Method and apparatus for embedding key information in an image

Abstract

To provide a method for embedding key information in an image.SOLUTION: The method comprises reserving a range of DMZ values, in a predetermined range of 2N values used for storing useful data in the image, the reserved range being used for storing key information associated with at least one coordinate in the image, with N>0 and DMZ<<2N.SELECTED DRAWING: Figure 2b

Description

[Technical field]

[0001] 1.Technical Field The field of the present disclosure is that of image and video processing.

[0002] More particularly, the present disclosure relates to methods for embedding key information in an image to optimize the decoding of the image. [Background technology]

[0003] 2.Background technology This section is intended to introduce the reader to various aspects of the art, which may be related to various aspects of the present disclosure, as described and / or claimed below. It is believed that this discussion is beneficial in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. As such, it should be understood that these discussions are to be read in this light, and not as admissions of prior art.

[0004] In the field of image coding and decoding, the decoding side needs to know / know some information so as to optimize the decoding of the image. For example, it is true that when a certain region in an image (for example, a color map or a distance map as shown in Fig. 1a) contains undefined values, it should not be decoded, for example at the risk of generating outliers.

[0005] A known solution for "informing" the decoder would in this case consist in transmitting the information whether a value should be decoded or not as an additional binary map, i.e. a "contour map" (e.g. as shown in Fig. 1b and corresponding to the color and distance maps of Fig. 1a), which would for example contain only predefined values ​​and thus clarify the task of the decoder. However, transmitting such an additional map may further complicate the global framework by adding extra streams, which in turn may increase the final bitrate quite significantly.

[0006] It is also known to reserve certain values ​​in the image to embed key information required at the decoding side, but the drawback of using such a solution is that this particular reserved value may be modified by the video encoding step, and the relevant information may be misinterpreted at the decoding side.

[0007] Therefore, there is a need for a new method of embedding key information in an image while optimizing the encoding and decoding complexity, as well as the final bit rate for image transmission. Summary of the Invention

[0008] 3. Overview of the Invention References in the specification to "one embodiment," "an embodiment," "an exemplary embodiment," or "a particular embodiment" indicate that the embodiment being described may include a particular feature, structure, or characteristic, but not every embodiment necessarily includes the particular feature, structure, or characteristic. Moreover, such phrases do not necessarily refer to the same embodiment. Moreover, when describing a particular feature, structure, or characteristic in relation to one embodiment, it is believed to be within the knowledge of one of ordinary skill in the art to affect such feature, structure, or characteristic in relation to other embodiments, whether or not explicitly stated.

[0009] The present disclosure relates to a method for embedding key information in an image, the method comprising: N reserving a range of DMZ values ​​within a predetermined range of N values, using the reserved range to store key information associated with at least one coordinate within the image, where N>0 and DMZ<<2. N It is.

[0010] Therefore, the present disclosure proposes a novel and inventive solution for embedding key information in an image in a robust and concise manner to optimize the decoding of said image.

[0011] For this purpose it is proposed to store the key information in a range of values ​​reserved for the predetermined range of values ​​used for storing useful data in the image, thus shortening the predetermined range of values ​​used for useful data to store the key information.

[0012] This solution first avoids sending an additional image to embed the key information, as in known solutions, and also avoids simply reserving a certain value for the key information, which may be modified by the encoding step and therefore may be misinterpreted at the decoding side.

[0013] For example, the solution may utilize a broad predefined range of values ​​for useful data to be stored.

[0014] According to a particular characteristic, the method further comprises encoding the image, the size of the reserved range DMZ being updated according to the level of video compression applied in the encoding.

[0015] Therefore, according to this embodiment, the size of the reserved range can be updated according to the level of video compression that will be used to encode the image. Such adaptive size therefore makes it possible to take into account encoding artifacts (encoding errors) and minimize the impact of embedding key information on the dynamic characteristics allowed for storing useful data.

