Encoding apparatus and method for encoding image frames including selective limitation of offset compression values ​​- Patents.com

Abstract

To provide a dynamic image encoding device and method for encoding an image frame comprising a plurality of pixel blocks.SOLUTION: A method 200 comprises the steps of: setting a respective offset compression value for each of a plurality of pixel blocks based on an interest level associated with the pixel block (S210), wherein each offset compression value defines an offset in relation to a reference compression value set for an image frame; identifying in the image frame one or more low contrast regions having a contrast which is below a predefined contrast threshold (S220); selectively restricting set offset compression values to be at most equal to a predefined offset compression threshold for pixel blocks within the one or more low contrast regions having the set offset compression values based on the associated interest levels higher than the predefined offset compression threshold (S230); and encoding the image frame using the set offset compression values (S240).SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to video coding, and in particular to the coding of image frames including selective limitation of offset compression values. [Background technology]

[0002] When encoding an image frame divided into multiple pixel blocks into a video, an offset compression value relative to a reference compression value, e.g., an offset quantization parameter (QP) value, may be set for each pixel block. The reference compression value may be set according to the importance or interest level of the region in which the pixel block is located. The importance or interest level of the region may be set based on different criteria depending on the application. A problem with this approach is that it may result in artifacts or other quality issues in the resulting video, especially in low-contrast regions. To this end, a method has been proposed in which the compression value of a pixel block in low-contrast regions is reduced by a fixed amount. However, this introduces other problems related to the encoding of low-contrast regions. Summary of the Invention

[0003] It is an object of the present invention to provide a method and encoding apparatus for encoding an image frame comprising a plurality of pixel blocks that overcomes or mitigates problems in known methods and systems.

[0004] According to a first aspect, a method for encoding an image frame comprising a plurality of pixel blocks includes: setting a respective offset compression value for each of the plurality of pixel blocks based on a level of interest associated with the pixel block, each offset compression value defining an offset relative to a reference compression value established for the image frame. The method further includes identifying one or more low-contrast regions in the image frame having a contrast below a predetermined contrast threshold, and selectively limiting, for pixel blocks within the one or more low-contrast regions having a set offset compression value based on the associated level of interest that is higher than the predetermined offset compression threshold, the set offset compression value to be at most equal to the predetermined offset compression threshold. The method further includes encoding the image frame using the set offset compression value.

[0005] Interest level refers to the relative level of interest or importance of different regions of an image frame. What is considered to be of relatively high interest or importance and what is considered to be of relatively low interest or importance will depend on the application.

[0006] Reference compression value refers to the compression value that is selected to be the reference against which the offset is set. The reference compression value may be, for example, an expected average or median compression value over time, a maximum compression value, a minimum compression value, etc. The offset may be negative, positive, or "0".

[0007] The contrast of a region refers to a measure of the variation in at least the luminance values ​​for the pixels within the region, and a low-contrast region therefore refers to a region whose contrast by that measure is below a contrast threshold. The measure of variation may be, for example, a variance or standard deviation. The measure may additionally or alternatively be a measure of the variation in saturation values ​​for the pixels within the region.

[0008] The inventors have realized that if the offset compression value of a pixel block is reduced by a fixed amount in all low-contrast regions, this will result in a high cost in terms of the bit rate after encoding for pixel blocks in some low-contrast regions, i.e., those pixel blocks located in regions that have been identified as important or interesting such that the pixel blocks have a relatively high interest level associated with them. For such low-contrast regions, the offset compression value is already set low even without being reduced by a fixed amount. Introducing a fixed amount of reduction exposes the risk that the offset compression value will be reduced too low, which may result in a high cost in terms of the bit rate after encoding.

