 870260047576, dated 05 / 19 / 2026, page 15 / 89 9 / 36

[028] According to a fifth aspect, a computer program product comprising program code instructions for executing the steps of the disclosed method when this program is run on a computer is disclosed.

[029] According to a sixth aspect, a processor-readable medium is disclosed, which has stored in it instructions to cause a processor to perform at least the steps of the disclosed method.

[030] According to a seventh aspect, a non-transient program storage device is disclosed, which is computer-readable, tangibly embodies a program of instructions executable by the computer to perform the disclosed method.

[031] Although not explicitly described, the present embodiments may be employed in any combination or subcombination. Furthermore, any feature or embodiment described for the method is compatible with a device intended to process the disclosed method and with a computer-readable storage medium storing program instructions. BRIEF DESCRIPTION OF THE DRAWINGS

[032] The drawings illustrate examples of these principles. They are: Figure 1 shows an end-to-end workflow supporting the production and distribution of content for HDR and SDR displays; Figure 2a shows the pre-processing stage in more detail; Figure 2b shows the HDR to SDR decomposition in more detail; Figure 3a shows the post-processing stage in more detail; Figure 3b shows the HDR reconstruction process in more detail; Figure 4a shows a block diagram of the steps of a method for invertible range mapping according to examples of the present principles; Figure 4b shows a block diagram of the geometric hue mapping step according to examples of the present principles; Petition 870260047576, dated 05 / 19 / 2026, page 16 / 89 10 / 36 Figure 5a illustrates the geometric hue mapping onto a chroma plane according to examples of the present principles; Figure 5b illustrates the geometric hue mapping onto a chroma plane according to examples of the present principles; Figure 5c illustrates the geometric hue mapping without area preservation on a chroma plane of a YUV color space according to examples of the present principles; Figure 5d illustrates the area-preserved geometric hue mapping on a chroma plane of a YUV color space according to examples of the present principles; Figure 6 shows an example of a device architecture according to an example of the present principles; and Figure 7 shows two remote devices communicating through a communication network according to an example of the present principles;

[033] The same elements or similar elements are referenced with the same reference numbers. DESCRIPTION OF AN EXAMPLE OF THESE PRINCIPLES

[034] These principles will be described in more detail hereafter with reference to the accompanying figures, in which examples of these principles are shown. These principles may, however, be incorporated in many alternative forms and should not be interpreted as limited to the examples presented here. Therefore, although these principles are susceptible to various modifications and alternative forms, specific examples thereof are shown by way of examples in the drawings and will be described here in detail. It should be understood, however, that there is no intention to limit these principles to the particular forms disclosed, but, on the contrary, the disclosure should cover all modifications, equivalents and alternatives encompassed by the spirit and scope of these principles. Petition 870260047576, dated 05 / 19 / 2026, page 17 / 89 11 / 36 present principles as defined by the claims.

[035] The terminology used here is intended to describe only particular examples and is not intended to limit the present principles. As used in this document, the singular forms “a” and “the” are also intended to include the plural forms, unless the context clearly indicates otherwise. It will also be understood that the terms “comprises”, “comprising”, “includes” and / or “including”, when used in this specification, specify the presence of declared resources, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other resources, integers, steps, operations, elements, components and / or groups thereof. Furthermore, when an element is referred to as being “responsive” or “connected” to another element, it may be directly responsive to or connected to the other element, or intervening elements may be present.On the other hand, when an element is referred to as being “directly responsive” or “directly connected” to another element, there are no intervening elements present. As used here, the term “and / or” includes any and all combinations of one or more of the listed associated items and may be abbreviated as “ / ”.

[036] It will be understood that, although the terms first, second, etc. may be used here to describe various elements, these elements should not be limited by these terms. These terms are used only to distinguish one element from another. For example, a first element may be called a second element, and similarly, a second element may be called a first element without departing from the teachings of the present principles.

[037] Although some of the diagrams include arrows in communication paths to show a primary direction of communication, it should be understood that communication can occur in the opposite direction to the arrows shown.

[038] Some examples are described with regard to block diagrams and Petition 870260047576, dated 05 / 19 / 2026, page 18 / 89 12 / 36 Operational flowcharts in which each block represents a circuit element, module, or piece of code comprising one or more executable instructions to implement the specified logical function(s). It should also be noted that, in other implementations, the function(s) shown in the blocks may occur out of the order mentioned. For example, two blocks shown in succession may, in fact, be executed substantially simultaneously, or the blocks may sometimes be executed in reverse order, depending on the functionality involved.

[039] The reference here to “according to an example” or “in an example” means that a particular feature, structure or characteristic described in connection with the example can be included in at least one implementation of these principles. Occurrences of the phrase “according to an example” or “in an example” in various locations in the report do not necessarily all refer to the same example, nor are they necessarily mutually exclusive separate or alternative examples of other examples.

[040] The reference numbers appearing in the claims are merely illustrative and shall not have a limiting effect on the scope of the claims.

[041] Although not explicitly described, the present examples and variants may be used in any combination or subcombination.

[042] The present principles are described for decoding an image, but they extend to decoding a sequence of images (video) because each image in the sequence is encoded / decoded sequentially, as described below.

[043] Figure 1 shows an end-to-end workflow supporting the production and distribution of content for HDR and SDR displays. It involves single-layer SDR / HDR encoding-decoding with side metadata, as defined, for example, in the ETSI recommendation: ETSI TS 103 433. The reader may also consult the IBC 2016 paper (“A single-Layer HDR video coding framework”). Petition 870260047576, dated 05 / 19 / 2026, page 19 / 89 13 / 36 with SDR compatibility”, E. François and L. Van de Kerkhof, IBC 2016) for more details.

[044] In a pre-processing stage, an incoming HDR video is decomposed into an SDR video and metadata. The SDR video is then encoded with any SDR video codec and an SDR bitstream is transported over an existing SDR distribution network with associated metadata transmitted on a specific channel or embedded in the SDR bitstream.

[045] Preferably, the encoded video is an HEVC codec, such as the H.265 / HEVC or H.264 / AVC codec.