[0016] According to a particular feature, the size of the reserved range DMZ is updated according to at least one characteristic of the image.

[0017] Therefore, according to this embodiment, the size of the reserved range can also be updated according to the image itself, for example according to the actual dynamic characteristics required to store useful data (e.g. images containing close and far distance objects, or only close distance objects, or only far distance objects).

[0018] According to a particular characteristic, the method further comprises: for at least one coordinate in the image: - if no useful data is associated with this coordinate, storing key information in a reserved area; - If useful data is associated with this coordinate, - applying a modified quantization to the useful data to provide modified useful data, the modified quantization taking into account the reserved range; and storing the modified useful data in a predetermined range of values ​​excluding a reserved range; Includes.

[0019] Thus, according to this embodiment, if no useful data is associated with a location / coordinate in the image (e.g. for an unused point in the corresponding point cloud), the key information is stored in the reserved range and the modified useful data (using the modified quantization) is stored in the remainder of the predefined range (excluding the reserved range).

[0020] Thus, on the decoding side, if a received value belongs to a reserved range, the corresponding point is discarded and considered unused.

[0021] For example, the image corresponds to a distance map obtained after projection onto a parametric surface of a point cloud representing at least a portion of an object in the multimedia content, the useful data corresponds to information representing, for at least one point in the point cloud, a distance between at least a portion of the object and the parametric surface corresponding to one coordinate in the image, the key information represents unused points, and the reserved range is 2 N The first DMZ value corresponds to a predetermined range of values, and the size DMZ is adaptive.

[0022] Thus, according to this embodiment, the method is applied to a distance map corresponding to an image (or object, or part of an object in the image) of multimedia content (e.g. video content) and useful data corresponding to the distance value at each location in the image is stored.

[0023] Indeed, this situation seems to result in some positions / coordinates in the distance map that have no useful value (e.g., when no projection occurs) and therefore are "unused" points that should not be decoded. These unused points must be known at the decoding side so that they can be discarded. To that end, we embed key information corresponding to these unused points using a range of values ​​reserved at the beginning of the range of distance values.

[0024] The method according to this embodiment exploits the fact that the range of distance map values ​​is generally (i.e. in most cases of multimedia content) broad with respect to the distance values ​​stored. Therefore, reserving a part of this predefined range of values ​​for key information is not disadvantageous. Moreover, since the distance values ​​are typically represented as a clipped inverse function of the distance to give more dynamic properties and favor close distances, reserving the beginning of the range values ​​is still not too disadvantageous, since the first values ​​are used for very distant objects.

[0025] Finally, the size of the reserved range is adaptive, for example to take advantage of the particularities of the image to be coded.

[0026] According to a particular embodiment, the key information is a value V k = 0, and the corrected useful data corresponds to V u =U DMZ (z)=floor((2 N -DMZ)Z(z)+DMZ), where z is the distance value,

number

[0027] The present disclosure also relates to a method for decoding an image corresponding to a representation of at least a portion of an object of multimedia content, the method comprising: for a value W read at at least one coordinate in the image: - discarding the read value W if W belongs to the reserved range provided by the method of embedding key information disclosed so far; and recovering useful data corresponding to a read value W by applying an inverse modified quantization to the read value W if W does not belong to the reserved range provided by the method of embedding key information disclosed so far, the inverse modified quantization taking into account the reserved range; Includes.

[0028] Therefore, according to this embodiment, at the decoding side, if the received value belongs to the reserved range, the corresponding point is discarded and considered as unused. On the other hand, if the received value is outside the reserved range (i.e., is relatively large), the corresponding point is considered valid and the associated distance value is recovered by modifying the received value and applying the inverse quantization as applied at the encoding side.

[0029] The present disclosure also relates to a stream carrying an image that embeds key information, the image being used to store useful data within the image. N storing key information in a range of DMZ values ​​reserved for a predetermined range of N values, the key information being associated with at least one coordinate of the image, where N>0 and DMZ<<2 N It is.