[0009] By selectively limiting the set offset compression value to at most the predetermined offset compression threshold for pixel blocks in one or more low-contrast regions that have a set offset compression value based on an associated level of interest that is higher than the predetermined offset compression threshold, it is ensured that the set offset compression value never exceeds the predetermined compression threshold for pixel blocks in one or more low-contrast regions. At the same time, the set offset compression value for pixel blocks in one or more low-contrast regions that have a set offset compression value based on an associated level of interest that is lower than the predetermined offset compression threshold is not affected by the method of the first aspect, and therefore, for such pixel blocks, the cost in terms of bit rate after encoding is not affected. Therefore, the risk of a very low offset compression value for such pixel blocks, which may result in a high cost in terms of bit rate after encoding, can be avoided.

[0010] The limiting operation may be performed together with the setting operation such that the setting operation includes setting a respective offset compression value for each of a plurality of pixel blocks based on an interest level associated with the pixel block, wherein for pixel blocks within one or more low contrast regions having a set offset compression value based on an associated interest level that is higher than a predetermined offset compression threshold, the set offset compression value is selectively limited to be at most equal to the predetermined offset compression threshold.

[0011] Alternatively, the setting operation may be performed before the limiting operation, and the limiting operation may include reducing, for pixel blocks within one or more low contrast regions having a set offset compression value based on an associated interest level that is higher than a predetermined offset compression threshold, the set offset compression value to be at most equal to the predetermined offset compression threshold.

[0012] In the method of the first aspect, the image frame may include a plurality of regions each having a respective level-of-interest, and for each of a plurality of pixel blocks, the level-of-interest associated with the pixel block is the level-of-interest of the region of the plurality of regions in which the pixel block is located.

[0013] In the method of the first aspect, the set offset compression value may decrease with increasing interest level.

[0014] In the method of the first aspect, the contrast may be for a region of the image frame comprising two or more pixel blocks, or may be for one pixel block.

[0015] In the method of the first aspect, the offset compressed value may be an offset quantized value, the reference compressed value may be a reference quantized value, and the predetermined offset compressed threshold may be a predetermined offset quantized threshold.

[0016] According to a second aspect, there is provided an encoding device configured to encode an image frame comprising a plurality of pixel blocks. The encoding device comprises a circuit and an encoder. The circuit is configured to perform a setting function, an identifying function, and a limiting function. The setting function is configured to set a respective offset compression value for each of the plurality of pixel blocks based on a level of interest associated with the pixel block, each offset compression value defining an offset relative to a reference compression value set for the image frame. The identifying function is configured to identify one or more low-contrast regions in the image frame having contrast below a predetermined contrast threshold. The limiting function is configured to selectively limit, for pixel blocks within the one or more low-contrast regions having a set offset compression value based on the associated level of interest that is higher than the predetermined offset compression threshold, the set offset compression value to be at most equal to the predetermined offset compression threshold. The encoder is configured to encode the image frame using the set offset compression values.

[0017] The above-mentioned optional features of the method according to the first aspect, where applicable, apply equally to this third aspect, and in order to avoid undue repetition, reference is made to the above.

[0018] According to a third aspect, there is provided a non-transitory computer-readable medium having stored thereon instructions for performing the method of the first aspect when executed on the encoding device of the second aspect, the instructions being, for example, in the form of computer-readable program code.

[0019] The above-mentioned optional features of the method according to the first aspect, where applicable, apply equally to this third aspect, and in order to avoid undue repetition, reference is made to the above.

[0020] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

[0021] Accordingly, it is to be understood that the present invention is not limited to the particular components of the apparatus described or the operation of the methods described, as such apparatus and methods may vary. It is also to be understood that the terminology used herein is merely for the purpose of describing particular embodiments and is not intended to be limiting. It should be noted that, as used in this specification and the appended claims, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of an element, unless the context clearly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several apparatuses, etc. Furthermore, the words "comprising," "including," "comprising," and similar phrases do not exclude other elements or steps.

[0022] These and other aspects of the present invention will now be described in more detail with reference to the accompanying drawings, which should not be considered limiting but are used for purposes of explanation and understanding, with like reference numerals referring to like elements throughout. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a schematic block diagram of an encoding device for encoding an image frame comprising a plurality of pixel blocks, according to the present disclosure; [Figure 2] 1 is a flowchart of a method for encoding an image frame comprising a plurality of pixel blocks according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which presently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided for thoroughness and completeness, and to convey the scope of the invention to those skilled in the art.