[046] Metadata is typically transported by SEI messages when used in conjunction with an H.265 / HEVC or H.264 / AVC codec.

[047] The SDR bitstream is decoded and a decoded SDR video is then available for an SDR Consumer Electronics (CE) display.

[048] Next, in a post-processing stage, which is functionally the reverse of the pre-processing stage, the HDR video is reconstructed from the decoded SDR video and the metadata obtained from a specific channel or SDR bitstream.

[049] Figure 2a shows the preprocessing stage in more detail.

[050] A core component of the preprocessing stage is the decomposition of HDR to SDR which generates an SDR video and dynamic metadata from the HDR video.

[051] More precisely, HDR to SDR decomposition aims to convert an HDR video represented in a specific input format into an SDR video represented in a specific output format according to the method disclosed below, but the present principles are not limited to the specific input / output format (color space or gamut).

[052] Optionally, the HDR video format, or the SDR video format, can be adapted to the aforementioned specific input format, Petition 870260047576, dated 05 / 19 / 2026, page 20 / 89 14 / 36 respectively, to the specific output format.

[053] The aforementioned adaptation to the input / output format may include color space conversion and / or color gamut mapping. Common format adaptation processes may be used, such as RGB to YUV or YUV to RGB conversion, BT.709-to-BT.2020 or BT.2020-to-BT.709, chroma component subsampling or oversampling, etc.

[054] HDR to SDR decomposition aims to convert an input linear light 4:4:4 RGB HDR video into an SDR-compatible version. The process uses static metadata, such as the primary colors and gamma of the HDR and SDR images.

[055] Optionally, the HDR video format can be pre-adapted to the predetermined input format of the pre-processing stage and / or a gamma mapping can be used when the HDR video (input of the HDR decomposition stage) and the SDR video (output of the HDR decomposition stage) are represented in different color spaces.

[056] Figure 2b shows the HDR to SDR decomposition in more detail.

[057] Next, HDR video samples are represented in the RGB color space (specific input format) and SDR video samples are represented in the RGB color space (specific output format).

[058] In step 1, the HDR video is analyzed frame by frame in order to derive a set of mapping parameters that will be used later to convert the HDR video into SDR video.

[059] In step 2, the luminance component L of a current HDR video image to be decomposed is the luminance mapped to an SDR luma component Yi. The resulting signal is the SDR luma (the SDR luma component Yi) given by: Petition 870260047576, dated 05 / 19 / 2026, page 21 / 89 15 / 36 ΓΛ1 = A 1 G B. Yl= TM[L] (2) where A = [A1A2A3]T is the canonical 3x3 R'G'B'-to-Y'CbCr conversion matrix (e.g., as specified in ITU-R Rec. BT.2020 or ITU-R Rec. BT.709 depending on the color space), A1A2A3 being 1x3 matrices and where the luminance mapping is based on a perceptual transfer function TM (e.g., also specified in ITU-R Rec. BT.2020 or ITU-R Rec. BT.709 depending on the color space).

[060] In step 3 of Figure 2b, the chroma components are derived as follows. First, the R, G, B values ​​of the input HDR video are scaled by the ratio (Kz / L), which results in a linear RGB light SDR version. Next, a square root is applied to reproduce a transfer function close to the OETF (Optoelectric Transfer Function) ITU-R Rec. BT.709. Note that the use of a square root ensures the reversibility of the process.

[061] The resulting R, G, B signal is converted into chroma components ( / , Vz: MlA2 * .A3. R Έ (3) B

[062] In step 4, a final color correction is applied in order to match the colors. SDR with HDR video colors. First, the chroma components are adjusted by a scaling factor of 1 / β(Yι), where β(Yι) is a function that allows control of the saturation and color hue of the resulting SDR video. UsDR VsDR. vil(4) -----* β(Hι)

[063] This step allows control of SDR colors and ensures their combination with HDR colors.

[064] In step 6, an invertible gamma mapping process can be applied when the input SDR image of the SDR to HDR reconstruction process is provided in a BT.709 color gamut (as specified by the variable Petition 870260047576, dated 05 / 19 / 2026, page 22 / 89 16 / 36 prePicColourSpace), and is different from the target BT.2020 color gamut of the HDR image (as specified by the recPicColourSpace variable). Backward color compatibility is defined so that the SDR CE receiver only supports the BT.709 color space, while the video to be distributed using SL-HDR1 can support the BT.2020 color space. When recPicColourSpace is not equal to prePicColourSpace, on the HDR-to-SDR decomposition side, the WCG HDR video must be converted to a standard color gamut SDR video (plus metadata), while the reverse process on the HDR reconstruction side reverses this conversion by rendering the WCG HDR video from the standard color gamut SDR video (plus metadata). The cascade of these two color processes must be visually lossless, while the standard color gamut SDR video must fully preserve the artistic intent of the original WCG HDR video with minimal deficiencies.Compression conversions (gamma mapping) and color reconstruction (reverse gamma mapping) are specified as reciprocals.

[065] In step 5, the metadata relating to luminance mapping (step 2), color correction (step 4) and invertible gamma mapping (step 6) are transmitted to the post-processing stage. The metadata is transmitted as static metadata (if the parameters are not changed with the image) or dynamic metadata (if the parameters are changed with the image as in luminance mapping).

[066] This metadata allows fine control of the texture and colors of the SDR version, and ensures a good fit to the intention of HDR.

[067] Figure 3a shows the post-processing stage in more detail.

[068] The core component of the post-processing stage is SDR to HDR reconstruction, which reconstructs an HDR video from an SDR video (decoded) and metadata.

[069] More precisely, HDR reconstruction aims to convert SDR video Petition 870260047576, dated 05 / 19 / 2026, page 23 / 89 17 / 36 represented in a specific input format in an HDR video represented in a specific output format according to the modality disclosed below, but the present principles are not limited to specific input / output formats (gamma or color space).