[0030] The present disclosure also relates to an apparatus configured to embed key information in an image, the apparatus comprising: N a memory associated with at least one processor configured to reserve a range of DMZ values ​​within a predetermined range of N values, the reserved range being used to store key information associated with at least one coordinate within the image, where N>0 and DMZ<<2;N It is.

[0031] The present disclosure also relates to an apparatus configured to decode an image corresponding to a representation of at least a portion of an object of multimedia content, the apparatus comprising: for a value W read at at least one coordinate in the image: - discarding the read value W if W belongs to a reserved range provided by a device configured to embed the previously disclosed key information; recovering useful data corresponding to the read value W by applying an inverse modified quantization to the read value W, if W does not belong to the reserved range provided by the device configured to embed the key information disclosed so far, said inverse modified quantization taking into account the reserved range; The at least one processor includes a memory associated with the at least one processor.

[0032] The present disclosure also relates to a computer program product comprising program code instructions for performing the steps of the method for embedding key information in an image as disclosed hereinbefore, when the program is executed on a computer or processor.

[0033] The present disclosure also relates to a non-transitory computer-readable carrier medium having stored therein instructions for enabling a processor to execute at least the above-mentioned method of embedding key information in an image when the program is executed on a computer.

[0034] 4. Brief description of the drawings Other characteristics and advantages of the embodiments will become apparent from the following description, given by way of illustrative and non-exhaustive example, and from the accompanying drawings, in which: [Brief description of the drawings]

[0035] [Figure 1a] 1 shows an example of an image with undefined values ​​according to the prior art. [Figure 1b]1 shows an example of a contour plot with undefined values ​​according to the prior art; [Figure 2a] 1 is a flow diagram of one embodiment of the disclosed method. [Figure 2b] 1 is an example of a reserved range of values ​​according to one embodiment of the disclosed method. [Figure 2c] 4 shows curves corresponding to a known quantization of useful data and to a modified quantization applied to the useful data, according to one embodiment of the disclosed method. [Diagram 3] FIG. 2 is a schematic diagram of a building block of an exemplary apparatus that can be used to implement the disclosed method, according to the embodiments disclosed in connection with FIGS. 2a-2c. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] 5. Description of the embodiment In all of the figures in this document, the same numerical references indicate similar elements and steps.

[0037] The subject matter will now be described with reference to the drawings, in which like reference numerals are used to refer to like elements throughout. For purposes of explanation in the following description, numerous specific details are set forth in order to provide a thorough understanding of the subject matter. It may be evident, however, that embodiments of the subject matter may be practiced without these specific details.

[0038] This description illustrates the principles of the present disclosure, and it will thus be appreciated that those skilled in the art can devise various arrangements which embody the principles of the present disclosure, although not explicitly described or shown herein.

[0039] The principles of the present invention will be explained with reference to a particular embodiment of a method for embedding key information within an image to optimize the decoding of this image, as shown in Figures 2a and 2b.

[0040] The general principle of the disclosed method is that two-dimensional image sensors are used to store useful data within an image. NA range of values ​​for storing key information (size DMZ<<2 N It is based on reserving a relatively narrow range of values ​​(for example, key information is associated with at least one coordinate in the image and must be recognized as a value to be discarded at the decoding end, rather than as useful data.

[0041] Since no specific value for embedding the key information is pre-set, but a range of values ​​is reserved and used, the disclosed method is therefore robust against encoding artifacts: in fact, even if the encoding modifies the key information, this information must not change outside the reserved range of values ​​and therefore can be recognized on the decoding side as values ​​that are discarded and not decoded.

[0042] To that end, the size of the reserved range DMZ may be adaptively refined depending on the compression level used to encode the image (so as to be robust enough to encode artifacts). Indeed, a too large value of DMZ (i.e., a too wide reserved range of values ​​of the key information) may waste part of the dynamic properties allowed for the useful data of the image, but a too small value (i.e., a too narrow reserved range of values ​​of the key information) may result in too many encoding artifacts (failure to transmit the key information).