[0025] 1 shows an encoding device 100 for encoding an image frame comprising a plurality of pixel blocks according to the present disclosure. The encoding device 100 comprises an encoder 110 and a circuit 120.

[0026] Circuit 120 is configured to perform functions of encoding device 100, such as functions related to setting compression values ​​and instructing encoder 110 to use the set compression values. Circuit 120 may include a processor 122, such as a central processing unit (CPU), a microcontroller, or a microprocessor. Processor 122 is configured to execute program code. The program code may be configured to perform functions of encoding device 100, for example.

[0027] Encoding device 100 may further include memory 130. Memory 130 may be one or more of a buffer, flash memory, a hard drive, removable media, volatile memory, non-volatile memory, random access memory (RAM), or another suitable device. In an exemplary configuration, memory 130 includes non-volatile memory for long-term data storage and volatile memory that serves as system memory for circuit 120. Memory 130 may communicate data with circuit 120 via a data bus. Associated control lines and address buses may also be present between memory 130 and circuit 120.

[0028] The functionality of encoding device 100 may be embodied in the form of executable logic routines (e.g., lines of code, software programs, etc.) stored in a non-transitory computer-readable medium (e.g., memory 130) of encoding device 100 and executed by circuitry 120 (e.g., using processor 122). Furthermore, the functionality of encoding device 100 may be a standalone software application or may form part of a software application that performs additional tasks related to encoding device 100. The described functionality may be considered as methods that a processing unit of circuitry 120, e.g., processor 122, is configured to execute. Also, while the described functionality may be implemented in software, such functionality may equally be performed via dedicated hardware or firmware, or some combination of hardware, firmware, and / or software.

[0029] The circuit 120 is configured to execute a setting function 142 configured to set a respective offset compression value for each of a plurality of pixel blocks based on an interest level associated with the pixel block, each offset compression value defining an offset relative to a reference compression value set for the image frame.

[0030] The offset compression value for a pixel block indicates how the compressed value of the pixel block is offset from the base compressed value. The offset compression value may be expressed, for example, as an additive offset. When the offset compression value is expressed as an additive offset, the compressed value of the pixel block is obtained by adding the offset compression value of the pixel block to the base compressed value. In this case, the compressed value of the pixel block, i.e., the compression of the pixel block, increases as the offset compression value increases.

[0031] The reference compression value may be, for example, an expected average or median compression value over time, an average or median compression value of all possible compression values, a maximum compression value of all possible compression values, a minimum compression value of all possible compression values, etc.

[0032] The setting function 142 may be configured to set a respective offset compression value for each of a plurality of pixel blocks in association with encoding based on dividing an image frame into a plurality of regions, each having a respective level of interest. For each of a plurality of pixel blocks, the level of interest associated with the pixel block is the level of interest of the region in which the pixel block is located, and the setting function 142 is configured to set the offset compression value for the pixel block according to the level of interest. Typically, the set offset compression value decreases as the level of interest increases. Thus, the higher the level of interest of the region in which the pixel block is located, the lower the offset compression value. Note that not only the absolute value of the offset compression value but also the sign of the offset compression value should be considered in relation to what is considered the lower of two offset compression values. Thus, a negative offset compression value corresponds to a higher level of interest than a positive compression value, regardless of their respective absolute values.

[0033] The division of the image frame into multiple regions, each having a respective interest level, is provided as an input to the setting function 142. What is considered to be of high interest and what is considered to be of low interest will depend on the application. For example, the division may be based on results from object detection, where regions containing identified objects classified as people or vehicles are considered to be of high interest, while other regions not containing such objects are considered to be of low interest. Alternatively or additionally, the division may be based on results from motion detection, where regions with detected motion are considered to be of high interest, while other regions without motion are considered to be of low interest. The interest levels may be divided into a set number of interest levels, in which case the offset compression level may be set based on the interest levels according to a corresponding set number of offset compression values; for example, the higher the interest level of the region in which the pixel block is located, the lower the offset compression value. The difference between the highest and lowest allowable offset compression levels may be based on the quality difference allowed between the highest and lowest interest regions.