[070] The aforementioned adaptation to the input or output format may include color space conversion and / or color gamut mapping. Common format adaptation processes may be used, such as RGB to YUV or YUV to RGB conversion, BT.709-to-BT.2020 or BT.2020-to-BT.709 etc. For example, see Appendix D of the ETSI recommendation: ETSI TS 103 433, which provides use cases for reverse gamut mapping.

[071] Optionally, the format of the reconstructed HDR video can be adapted to specific system characteristics (e.g., a set-top box type decoder, a connected TV) and / or reverse gamma mapping can be used when the decoded SDR video (input to the HDR reconstruction stage) and the reconstructed HDR video (output to the HDR reconstruction stage) are represented in different color spaces and / or gamut.

[072] Figure 3b shows the HDR reconstruction process in more detail.

[073] HDR reconstruction is the functional inverse of HDR decomposition to SDR (Figure 2b). However, for reasons of implementation complexity, some operations are concatenated or applied in a different order.

[074] In step 31, dynamic and / or static metadata is obtained, for example, from the SDR bitstream or a specific channel.

[075] In step 32, a lutMapY luminance mapping lookup table (a 1D lookup table) is derived from the obtained metadata. This luminance mapping lookup table corresponds to the inverse of the square root of the luminance mapping curve. Petition 870260047576, dated 05 / 19 / 2026, page 24 / 89 18 / 36

[076] In step 33, a lutCC color correction lookup table is derived from the obtained dynamic metadata. The lutCC color correction lookup table is linked to the βρ(Yι) preprocessing color correction (equation 4) and the lutMapY luminance mapping lookup table by the following equation: β[Y] = 2bx lutMapY [Y] x lutCC [Y] (5) where B is the bit depth of the luma component of the decoded SDR image.

[077] In step 34, an image of the reconstructed HDR video (linear light HDR video) is reconstructed by applying an HDR reconstruction to an image of the decoded SDR video using the luma-related lookup table derived from lutMapY and the color correction lookup table derived from lutCC. In step 36, the reverse gamma mapping or color reconstruction process allows generating a wide color gamut image from a standard color gamut image with associated metadata. This process is defined for a 4:4:4 chroma sampling and a full-range YUV linear light signal. The input YUV signal comes from converting an input RGB linear light signal (output of the SDR to HDR 34 reconstruction process) into a YUV color space, thanks to the canonical R'G'B'-Y-to-CCbCr matrix (computed thanks to SMPTE RP 177 [i.8]).This process also includes chroma remapping, luminosity remapping, and hue remapping according to the modalities described.

[078] Post-processing operates on a lutMapY luminance mapping lookup table (step 32), a lutCC color correction lookup table (step 33), and reverse gamma mapping parameters (step 35) defining a preservation area that keeps the hue unchanged for less saturated colors, and the amount of color correction needed per primary and secondary color for the other colors. The respective table and parameters are derived from metadata (step 31).

[079] The aforementioned metadata can be transmitted (step 5) in accordance with Petition 870260047576, dated 05 / 19 / 2026, page 25 / 89 19 / 36 the so-called parameter-based mode or table-based mode, in order to derive the lutMapY luminance mapping lookup table (step 32), the lutCC color correction lookup table (step 33) and the reverse gamma mapping (step 35) from the obtained metadata (step 31).

[080] Some metadata to be transmitted in step 5 are hue mapping parameters representative of a preservation area that maintains the hue unchanged for less saturated colors, and a hue color correction by primary and / or secondary colors for colors with higher saturation (ETSI recommendation: ETSI TS 103 433, section 6.3.10). Therefore, the following parameters are described: hueAdjMode, hueGlobalPreservationRatio, huePreservationRatio, hueAlignCorrectionPresentFlag, hueAlignCorrection. hueAdjMode

[081] This variable indicates the hue adjustment mode used by invertible gamma mapping. The value of hueAdjMode must be as defined in the following table: Value of hueAdjMode Definition 0 Hue adjustment disabled 1 Global linear hue adjustment method 2 Patched hue adjustment with globally preserved area 3 Patched hue adjustment with area preservation by primary and secondary colors hueGlobalPreservationRatio

[082] This variable indicates the percentage of global preservation for hue adjustment. This variable should only be invoked when hueAdjMode equals 2. The value of hueGlobalPreservationRatio must be in the limited range [0 to 0.875] and in multiples. Petition 870260047576, dated 05 / 19 / 2026, page 26 / 89 20 / 36 of (1 - 8). huePreservationRatio

[083] This array of six variables indicates the preservation ratio to be applied to each primary or secondary color during the hue adjustment process. This array should only be invoked when hueAdjMode equals 3. The index value c equal to 0 should correspond to the red primary, c equal to 1 should correspond to the magenta secondary, c equal to 2 should correspond to the blue primary, c equal to 3 should correspond to the cyan secondary, c equal to 4 should correspond to the green primary, c equal to 5 should correspond to the yellow secondary. The value of huePreservationRatio[ c ] should be in the limited range [0 to 0.875] and in multiples of (1 + 8). hueAlignCorrectionPresentFlag

[084] This variable indicates the presence of hue alignment correction parameters for each primary and secondary color. A hueAlignCorrectionPresentFlag value of 0 indicates that the parameters are not present. A hueAlignCorrectionPresentFlag value of 1 indicates that the parameters are present. hueAlignCorrection

[085] This array of five variables indicates the hue correction angle associated with each primary or secondary color during the hue adjustment process. This array should be invoked when hueAlignCorrectionPresentFlag equals 1 and when hueAdjMode does not equal 0. The index value c equal to 0 should correspond to the red primary, c equal to 1 should correspond to the magenta secondary, c equal to 2 should correspond to the blue primary, c equal to 3 should correspond to the cyan secondary, c equal to 4 should correspond to the green primary, c equal to 5 should correspond to the yellow secondary. The value of hueAlignCorrection[ c ] should be in the limited range [0 to 0.875] and in multiples of (1 + 8).

[086] This dynamic metadata can be transmitted using the message Petition 870260047576, dated 05 / 19 / 2026, p. 27 / 89 21 / 36 SEI is registered in the HEVC Color Volume Reconstruction Information (CVRI) user data, whose syntax is based on the SMPTE ST 2094-20 specification (ETSI recommendation: ETSI TS 103 433, Appendix A.3).