[0043] Depending on the image itself, i.e. depending on the actual dynamic characteristics required to store useful data within the image, the size of the reserved range DMZ may be adaptively refined.

[0044] Therefore, a multi-pass strategy can be considered to set the optimal value of DMZ for a given image and a given level of compression. For each pass, on the encoding side as well as on the decoding side, candidate values ​​are tested starting from a minimal initialization and gradually increased if too many coding artifacts are observed. In any case, the DMZ value must be known at the time of decoding in order to properly decode the image information.

[0045] For example, such a multi-pass strategy may be implemented to adapt the size DMZ for each GOP (group of pictures).

[0046] According to a particular embodiment of the disclosed method, the image embedding the key information as well as the useful data corresponds to a distance map obtained, for example, from the previous step of encoding the source image. N The predetermined range of values ​​corresponds to the dynamic range of values ​​used to store the distance values ​​for each coordinate in the image that corresponds to a useful distance value.

[0047] However, since this distance map / image may present some regions containing undefined values ​​that should not be decoded (e.g., if no projection occurred), the reserved range of size DMZ therefore corresponds to a portion of the dynamic characteristic of this distance map, which will be used to store key information for each coordinate in the image without an associated distance value.

[0048] A distance map / image may be obtained after projection of a 3D point cloud representing at least a portion of an object in the multimedia content onto a parametric surface. Such a distance map / image contains information of the distance z between the projected points and the original 3D points. This information of the distance z, when present, is required at the decoding side to properly recover the point cloud and is typically represented as a clipped inverse function of the distance Z(z) to give additional dynamic properties and favor close distances. For example, this clipped inverse function Z(z) can be shown as follows:

number

[0049] Here, z min and z max defines the clipping range, and Z(z min )=1 and Z(z max )=0.

[0050] This quantity Z(z) is then stored as an unsigned integer U(z), quantified on N (typically 10 or 12) bits, as follows: U(z)=floor(2 N Z(z)

[0051] For example, N=12, z min = 1, and z max = 5, and U(z) is shown as a solid line in Figure 2c.

[0052] Therefore, the disclosed method proposes to use part of this dynamic property given to U(z) to store key information when no useful data is associated with the coordinates in the image.

[0053] For example, in a first variant shown in FIG. 2b, a reserved range of DMZ values ​​is defined at the beginning of the permissible dynamic characteristics, where DMZ<<2 N It is.

[0054] Indeed, as mentioned above, reserving the start of the range value is still not very disadvantageous, since the first value (according to the clipped inverse function Z(z)) is used for distant objects.

[0055] In another variation, for example if the image does not correspond to a distance map, it may be better to use the end of the range values ​​if the last values ​​are used to store less important information.

[0056] As already mentioned, the size DMZ may be adapted to anticipate the fact that the encoding step will cause the values ​​stored in the distance map to vary within a maximum range of, for example, + / -(DMZ / 2). Depending on the compression level, this maximum range of variation is adaptively adjusted. This is illustrated in FIG. 2b in relation to the first variant, where the vertical dotted lines indicate the 2 Nrepresents a value DMZ that may vary to split the predefined range of values ​​into two ranges: a reserved range and a useful data range. Thus, the useful data stored in the range image ranges from (DMZ+1) to (2 N -1) may take any value.

[0057] Then, once the size DMZ is defined, a dedicated encoding and decoding process is performed.

[0058] For example, on the encoding side, for a given position / coordinate in an image, look at a value V that is stored in a distance map / image. As already mentioned, the value V may be useful data corresponding to distance information associated with the coordinate, or key information about the undefined region.

[0059] Therefore, if no useful data is known at the current coordinate (for example, no projection occurs in this particular part of the image), we use the key information V k is stored in the distance image, and in a first variant, 0 is a value belonging to a reserved range of size DMZ, at the beginning of the dynamic range of distance values.