[0034] The circuit 120 is further configured to perform an identification function 144 configured to identify one or more low contrast regions in the image frame having a contrast below a predetermined contrast threshold.

[0035] Contrast may be determined based on a measure of variation in luminance values ​​of pixels within a region. Additionally or alternatively, contrast may be determined based on a measure of variation in saturation values ​​of pixels within a region. Whether contrast is based on a measure of variation in luminance values, saturation values, or both typically depends on whether the level of interest is set taking into account luminance or saturation values ​​of both. For example, variance or standard deviation may be used as a measure of variation. When determining the contrast of an image for use in identifying one or more low-contrast regions, contrast may be determined for regions containing only one pixel block or for regions containing two or more pixel blocks.

[0036] For a region containing only one pixel block, the contrast may be determined for that pixel block based only on the pixels of the pixel block, for example as the variance or standard deviation of the luminance of the pixels of the pixel block. Alternatively, the contrast may be determined by first determining the contrast of the pixel block and each of the neighboring pixel blocks based only on the pixels of the respective pixel block. The final contrast of the pixel block may then be determined by also taking into account the contrast of the neighboring pixel blocks, for example by applying a filter to the pixel block and the neighboring pixel blocks.

[0037] For a region including two or more pixel blocks, the contrast may be determined based on a calculation of the two or more pixel blocks together, e.g., as the variance or standard deviation of the luminance of all pixels of the two or more pixel blocks. Alternatively, a separate contrast may first be determined for each pixel block of the two or more pixel blocks based on a separate calculation performed for each pixel block, e.g., as the variance or standard deviation of the luminance of the pixels of each pixel block, and then the local contrast for the region may be determined as the average of the results of the separate pixel blocks, e.g., as the average of the variances or standard deviations of each pixel block.

[0038] The pixel blocks may be, for example, 8x8, 16x6, 32x32, or 64x64 pixels in size.

[0039] The circuit 120 is further configured to execute a limiting function 146 configured to selectively limit, for pixel blocks within the identified low-contrast region or regions having a set offset compression value based on an associated level of interest that is higher than a predetermined offset compression threshold, the set offset compression value to be at most equal to the predetermined offset compression threshold. Thus, the offset compression values ​​of pixel blocks within the identified low-contrast region or regions are allowed to be equal to the predetermined offset compression threshold, but are not allowed to be higher than the predetermined offset compression threshold. This is achieved by actively and selectively limiting the offset compression values ​​of pixel blocks within the low-contrast region or regions that may be set higher than the offset compression threshold according to the level of interest associated with those pixel blocks. In other words, the offset compression value of each pixel block is set to the lower of the offset compression values ​​based on the associated level of interest and the predetermined offset compression threshold.

[0040] This selective restriction of the set offset compression value ensures that the set offset compression never exceeds the predetermined compression threshold for pixel blocks in one or more low-contrast regions. At the same time, the set offset compression value for pixel blocks in one or more low-contrast regions that have a set offset compression value based on an associated interest level that is lower than the predetermined offset compression threshold is not affected by the method of the first aspect, and therefore, for such pixel blocks, the cost in terms of bit rate after encoding is not affected. Therefore, the risk of reducing the offset compression value too much for such pixel blocks, which could result in a high cost in terms of bit rate after encoding, can be avoided. The selective restriction achieves a reduction in the offset compression value only for pixel blocks that would otherwise risk introducing artifacts or other quality issues, but not enough to result in a high cost in terms of bit rate after encoding.

[0041] Furthermore, the predetermined offset compression threshold can be set to avoid artifacts or other quality issues in the resulting video that may occur in low-contrast regions. To identify such a threshold for offset compression, empirical tests can be performed using different offset compression thresholds and evaluating the encoded video for the presence of artifacts or other quality issues. The predetermined offset compression threshold can be defined within each low-contrast region based on the contrast within the respective low-contrast region. Typically, if a first low-contrast region has lower contrast than a second low-contrast region, the predetermined offset compression threshold will be set lower in the first low-contrast region than the predetermined offset compression threshold set in the second low-contrast region. The predetermined offset compression threshold can also depend on a reference compression value.