[087] In step 31, the SEI CVRI message is thus analyzed to obtain the luminance mapping parameters, the color correction parameters and the reverse gamma parameters.

[088] In step 32, the lutMapY luminance mapping lookup table is reconstructed (derived) from the obtained luminance mapping parameters (see ETSI TS 103 433, clause 7.2.3.1 for more details).

[089] In step 33, the lutCC color correction lookup table is reconstructed (derived) from the obtained color correction parameters (see ETSI TS 103 433, clause 7.2.3.2 for more details).

[090] In step 35, the geometric features (namely the rotated gamma and the preserved gamma) for reverse hue mapping are determined (derived) from the reverse gamma mapping parameters, as described below with the gamma mapping method.

[091] This metadata can be transmitted as dynamic metadata using the HEVC Color Remapping Information (CRI) SEI message, whose syntax is based on the SMPTE ST 2094-30 specification (ETSI recommendation: ETSI TS 103 433, Appendix A.4).

[092] Note that static metadata can also be used by the post-processing stage and transmitted via SEI message. For example, the selection of the hue mapping method can be performed by the SEI message recorded in the TS 103 433 Information (TSI) user data (payloadMode), as defined by ETSI TS 103 433 (clause A.2.2). Static metadata, such as, for example, primary colors or the dominant display luminance of the maximum display, are transmitted by a Color Volume SEI message. Petition 870260047576, dated 05 / 19 / 2026, page 28 / 89 22 / 36 Dominant Display (MDCV), as defined in AVC, HEVC.

[093] Figure 4a shows a block diagram of the steps of a method for invertible gamma mapping according to examples of the present principles. This method is compatible with any HDR-to-SDR decomposition process that produces an SDR image and metadata. For illustrative purposes, the gamma mapping method is part (step 6) of the HDR-to-SDR decomposition process, as described in relation to Figure 2b. This process is particularly suited when wide-gamut HDR video needs to be converted to a standard-gamut SDR video. However, since this method is fully invertible, the method is also compatible with any SDR-to-HDR reconstruction process that produces an HDR image. For illustrative purposes, the gamma mapping method is also part (step 36) of the SDR-to-HDR reconstruction process, as described in relation to Figure 3b.This process is particularly well-suited when standard color gamut SDR video needs to be converted back into wide color gamut HDR video. However, the method is also compatible with any process involving inverse and / or direct gamut mapping from a first color gamut to a second color gamut. HDR video samples and SDR video samples can be represented in any color space, such as YUV, CIELUV, L*a*b*, or IPT. For illustrative purposes, an HDR video sample, called a color, is represented by a representative angle of a hue and by a representative absolute value of a saturation in polar coordinates of a chroma plane with a constant luminance of an L*a*b* color space, as shown in Figure 5a.

[094] In a preliminary step 41, a preserved key color (prsv_color ko, prsv_color ki) and a hue alignment correction angle (θκο, Θκι) for the key color (color ko, color ki) are obtained. The key color (color ko, color ki) is selected from a group of primary and secondary colors. The key color (color ko, color ki) belongs Petition 870260047576, dated 05 / 19 / 2026, page 29 / 89 23 / 36 to the outline of the first range. A corresponding mapped key color (map_color ko) belongs to the outline of the second range. The hue alignment correction angle (θκο, Θκι) is defined between the hue line (hue_line ko, hue_line ki) of the key color (color ko, color ki) and the hue line (rot_hue_line ko) of the corresponding mapped key color (map_color ko). The hue alignment correction angle (θκο, Θκι) is thus representative of the amount of hue correction for a key color between the first and second ranges. For example, the hue alignment correction angle (θκο, Θκι) is defined so that the perceived hue of the mapped key color (including other color gamut mapping steps) is the same as the perceived hue of the source key color. At least 3 key colors are defined, and up to 6 key colors, as shown in Figure 5a.Preserved key colors (prsv_color ko, prsv_color ki) define a preserved area (prsv_area) or preserved gamut around the lightness axis and correspond to less saturated colors. In specific alternative variants, the preserved key color is on the key hue line (hue_line ko) defined between the center (i.e., origin) of the chroma plane and the key color (color ko), on the rotated key hue line (rot_hue_line ko) defined as the key hue line rotated by the hue rotation angle (θκο), or on a median key hue line defined as the key hue line rotated by half the hue rotation angle (θκο). In fact, the preserved key color should be anywhere on the chroma plane, preferably in the section between the key hue line (hue_lineKo) and the rotated key hue line (rot_hue_line ko) as shown in Figure 5b.The preserved key color is, for example, defined by an angle (between 0 and θκο) relative to the key hue line (hue_lineKo) and a saturation (between 0 and colorKo saturation). According to a particular feature, the colors in the preserved area are not modified (meaning, in this case, that the hue remains unchanged), and the colors that are not in the preserved area are hue-mapped according to reversible geometric hue mapping, as detailed below. In one. Petition 870260047576, dated 05 / 19 / 2026, page 30 / 89 In the 24 / 36 HDR-to-SDR decomposition process, key colors and corresponding preserved key colors, and hue alignment correction angles are defined and encoded as parameters for invertible gamma mapping, as described above. The hue mapping parameters thus define a preservation area (prsv_area) that keeps the hue unchanged for less saturated colors and the amount of color correction needed per primary and secondary color for other colors. The hue mapping parameters used for invertible gamma mapping are then sent as static or dynamic metadata for reverse gamma mapping. In the SDR-to-HDR reconstruction process, metadata relating to parameters used for invertible gamma mapping is received, and key colors and corresponding preserved key colors, and hue alignment correction angles are derived from the received parameters.