[0060] Otherwise, if the distance value is known at the current coordinates (i.e., the projection occurs in this particular part of the image), then it is necessary to calculate the distance from the image within the useful data range, i.e., 2 N The useful data must be stored within a given range of values. Therefore, to obtain the values ​​to be stored from the distance information, a modified quantization is applied to Z(z) to obtain the values ​​V u is stored as follows: V u =U DMZ (z)=floor((2 N -DMZ)Z(z)+DMZ)

[0061] For example, U DMZ (z) is also N=12, z min = 1, and z max The case of =5 is shown by the dotted line in Figure 2c.

[0062] Thus, according to the disclosed method, for each coordinate in the distance image, depending on whether a projection occurred for the corresponding point or not, the value V u or V k (equal to 0 in the first variant) is stored. Then, using an image or video compression method, the encoded image is compressed.

[0063] On the decoding side, for the received and decompressed image, in some cases paying attention to the read value W that is affected by compression artifacts. For the received image, the value DMZ is known to the decoder. The value DMZ may have been transmitted in the stream in relation to the image. In one variant, a default value is set for DMZ. In another variant, the decoder implements a method for determining the value DMZ according to the received image.

[0064] According to the disclosed method, if the value W belongs to the reserved range of size DMZ (i.e., in the first variant when W < DMZ), the current value W may correspond to an undefined / unused point, or may result from an encoding error of a "true" point (i.e., close to the reserved range before encoding, but modified by compression and belonging to the reserved range when decompressed by the decoder). As a result, such points are discarded and considered unused by the decoder. Certainly, even if a "true" point is discarded due to compression artifacts, if a few more useful data correspond to distant objects, it is not as disadvantageous to "liberate" this few useful data than to interpret undefined data as useful data.

[0065] Conversely, if the value W does not belong to the reserved range (i.e., in the first variant when W > DMZ), the current point is considered a "valid" point, and the associated distance value is recovered by calculating U DMZ -1 (W) (i.e., inverse quantization as quantization applied to the encoded size).

[0066] Thus, according to the disclosed method, for each coordinate in the received range image, the value W can be interpreted as useful data or can be discarded if key information is recognised.

[0067] Thus, the disclosed method allows for compact storage of key information within an image while providing robustness to the encoding of content / image artifacts and a level of compression adaptation schemes.

[0068] To that end, the disclosed method may take advantage of a dynamic range of values ​​that can be considered large with respect to the values ​​to be stored and / or the image itself, which allows embedding key information in the image without transmitting additional images, thus optimizing the final bit rate of the image transmission.

[0069] Referring now to FIG. 3, there is shown a building block of an exemplary apparatus that can be used to implement a method for embedding key information in an image according to any of the embodiments disclosed above in relation to FIGS. 2a-2c.

[0070] In one embodiment, an apparatus 300 for implementing the disclosed method comprises a non-volatile memory 303 (e.g., a read-only memory (ROM) or a hard disk), a volatile memory 301 (e.g., a random access memory, or RAM), and a processor 302. The non-volatile memory 303 is a non-transitory computer-readable carrier medium. It stores executable program code instructions, which are executed by the processor 302 to enable the implementation of the methods described above in various embodiments of the methods.

[0071] Once initialized, the aforementioned program code instructions are transferred from the non-volatile memory 303 to the volatile memory 301 and executed by the processor 302. Similarly, the volatile memory 301 contains registers for storing variables and parameters required for this execution.

[0072] Of course, the present disclosure is not limited to the above-described embodiments.

[0073] In particular, the present disclosure is not limited to methods and apparatus for embedding key information in an image, but extends to a method for encoding / decoding packets containing data representing objects of multimedia content, as well as any apparatus for implementing this method, in particular any apparatus including at least one CPU and / or at least one GPU.

[0074] For example, the disclosed methods may be applied to stereoscopic or immersive video.