[0042] The encoder 110 is then configured to encode the image frame using the set compression level resulting from the setting function 142, the identification function 144, and the limiting function 146. The encoder 110 may be adapted to encode according to, for example, the H.264 or H.265 video compression standard.

[0043] The setting function 142 and the limiting function 146 may be combined functions configured to set a respective offset compression value for each of a plurality of pixel blocks based on a level of interest associated with the pixel block, and selectively limit the set offset compression value for pixel blocks in one or more low-contrast regions that have a set offset compression value based on the associated level of interest higher than a predetermined offset compression threshold to be at most equal to the predetermined offset compression threshold. Thus, when the offset compression value is set, pixel blocks in one or more low-contrast regions are limited such that if a pixel block in one or more low-contrast regions has a set offset compression value based on the associated level of interest higher than the predetermined offset compression threshold, the offset compression value is set to the predetermined offset compression threshold for that pixel block. In other words, the combined function is configured to set the offset compression value for each pixel block in the low-contrast region to the lower of the offset compression threshold and the offset compression value based on the level of interest associated with the pixel block. Furthermore, the setting function 142 may be further combined with an increasing function (not shown) to form a further combined function in which the set offset compression value is selectively increased to be at most equal to the predetermined offset compression threshold. Thus, when the offset compression value is set, pixel blocks within one or more low contrast regions are also further increased such that if a pixel block within one or more low contrast regions has an offset compression value set based on the associated level of interest that is lower than a predetermined offset compression threshold, the offset compression value is set to an offset compression value that is higher than the offset compression value set based on the associated level of interest but lower than the predetermined offset compression threshold for that pixel block. For example, the increase may be by a percentage of the difference between the offset compression threshold and the offset compression value set based on the associated level of interest.Alternatively, the increase in the offset compression value may be up to a predetermined offset compression value that is lower than a predetermined offset compression threshold for that pixel block, in which case the increase is applied only if the offset compression value set based on the associated interest level is lower than the predetermined offset compression value.

[0044] Alternatively, the setting function 142 and the limiting function 146 are separate functions. In this case, the limiting function 146 is configured to selectively reduce, for pixel blocks within one or more low-contrast regions having an offset compression value set by the setting function 142 based on an associated level of interest that is higher than a predetermined offset compression threshold, the set offset compression value to be at most equal to the predetermined offset compression threshold. Again, the offset compression value is ultimately set to the lower of the offset compression threshold and the offset compression value based on the level of interest associated with the pixel block. Additionally, a separate increasing function (not shown) may be added. The increasing function is configured to selectively increase, for pixel blocks within one or more low-contrast regions having an offset compression value set by the setting function 142 based on an associated level of interest that is lower than the predetermined offset compression threshold, the set offset compression value to be equal to the predetermined offset compression threshold.

[0045] The compression value may be associated with different measures and characteristics indicating the level of compression. Compression may be specified, for example, by a quantization parameter (QP), in which case the reference compression value is a reference QP value where the offset compression value is defined in the form of an offset QP value. In that case, the limiting function 146 is configured to selectively limit the set offset QP value to a predetermined offset QP threshold. The offset QP value for each pixel block set in this manner may be provided to a quantization parameter map (QMAP) used to instruct the encoder 110 to encode the image frame using the set offset QP value according to the QMAP. The offset QP value may be expressed, for example, as an additive offset. In such a case, the QP value of the pixel block is obtained by adding the pixel block's offset QP value to the reference QP value.

[0046] A method 200 for encoding an image frame comprising a plurality of pixel blocks according to the present disclosure will be discussed with reference to Figure 2. The steps of the method 200 may be performed by the encoding device 100 described above.