[095] In a step 42, a hue mapping of the current color (color) from the first color range to the second color range is applied. In a substep 426, the condition of the current color belonging to the preserved area is tested. If the current color (color) belongs to the preserved area (true condition) defined by the preserved key colors (prsv_color ko, prsv_color ki), the hue-mapped current color is the current color, no hue alignment (428) is performed for this, the hue of the current color is not changed. If the current color is outside the preserved area (false condition), the hue-mapped current color is calculated from the hue alignment angle (θκο, Θκι) of two adjacent key colors of the current color. The hue mapping is called geometric hue mapping as it is based on the geometric construction in the chroma plane. Advantageously, the hue compensation that is applied in the HDR to SDR decomposition process is used to improve the perceived hue for some saturated colors.This compensation is reversed in the reverse gamma mapping process during SDR to HDR reconstruction. Hue compensation keeps the luminance unchanged. This operation is almost equivalent to a rotation in the... Petition 870260047576, dated 05 / 19 / 2026, page 31 / 89 25 / 36 chroma plane, but with a significant portion of the narrow (pattern) gamut being preserved (i.e., no remapping). However, the preserved area feature is presented as part of the gamma mapping method; geometric hue mapping can be applied to the entire chroma plane without preserved gamma, as illustrated in Figure 5c. Those versed in the technique will appreciate that, in this case, the preserved color, and therefore the last defined forward and reverse anchor, are then located at the origin of the chroma plane.

[096] Figure 4b shows a block diagram of the geometric hue mapping step according to examples of the present principles. References for geometric construction are shown in Figure 5b, which illustrates geometric hue mapping on a chroma plane with polar coordinates. Step 42 of the current color hue mapping is now described.

[097] In a substep 421, 2 adjacent key colors (color ko, color ki) from among at least 3 key colors are determined for the current color. As shown in Figure 5a, the chroma plane is divided into sections, each section defined by two key colors. The adjacent key colors are the key colors defining the section to which the current color belongs. In other words, the adjacent key colors or neighboring key colors are the 2 key colors (color ko, color ki) such that the current color (color) is surrounded by the corresponding key hue lines in the chroma plane. Such adjacent key colors are, for example, determined based on the hue angle of the current color compared to the hue angle of key colors in the plane. In a variant, cross products are used to determine the position of the current color relative to the direct hue lines (since the sign of the cross vector product allows determining the relative position of a point with respect to a line).

[098] In a substep 422, for each adjacent key color (color ko), a rotated key color (rot_color ko) is determined by projecting the key color (color ko) onto a rotated hue line (rot_hue_line ko) being the hue line of the key color. Petition 870260047576, dated 05 / 19 / 2026, page 32 / 89 26 / 36 rotated by the hue alignment angle parameter (θκο). According to different variants, the projection is one of a rotation around the center of the chroma plane, an oblique projection (as shown in Figure 5b), an orthogonal projection, or a rotation around the preserved key color. In the variant of a hue rotation in the chroma plane (i.e., around the gray axis or origin of polar coordinates), the rotated key color is defined as the intersection between a circle centered on the gray axis (i.e., lightness axis) passing through the key color (color k0) with the rotated key hue line defined by the hue alignment angle (θκο). This variant corresponds to a true hue rotation, and the saturation is not altered by the hue mapping.According to the oblique projection variant illustrated in Figure 5b, the rotated key color (rot_color k0) is defined as the intersection of the line perpendicular to the key hue line (hue_linera) passing through the key color (color ko) with the rotated key hue line defined by the hue alignment angle (θκο) (oblique projection of the key color onto the rotated key hue line). According to the orthogonal projection variant, the rotated key color (rot_color ko) is defined as the intersection of the rotated key hue line with the line passing through the key color (color ko) and perpendicular to the rotated key hue line (rot_hue_line ko) (orthogonal projection of the key color onto the rotated key hue line). Yet another variant involves a local rotation around the preserved key color (prsv_colorKo).The rotated key color (rot_color ko) is defined as the intersection of a circle centered on the preserved key color (prsv_color ko) passing through the key color (color ko) with the rotated key hue line defined by the hue alignment angle (θκο). Unlike the variant of rotation around the center of the chroma plane, those last 3 variants induce saturation modification. However, for small alignment angles, these transformations are close to the actual hue rotation (i.e., around the gray axis) with low saturation modification. Petition 870260047576, dated 05 / 19 / 2026, page 33 / 89 27 / 36

[099] In a substep 423, for each adjacent key color (color ko, color ki), a forward hue line (forward hue lineKo, forward hue lineK1) being a line through the key color (color ko, color ki) and preserved key color (prsv_color ko, prsv_color ki) is determined. Additionally, in substep 423, for each adjacent key color (color ko, color ki), a reverse hue line (backward hue lineKo, backward hue lineK1) being a line through the rotated key color (rot_colorKo, rotated_color ki) and preserved key color (prsv_color ko, prsv_color ko) is also determined.

[0100] Those 2 adjacent forward hue lines (forward hue lineKo, forward hue lineK1) and backward hue lines (backward hue lineKo, backward hue lineK1) determine, in a substep 424, a forward anchor and a backward anchor. The forward anchor is defined as the intersection between the forward hue lines (forward hue lineKo, forward hue lineK1) of the 2 adjacent key colors (color ko, color ki). The backward anchor is defined as the intersection between the backward hue lines (backward hue lineKo, backward hue lineK1) of the 2 adjacent key colors (color ko, color ki). Those skilled in the art will observe that, in the variant where the preserved key color (prsv_colorKo) is on the hue line (hue_lineKo), the forward anchor is the center of the chroma plane; In the variant where the preserved key color (prsv_colorKo) is on the rotated key hue line (rot_hue_lineKo), the reverse anchor is the origin of the chroma plane.