[0075] The implementations described herein may be implemented, for example, in a method or process, an apparatus, a computer program product, a data stream, or a signal. Even if discussed in the context of a single form of embodiment (e.g., discussed only as a method or an apparatus), the implementations of the discussed functionality may be implemented in other forms (e.g., a program). An apparatus may be implemented, for example, in appropriate hardware, software, or firmware. For example, each method may be implemented in an apparatus, such as, for example, a processor, which generally refers to a processing device including, for example, a computer, a microprocessor, an integrated circuit, or a programmable logic device. Processors also include communication devices, such as, for example, smartphones, tablets, computers, mobile phones, portable / personal digital assistants ("PDAs"), and other devices that facilitate communication of information between end users.

[0076] Implementations of the various processes and features described herein may be implemented in a variety of different devices or applications, particularly those related to data encoding, data decoding, view generation, texture processing, and other processing of images and associated texture and / or depth information, for example. Examples of such devices include encoders, decoders, post-processors that process output from decoders, pre-processors that input to encoders, video encoders, video decoders, video codecs, web servers, set-top boxes, laptops, personal computers, cell phones, PDAs, and other communications devices. As will become apparent, the devices may be mobile and may be located in moving vehicles.

[0077] Furthermore, the method may be implemented by instructions executed by a processor, and such instructions (and / or data values ​​produced by implementing them) may be stored on a processor-readable medium, such as, for example, an integrated circuit, a software carrier medium, or other storage device, such as, for example, a hard disk, a compact diskette ("CD"), an optical disk (such as, for example, a DVD, often referred to as a digital versatile disk or digital video disk), a random access memory ("RAM"), or a read-only memory ("ROM"). The instructions may form an application program tangibly embodied on the processor-readable medium. For example, the instructions may be hardware, firmware, software, or a combination thereof. The instructions may be found, for example, in an operating system, a separate application, or a combination of the two. A processor may thus be characterized as both, 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 for executing a process. Furthermore, a processor-readable medium may store data values ​​produced by an implementation in addition to or in place of instructions.

[0078] As will be apparent to one skilled in the art, implementations may generate various signals formatted to carry information that may be stored or transmitted, for example. This information may include, for example, instructions for performing a method or data generated by one of the described implementations. For example, a signal may be formatted to carry as data rules for reading and writing the syntax of the described embodiments, or to carry as data the actual syntax values ​​written by the described embodiments. For example, such a signal may be formatted as an electromagnetic wave (e.g., using the radio frequency portion of the spectrum) or as a baseband signal. Formatting may include, for example, encoding a data stream and modulating a carrier wave with the encoded data stream. The information that the signal carries may be, for example, analog or digital information. As is known, the signal may be transmitted over a variety of different wired or wireless links. The signal may be stored on a processor-readable medium.

[0079] Several implementations have been described. It will be understood, nevertheless, that various modifications may be made. For example, elements of various implementations may be combined, added, modified, or removed to produce other implementations. Moreover, those skilled in the art will understand that other structures and processes may be substituted for those disclosed, such that the resulting implementations perform at least substantially the same functions in at least substantially the same manner to achieve at least substantially the same results as the disclosed implementations. Thus, these and other implementations are contemplated by the present application.

Claims

1. 1. A method for encoding an image corresponding to a map obtained after projection onto a parametric surface of a cloud of points representative of at least a portion of an object of a multimedia content, comprising: reserving a first range of values ​​within a predetermined second range of values ​​used for storing values ​​within pixels of the image, the first range being used for storing key values ​​indicating that the pixel is a non-useful pixel, and the second range of values, excluding the first range of values, being used for storing useful values ​​of the pixels of the image; encoding the image and information representative of the first range in a data stream; A method comprising:

2. The method of claim 1 , wherein the first range is a beginning of the second range.

3. The method of claim 1 , wherein the first range is an end of the second range.

4. For at least one pixel in the image, storing a key value within the first range if the pixel is associated with a non-useful value; If the pixel is associated with a utility value, applying a modified quantization to the useful value determined in accordance with a number of values ​​in the first range to provide a modified useful value; and storing said modified useful value within said pixel; The method of claim 1 , comprising:

5. The first range is: (a) encoding the image according to the first range; (b) decoding the image; and (c) if the decoded image contains more than a given number of artifacts, expanding the first range and repeating (a), (b) and (c). The method of claim 1 , wherein the

6. 1. A method for decoding an image corresponding to a map obtained after projection onto a parametric surface of a cloud of points representative of at least a portion of an object of a multimedia content, the method comprising: decoding from a data stream said image and information representative of a reserved range provided for storing key information indicating that a pixel is a non-useful pixel; For a value W read for at least one pixel in the image, If the value W falls within the reserved range, discard the value W; if said value W does not belong to said reserved range, recovering a useful value corresponding to said value W by applying to said value W an inverse modified quantization determined according to said reserved range; A method comprising:

7. A non-transitory computer-readable storage medium containing instructions that, when executed by a processor, cause the processor to: generating data representing an image embedding a depth value, said image storing a key value within a first range reserved within a second range used for storing useful values ​​within said image, said key value being associated with at least one pixel of said image, said second range of values ​​excluding the first range of values ​​being used for storing values ​​of said pixel of said image; generating information representative of the first range used to store key values ​​that indicate pixels are non-useful pixels; A non-transitory computer-readable storage medium that causes

8. 1. An apparatus configured to encode an image corresponding to a map obtained after projection of a cloud of points representing at least a portion of an object of multimedia content onto a parametric surface, the apparatus comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus to: reserving a first range of values ​​within a second range of values ​​used for storing useful values ​​within pixels of the image, the first range being used for storing key values ​​indicating that the pixel is a non-useful pixel, and the second range of values ​​excluding the first range of values ​​being used for storing useful values ​​of the pixels of the image; encoding the image and information representative of the first range in a data stream; An apparatus that operates to perform the above.

9. The instructions further include, for at least one pixel in the image: storing a key value within the first range if the pixel is associated with a non-useful value; If the pixel is associated with a utility value, applying a modified quantization to the useful value determined in accordance with a number of values ​​in the first range to provide a modified useful value; and storing said modified useful value within said pixel; The apparatus of claim 8 , further comprising:

10. The apparatus of claim 8 , wherein the first range is a beginning of the second range.

11. The apparatus of claim 8 , wherein the first range is an end of the second range.

12. The instructions further include causing the device to: (a) encoding the image according to the first range; (b) decoding the image; and (c) if the decoded image contains more than a given number of artifacts, expanding the first range and repeating (a), (b) and (c). The apparatus of claim 8 , wherein the first range is determined by:

13. 1. An apparatus configured to decode an image corresponding to a map obtained after projection of a cloud of points representing at least a portion of an object of multimedia content onto a parametric surface, the apparatus comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the apparatus to: decoding from a data stream said image and information representative of a reserved range provided for storing key information indicating that a pixel is a non-useful pixel; For a value W read for at least one pixel in the image, If the value W falls within the reserved range, discard the value W; if said value W does not belong to said reserved range, recovering a useful value corresponding to said value W by applying to said value W an inverse modified quantization determined according to said reserved range; An apparatus that operates to perform the above.

14. A non-transitory computer-readable storage medium comprising instructions that, when executed by a processor, cause the processor to encode an image corresponding to a map obtained after projection of a point cloud representing at least a portion of an object of multimedia content onto a parametric surface, the encoding comprising: reserving a first range of values ​​within a predetermined second range of values ​​used for storing values ​​within pixels of the image, the first range being used for storing key values ​​indicating that the pixel is a non-useful pixel, and the second range of values, excluding the first range of values, being used for storing useful values ​​of the pixels of the image; encoding the image and information representative of the first range in a data stream; A non-transitory computer readable storage medium comprising:

Images