[0047] The method includes setting S210 a respective offset compression value for each of a plurality of pixel blocks based on a level of interest associated with the pixel block, each offset compression value defining an offset relative to a reference compression value established for the image frame. The method further includes S220 identifying one or more low-contrast regions in the image frame having local contrast below a predetermined contrast threshold, and selectively limiting S230, for pixel blocks within the one or more low-contrast regions having a set offset compression value based on the associated level of interest that is higher than the predetermined offset compression threshold, the set offset compression value to be at most equal to the predetermined offset compression threshold. The method further includes S240 encoding the image frame using the set offset compression value.

[0048] Setting the offset compression value S210 and selectively limiting the set offset compression value S230 may occur simultaneously such that a respective offset compression value for each of a plurality of pixel blocks is set based on a level of interest associated with the pixel block, and for pixel blocks in one or more low contrast regions having a set offset compression value based on an associated level of interest that is higher than a predetermined offset compression threshold, the set offset compression value is selectively limited to be at most equal to the predetermined offset compression threshold. Thus, when the offset compression value is set, pixel blocks in one or more low contrast regions are limited such that if a pixel block in one or more low contrast regions has a set offset compression value based on an associated level of interest that is higher than the predetermined offset compression threshold, the offset compression value will be set to the offset compression threshold for that pixel block.

[0049] Alternatively, setting the offset compression value S210 may be performed separately from and prior to selectively limiting the set offset compression value S230, in which case selectively limiting the set offset compression value includes reducing the set offset compression value for pixel blocks in one or more low contrast regions that have an associated level-of-interest-based set offset compression value higher than a predetermined offset compression threshold to be at most equal to the predetermined offset compression threshold.

[0050] It should be noted that identifying one or more contrast regions S210 may be performed at any time prior to selectively limiting the offset values ​​set for pixel blocks within one or more low contrast regions S220, e.g., before or after setting the offset compression value S210.

[0051] As shown in connection with FIG. 1 , compression may be specified, for example, by a quantization parameter (QP), in which case the reference compression value is a reference QP value, where the offset compression value is defined in the form of an offset QP value. Next, the offset QP values ​​for pixel blocks within one or more low-contrast regions identified in S220 are selectively limited to a predetermined offset QP threshold in S230. In this scenario, the QP offset values ​​are expressed as additive offsets, the interest levels of the region are divided into 16 levels, the QP values ​​can range from 0 to 51, the QP reference value is set to 25, and each interest level is set to correspond to an offset QP value in steps of 3. In this scenario, a zero interest level corresponds to an offset QP value of 24, and therefore the equivalent QP value is 25 plus 24, i.e., 49. A level of interest of 16 corresponds to an offset QP value of −24, and therefore the equivalent QP value is 25 plus −24, i.e., 1. If the offset QP threshold is set to 5, this means that all pixel blocks within low contrast regions (which are also within regions with a level of interest of 6 or less) will have their offset QP value selectively limited to 5 according to the method 200 of the present disclosure. All pixel blocks within low contrast regions (which are also within regions with a level of interest of 7 or more) will not be affected by the selective limitation according to the method 200 of the present disclosure, and will instead be set to 3 or less according to their associated level of interest.

[0052] It should be noted that the above scenario is merely an example. In other scenarios, a different number of interest levels may be used, an offset QP value may be selectable so that a smaller range of possible QP values ​​is used, and the compression value may be defined by some other parameter other than QP.

[0053] The method 200 and its included steps may further be adapted as the encoding device 100 and its included functions described in relation to FIG.