[0101] In a substep 425, a preserved current color (prsrv_color) is defined as the intersection of a direct hue line defined between the current color (color) and the direct anchor and a preservation line (prsv_line) defined between the preserved key color (prsv_color ko, prsv_color ki) of the aforementioned 2 adjacent key colors (color ko, color ki). This preserved current color (prsrv_color) allows the determination of whether the current color is in the preserved area or not. According to different variants, the preservation line defined between the preserved key colors (prsv_color ko, prsv_color ki) is either a straight line (as shown in Figure 5b), or a curved line. Petition 870260047576, dated 05 / 19 / 2026, page 34 / 89 28 / 36

[0102] Next, in substep 426, the saturation of the current color (color) is compared to the saturation of the preserved current color (prsrv_color). If (true branch t) the saturation of the current color is below the saturation of the preserved current color (prsrv_color), then the hue-mapped current color is the current color. Otherwise, the hue-mapped current color is obtained by projecting the current color onto the reverse hue line defined by the preserved color (prsv_color) and the reverse anchor. The projection is the same as that used in step 422. Those versed in the technique will understand that, for reverse gamma mapping, forward and reverse features are inverted. Thus, the reverse hue-mapped current color is obtained by projecting a current color onto a forward hue line defined by the preserved color (prsv_color) and the forward anchor.

[0103] This method is based on any HDR reconstruction process that requires an SDR image and dynamic metadata.

[0104] For illustrative purposes, the HDR reconstruction process can be the HDR reconstruction process as described in Figure 3b. In this case, the HDR image is reconstructed from a decoded SDR image. However, the SDR image, used for HDR image reconstruction, can also be stored with compression and obtained without the need for decoding.

[0105] So, the method obtains, for example, by decoding an SDR bitstream, an SDR image (decoded) whose dynamic range of luminance values ​​is smaller than the dynamic range of luminance values ​​of the HDR image to be reconstructed.

[0106] In Figures 1-4b, modules are functional units, which may or may not be related to distinguishable physical units. For example, these modules, or some of them, may be assembled into a single component or circuit, or contribute to the functionalities of a software. On the other hand, some modules may potentially be composed of separate physical entities. Devices that are compatible with the present principles are implemented using pure hardware, for example, using dedicated hardware such as ASICs or FPGAs or VLSIs, respectively. Petition 870260047576, dated 05 / 19 / 2026, page 35 / 89 29 / 36 "Application-Specific Integrated Circuit", "Field Programmable Gate Array", "Very Large Scale Integration", or from several integrated electronic components incorporated into a device or from a mixture of hardware and software components.

[0107] Figure 6 represents an exemplary architecture of a device 60 that can be configured to implement a method described in relation to Figure 1-4b.

[0108] Device 60 comprises the following elements, which are connected by a data and address bus 61: - a 62 microprocessor (or CPU), which is, for example, a DSP (or Digital Signal Processor); - a ROM (or Read-Only Memory) 63; - a 64-bit RAM (or Random Access Memory); - an I / O interface 65 for receiving data to be transmitted from an application; and - a 66 battery.

[0109] According to an example, battery 66 is external to the device. In each memory mentioned, the word "register" used in the report may correspond to a small capacity area (a few bits) or a very large area (for example, an entire program or a large amount of received or decoded data). ROM 63 comprises at least one program and parameters. ROM 63 may store algorithms and instructions to perform techniques according to the described modalities. When powered on, CPU 62 loads the program into RAM and executes the corresponding instructions.

[0110] RAM 64 comprises, in a register, the program executed by CPU 62 and loaded after the initialization of device 60, input data in a register, intermediate data in different states of the method in a register, and other variables. Petition 870260047576, dated 05 / 19 / 2026, page 36 / 89 30 / 36 are used for executing the method on a record.

[0111] The implementations described in this document can be implemented, for example, in a method or process, a device, a software program, a data stream, or a signal. Even if only discussed in the context of a single implementation form (for example, discussed only as a method or a device), the implementation of the discussed features can also be implemented in other forms (for example, a program). A device can be implemented, for example, in appropriate hardware, software, and firmware. Methods can be implemented, for example, in a device such as, for example, a processor, which refers to processing devices in general, including, for example, a computer, a microprocessor, an integrated circuit, or a programmable logic device.Processors also include communication devices, such as, for example, computers, mobile phones, portable / personal digital assistants (“PDAs”), and other devices that facilitate the communication of information between end users.

[0112] According to an encoding example or encoder, the HDR video or an HDR image from an HDR video is played from a source. For example, the source belongs to a set comprising: - a local memory (63 or 64), for example, a video memory or a RAM (or Random Access Memory), a flash memory, a ROM (or Read Only Memory), a hard disk; - a storage interface (65), for example, an interface with a mass storage, a RAM, a flash memory, a ROM, an optical disk or a magnetic media; - a communication interface (65), for example, a wired interface (for example, a bus interface, a wide area network interface, a local area network interface) or a wireless interface (such as an IEEE interface) Petition 870260047576, dated 05 / 19 / 2026, page 37 / 89 31 / 36 802.11 or a Bluetooth® interface); and - an image capture circuit (for example, a sensor, such as a CCD (or Charge-Coupled Device) or CMOS (or Complementary Metal Oxide Semiconductor)). [ 0113] According to an example of decoding or decoder, the decoded SRD video or reconstructed HDR video is sent to a destination; specifically, the destination belongs to a set comprising: - a local memory (63 or 64), for example, a video memory or RAM, a flash memory, a hard drive; - a storage interface (65), for example, an interface with a mass storage, a RAM, a flash memory, a ROM, an optical disk or a magnetic media; - a communication interface (65), for example, a cable interface (for example, a bus interface (for example, USB (or Universal Serial Bus)), a wide area network interface, a local area network interface, an HDMI (High Definition Multimedia Interface) interface or a wireless interface (such as the IEEE 802.11, WiFi® or Bluetooth® interface); and - a display.

[0114] According to encoding examples or encoder, the SDR bitstream and / or the other bitstream carrying the metadata is sent to a destination. For example, one or both of these bitstreams are stored in local or remote memory, for example, video memory (64) or RAM (64), a hard disk (63). In a variant, one or both bitstreams are sent to a storage interface (65), for example, an interface with mass storage, flash memory, ROM, an optical disk or magnetic media and / or transmitted through a communication interface (65), for example, an interface with a point-to-point link, a communication bus, a point-to-multipoint link Petition 870260047576, dated 05 / 19 / 2026, page 38 / 89 32 / 36 or a broadcasting network.

[0115] According to decoding or decoder examples, the SDR bitstream and / or other bitstream carrying metadata is obtained from a source. For example, the bitstream is read from local memory, for example, video memory (64), RAM (64), ROM (63), flash memory (63) or a hard disk (63). In a variant, the bitstream is received from a storage interface (65), for example, an interface with mass storage, RAM, ROM, flash memory, optical disk or magnetic media and / or received from a communication interface (65), for example, an interface with a point-to-point link, a bus, a point-to-multipoint link or a broadcast network.

[0116] According to examples, device 60 being configured to implement an encoding method, as described above, belongs to a set comprising: - a mobile device; - a communication device; - a gaming device; - a tablet (or tablet computer); - a laptop; - a still image camera; - a video camera; - an encoding chip; - a static image server; and - a video server (for example, a broadcast server, a video-on-demand server, or a web server).

[0117] According to examples, device 60 being configured to implement a decoding method, as described above, belongs to a set comprising: Petition 870260047576, dated 05 / 19 / 2026, page 39 / 89 33 / 36 - a mobile device; - a communication device; - a gaming device; - a set-top box type decoder; - a TV set; - a tablet (or tablet computer); - a laptop; - a display; and - a decoding chip.

[0118] According to an example of the present principles, illustrated in Figure 7, in a transmission context between two remote devices A and B through a NET communication network, device A comprises a processor related to RAM and ROM memory, which are configured to implement a method for encoding an image as described above, and device B comprises a processor related to RAM and ROM memory, which are configured to implement a decoding method as described above.

[0119] According to an example, the network is a broadcasting network, adapted to transmit still images or video images from device A to decoding devices, including device B.

[0120] A signal, intended to be transmitted by device A, carries the SDR bitstream and / or the other bitstream that carries the metadata. The SDR bitstream comprises an encoded SDR video, as explained previously. This signal also comprises metadata relating to parameter values ​​used to reconstruct an HDR video from said decoded SDR video.

[0121] The implementations of the various processes and features described in this document can be incorporated into a variety of different equipment or applications. Examples of such equipment include an encoder, a decoder, Petition 870260047576, dated 05 / 19 / 2026, page 40 / 89 34 / 36 a post-processor that processes the output of a decoder, a pre-processor that provides input to an encoder, a video encoder, a video decoder, a video codec, a web server, a set-top box, a laptop, a personal computer, a mobile phone, a PDA, and any other device for processing an image or video, or other communication devices. As should be clear, the equipment can be mobile and even installed in a mobile vehicle.

[0122] Furthermore, methods can be implemented by instructions being executed by a processor, and these instructions (and / or data values ​​produced by an implementation) can be stored in a computer-readable storage medium. A computer-readable storage medium can take the form of a computer-readable program product embedded in one or more computer-readable media and having computer-readable program code embedded therein executable by a computer. A computer-readable storage medium, as used herein, is considered a non-transient storage medium, given the inherent ability to store the information contained therein, as well as the inherent ability to provide retrieval of the information from it.A computer-readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination thereof. It should be noted that the following, while providing more specific examples of computer-readable storage media to which the present principles may be applied, is merely an illustrative and not exhaustive list, as is readily apparent to one skilled in the art: a portable computer floppy disk; a hard disk; a read-only memory (ROM); an erasable programmable read-only memory (EPROM or Flash memory); a portable compact disc (CD-ROM) read-only memory; an optical storage device; a magnetic storage device; or... Petition 870260047576, dated 05 / 19 / 2026, page 41 / 89 35 / 36 any suitable combination of the above items.

[0123] The instructions can form an application program tangibly embedded in a processor-readable medium.

[0124] Instructions can be, for example, in hardware, firmware, software, or a combination thereof. Instructions can be found, for example, in an operating system, a separate application, or a combination of both. A processor can therefore be characterized as, for example, a device configured to execute a process and a device that includes a processor-readable medium (such as a storage device) having instructions to execute a process. Furthermore, a processor-readable medium may store, in addition to or instead of instructions, data values ​​produced by an implementation.

[0125] As will be evident to one skilled in the art, implementations can produce a variety of formatted signals to carry information that can, for example, be stored or transmitted. The information may include, for example, instructions for carrying out a method or data produced by one of the described implementations. For example, a signal may be formatted to carry as data the rules for writing or reading the syntax of a described example of the present principles, or to carry as data the actual syntax values ​​written by a described example of the present principles. This signal may be formatted, for example, as an electromagnetic wave (for example, using a portion of the radio frequency spectrum) or as a baseband signal. The formatting may include, for example, encoding a data stream and modulating a carrier with the encoded data stream.The information that the signal carries can be, for example, analog or digital information. The signal can be transmitted through a variety of wired or wireless links, as is known. The signal can be stored on a processor-readable medium.

[0126] A number of implementations have been described. However, it will be understood Petition 870260047576, dated 05 / 19 / 2026, page 42 / 89 36 / 36 that various modifications can be made. For example, elements of different implementations can be combined, supplemented, modified, or removed to produce other implementations. Furthermore, one skilled in the art will understand that other structures and processes can be substituted for those disclosed and the resulting implementations will perform at least substantially the same function(s), at least substantially in the same manner(s), to achieve at least substantially the same result(s) as the disclosed implementations. Therefore, these and other implementations are contemplated by this application. Petition 870260047576, dated 05 / 19 / 2026, page 43 / 89