[0054] Those skilled in the art will understand that the invention is not limited to the above-described embodiments. On the contrary, many modifications and variations are possible within the scope of the appended claims. Such modifications and variations can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Claims

1. 1. A method for encoding an image frame comprising a plurality of pixel blocks, the method comprising: establishing a respective offset compression value for each of the plurality of pixel blocks based on a level of interest associated with the pixel block, each offset compression value defining an offset relative to a reference compression value established for the image frame; identifying one or more low-contrast regions in the image frame having a contrast below a predetermined contrast threshold; selectively limiting, for pixel blocks within the one or more low contrast regions having a set offset compression value based on the associated level of interest that is higher than a predetermined offset compression threshold, the set offset compression value to be at most equal to the predetermined offset compression threshold; encoding the image frame using the set offset compression value; A method comprising:

2. The setting operation is setting the respective offset compression value for each of the plurality of pixel blocks based on a level of interest associated with the pixel block, such that for pixel blocks within the one or more low contrast regions having a set offset compression value based on the associated level of interest that is higher than the predetermined offset compression threshold, the set offset compression value is selectively limited to be at most equal to the predetermined offset compression threshold; the limiting operation is performed together with the setting operation; The method of claim 1.

3. The setting operation is performed before the limiting operation, and the limiting operation is for pixel blocks within the one or more low contrast regions having a set offset compression value based on the associated level of interest that is higher than the predetermined offset compression threshold, reducing the set offset compression value to be at most equal to the predetermined offset compression threshold. The method of claim 1.

4. 2. The method of claim 1, wherein the image frame includes a plurality of regions each having a respective level of interest, and for each of the plurality of pixel blocks, the level of interest associated with the pixel block is the level of interest of the region of the plurality of regions in which the pixel block is located.

5. The method of claim 1 , wherein the set offset compression value decreases with increasing interest level.

6. The method of claim 1 , wherein the contrast is for a region of the image frame comprising two or more pixel blocks, or for a single pixel block.

7. The method of claim 1 , wherein the offset compressed value is an offset quantized value, the reference compressed value is a reference quantized value, and the predetermined offset compressed threshold is a predetermined offset quantized threshold.

8. 1. An encoding device configured to encode an image frame comprising a plurality of pixel blocks, the encoding device comprising: A circuit comprising: a setting function configured to set a respective offset compression value for each of the plurality of pixel blocks based on a level of interest associated with the pixel block, each offset compression value defining an offset relative to a reference compression value established for the image frame; an identification function configured to identify one or more low-contrast regions in the image frame, the low-contrast regions having a contrast below a predetermined contrast threshold; and a limiting function configured to selectively limit, for pixel blocks within the one or more low-contrast regions having a set offset compression value based on the associated level of interest that is higher than a predetermined offset compression threshold, the set offset compression value to be at most equal to the predetermined offset compression threshold. and a circuit configured to perform an encoder configured to encode the image frame using the set offset compression value; An encoding device comprising:

9. 9. The encoding device of claim 8, wherein the setting function and the limiting function are combined functions configured to set the respective offset compression value for each of the plurality of pixel blocks based on an interest level associated with the pixel block, and wherein for pixel blocks within the one or more low contrast regions having a set offset compression value based on the associated interest level that is higher than the predetermined offset compression threshold, the set offset compression value is selectively limited to be at most equal to the predetermined offset compression threshold.

10. 9. The encoding device of claim 8, wherein the setting function and the limiting function are separate functions, and the limiting function is configured to reduce, for pixel blocks in the one or more low contrast regions having a set offset compression value based on the associated level of interest that is higher than the predetermined offset compression threshold, the set offset compression value to be at most equal to the predetermined offset compression threshold.

11. 9. The encoding device of claim 8, wherein the image frame includes a plurality of regions each having a respective level of interest, and for each of the plurality of pixel blocks, the level of interest associated with the pixel block is the level of interest of the region of the plurality of regions in which the pixel block is located.

12. 9. The encoding device of claim 8, wherein the set offset compression value decreases with increasing interest level.

13. 9. The encoding device of claim 8, wherein the contrast is a contrast within a region of the image frame comprising two or more pixel blocks, or a local contrast for one pixel block.

14. The encoding device according to claim 8 , wherein the offset compressed value is an offset quantized value, the reference compressed value is a reference quantized value, and the predetermined offset compressed threshold is a predetermined offset quantized threshold.

15. A non-transitory computer-readable storage medium storing instructions for performing the method of any one of claims 1 to 7 when executed by an encoding device of claim 8.

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