Claims

1 / 5 CLAIMS 1. A method for gamut mapping from a first color gamut to a second color gamut, wherein said first color gamut and said second color gamut belong to a color space where a current color is defined by a triplet comprising a luminance value and two chroma values, said two chroma values ​​comprising a hue representative angle and a saturation representative absolute value on a chroma plane, the method comprising: determining at least three key colors in the first color gamut; for each key color (colorra) of the at least three key colors in the first color gamut, obtaining a preserved key color (prsv_color ko) and a hue alignment angle (θκο) for the key color (colorKo), said hue alignment angle (θκο) corresponding to the angle formed between the mapped key color in the second color gamut and the key color in the first color gamut with respect to an origin on the chroma plane;and perform a hue mapping of the current color (color) from the first color gamut to the second color gamut, CHARACTERIZED in that the hue mapping comprises: determining two adjacent key colors (color ko, color ki) from among the at least three key colors of the current color; for each adjacent key color (color ko, color ki), determining a rotated key color (rotated_color ko, rotated_color ki) by projecting the key color (color ko, color ki) onto a rotated hue line being a hue line through the rotated key color by the hue alignment angle from the origin in the chroma plane; for each adjacent key color (color ko, color ki), determining a direct key hue line being a line through the key color and the preserved key color (prsv_color ko, prsv_color ki);Petition 870260047576, dated 05 / 19 / 2026, page 44 / 89 2 / 5 for each adjacent key color (color ko, color ki), determine a reverse key hue line being a line through the rotated key color and the preserved key color (prsv_color ko, prsv_color ki); determine a direct anchor at an intersection between the direct key hue line of the two adjacent key colors; determine a reverse anchor at an intersection between the reverse key hue lines of the two adjacent key colors; determine a preserved current color (prsrv_color) at an intersection of a direct hue line between the current color and the direct anchor and a preservation line between the preserved key color of the two adjacent key colors; and determine the current hue-mapped color, where if the current color (color) is below a saturation of the preserved current color (prsrv_color), the current hue-mapped color is the current color;And if the current color is not below the saturation of the preserved current color, the hue-mapped current color is obtained by projecting the current color onto a reverse hue line defined by the preserved current color and the reverse anchor.

2. Method, according to claim 1, CHARACTERIZED in that the preserved key color (prsv_color ko) is located on one of: a key hue line between the origin in the chroma plane and the key color (colorKo), a rotated key hue line being the key hue line rotated by the hue rotation angle (θκο), a median key hue line being the key hue line rotated by half the hue rotation angle (θκο).

3. Method, according to claim 1 or 2, CHARACTERIZED in that a key color is selected from a group of primary colors and a group of secondary colors in the first color gamut.

4. Method, according to any one of claims 1 to 3, CHARACTERIZED in that projecting the key color is one of: an oblique projection, an orthogonal projection, a rotation about the origin in the chroma plane, a rotation about the preserved key color.

5. A method, according to any one of claims 1 to 4, CHARACTERIZED in that obtaining a preserved key color (prsv_color ko) and a hue alignment angle (θκο) of the key color (color ko) comprises receiving metadata relating to parameters used for invertible gamut mapping.

6. Method, according to claim 1, CHARACTERIZED in that the color space is the linear light YUV color space.

7. Device for gamut mapping from a first color gamut to a second color gamut, wherein said first color gamut and second color gamut belong to a color space where a current color is defined by a triplet comprising a luminance value and two chroma values, said chroma values ​​comprising a representative angle of a hue and a representative absolute value of a saturation on a chroma plane, CHARACTERIZED in that the device further comprises means for determining at least three key colors in the first color gamut;means for obtaining a preserved key color (prsv_color ko) and a hue alignment angle (θκο) for each key color (color ko) of the at least three key colors in the first color gamut, said hue alignment angle (θκο) corresponding to the angle formed between the key color mapped in the second color gamut and the key color in the first color gamut with respect to an origin in the chroma plane; and means for performing a hue mapping of the current color (color) from the first color gamut to the second color gamut further comprise: means for determining two adjacent key colors (color ko, color ki) from among the at least three key colors of the current color;for each adjacent key color (color ko, color ki), means to determine a rotated key color (rot_color ko, rot_color ki) projecting the key color (color ko, color ki) Petition 870260047576, dated 05 / 19 / 2026, page 46 / 89 4 / 5 onto a rotated hue line being a hue line through the key color rotated by the hue alignment angle from the origin in the chroma plane; for each adjacent key color (color ko, color ki), means to determine a direct key hue line being a line through the key color and preserved key color (prsv_color ko, prsv_color ki); for each adjacent key color (color ko, color ki), means to determine a reverse key hue line being a line through the rotated key color (rot_colorKo, rot_color ki) and preserved key color (prsv_color ko, prsv_color ki); means to determine a direct anchor at an intersection between the direct key hue line of said two adjacent key colors;means for determining a reverse anchor at an intersection between the reverse key hue lines of said two adjacent key colors; means for determining a preserved current color (prsrv_color) at an intersection of a direct hue line between the current color and the direct anchor and a preservation line between the preserved key color of said two adjacent key colors; and means for determining the hue-mapped current color, wherein if the current color (color) is below the saturation of said preserved current color (prsv_color), the hue-mapped current color is the current color; and if the current color is not below the saturation of said preserved current color, the hue-mapped current color is obtained by projecting the current color onto a reverse hue line defined by the preserved color and the reverse anchor.

8. Device for gamma mapping, according to claim 7, CHARACTERIZED in that the preserved key color (prsv_color ko) is located on one of: a key hue line between the origin in the chroma plane and the key color (color ko), a rotated key hue line being the key hue line rotated by the hue rotation angle (θκο), a median key hue line Petition 870260047576, dated 05 / 19 / 2026, page 47 / 89 5 / 5 being the key hue line rotated by half the hue rotation angle (θκο).

9. Device for gamut mapping, according to claim 7 or 8, CHARACTERIZED in that a key color is selected from a group of primary colors and a group of secondary colors in the first color gamut.

10. Device for gamma mapping, according to claim 7, CHARACTERIZED in that projecting the key color is one of: an oblique projection, an orthogonal projection, a rotation about the origin in the chroma plane, a rotation about the preserved key color.

11. Device for gamma mapping, according to any one of claims 7 to 10, CHARACTERIZED in that obtaining a preserved key color (prsv_color K0) and a hue alignment angle (Θκ0) of the key color (color K0) comprises means for receiving metadata relating to parameters used for invertible gamma mapping.

12. Device for gamma mapping, according to any one of claims 7 to 11, CHARACTERIZED in that the color space is the linear light YUV color space. Petition 870260047576, dated 05 / 19 / 2026, p. 48 / 89

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