QP derivation and offset for adaptive color transform in video coding

A device for decoding video data is configured to determine, based on a chroma sampling format for the video data, that adaptive color transform is enabled for one or more blocks of the video data; determine a quantization parameter for the one or more blocks based on determining that the adaptive color transform is enabled; and dequantize transform coefficients based on the determined quantization parameter. A device for decoding video data is configured to determine for one or more blocks of the video data that adaptive color transform is enabled; receive in a picture parameter set, one or more offset values in response to adaptive color transform being enabled; determine a quantization parameter for a first color component of a first color space based on a first of the one or more offset values; and dequantize transform coefficients based on the quantization parameter.

TECHNICAL FIELD

This disclosure relates to video coding, such as video encoding or video decoding.

BACKGROUND

SUMMARY

This disclosure describes techniques related to determining quantization parameters when color-space conversion coding is used and, furthermore, this disclosure describes techniques for generating and parsing various syntax elements, in an encoded bitstream of video data, used for signaling quantization parameters when color-space conversion coding is used.

In one example, a method of decoding video data includes determining, based on a chroma sampling format for the video data, that adaptive color transform is enabled for one or more blocks of the video data; determining a quantization parameter for the one or more blocks based on determining that the adaptive color transform is enabled; and dequantizing transform coefficients based on the determined quantization parameter.

In another example, a device for decoding video data includes a video data memory; and one or more processors configured to determine, based on a chroma sampling format for the video data, that adaptive color transform is enabled for one or more blocks of the video data; determine a quantization parameter for the one or more blocks based on determining that the adaptive color transform is enabled; and dequantize transform coefficients based on the determined quantization parameter.

In another example, an apparatus for decoding video data includes means for determining, based on a chroma sampling format for the video data, that adaptive color transform is enabled for one or more blocks of the video data; means for determining a quantization parameter for the one or more blocks based on determining that the adaptive color transform is enabled; and means for dequantizing transform coefficients based on the determined quantization parameter.

In another example, a computer readable medium stores instructions that when executed by one or more processors cause the one or more processors to determine, based on a chroma sampling format for the video data, that adaptive color transform is enabled for one or more blocks of the video data; determine a quantization parameter for the one or more blocks based on determining that the adaptive color transform is enabled; and dequantize transform coefficients based on the determined quantization parameter.

In another example, a method of decoding video data includes determining for one or more blocks of the video data that adaptive color transform is enabled; receiving in a picture parameter set, one or more offset values in response to adaptive color transform being enabled; determining a quantization parameter for a first color component of a first color space based on a first of the one or more offset values; and dequantizing transform coefficients based on the quantization parameter.

In another example, a device for decoding video data includes a video data memory; one or more processors configured to determine for one or more blocks of the video data that adaptive color transform is enabled; receive in a picture parameter set, one or more offset values in response to adaptive color transform being enabled; determine a quantization parameter for a first color component of a first color space based on a first of the one or more offset values; and dequantize transform coefficients based on the quantization parameter.

An apparatus for decoding video data includes means for determining for one or more blocks of the video data that adaptive color transform is enabled; means for receiving in a picture parameter set, one or more offset values in response to adaptive color transform being enabled; means for determining a quantization parameter for a first color component of a first color space based on a first of the one or more offset values; and means for dequantizing transform coefficients based on the quantization parameter.

In another example, a computer readable medium stores instructions that when executed by one or more processors cause the one or more processors to determine for one or more blocks of the video data that adaptive color transform is enabled; receive in a picture parameter set, one or more offset values in response to adaptive color transform being enabled; determine a quantization parameter for a first color component of a first color space based on a first of the one or more offset values; and dequantize transform coefficients based on the quantization parameter.

DETAILED DESCRIPTION

This disclosure describes techniques related to adaptive color transform quantization parameter derivations. This disclosure identifies various issues related to how quantization parameter derivation when adaptive color transform is used and proposes solutions to address these issues. This disclosure describes video coding techniques, including techniques related to emerging screen content coding (SCC) extensions and range extensions (RExt) of the recently finalized high efficiency video coding (HEVC) standard. The SCC and range extensions are being designed to potentially support high bit depth (e.g. more than 8 bit) and/or different chroma sampling formats such as 4:4:4, 4:2:2, 4:2:0, 4:0:0, etc, and are therefore being designed to include new coding tools not included in the base HEVC standard.

One such coding tool is color-space conversion coding. In color-space conversion coding, a video encoder may convert residual data from a first color space (e.g. YCbCr) to a second color space (e.g. RGB) in order to achieve better coding quality (e.g. a better rate-distortion tradeoff). Regardless of the color space of the residual data, a video encoder typically transforms the residual data into transform coefficients and quantizes the transform coefficients. A video decoder performs the reciprocal processes of dequantizing the transform coefficients and inverse transforming the transform coefficients to reconstruct the residual data. The video encoder generates, for inclusion in the encoded bitstream of video data, a quantization parameter indicating an amount of scaling used in quantizing the transform coefficient levels. The video decoder parses the bitstream to determine the quantization parameter used by the video encoder. The quantization parameter may also be used by other video coding processes, such as deblock filtering.

This disclosure describes techniques related to determining quantization parameters when color-space conversion coding is used and, furthermore, this disclosure describes techniques for signaling, from an encoder to a decoder as part of an encoded bitstream of video data, quantization parameters when color-space conversion coding is used.

FIG. 1is a block diagram illustrating an example video encoding and decoding system10that may utilize the techniques described in this disclosure, including techniques for coding blocks in an IBC mode and techniques for parallel processing. As shown inFIG. 1, system10includes a source device12that generates encoded video data to be decoded at a later time by a destination device14. Source device12and destination device14may comprise any of a wide range of devices, including desktop computers, notebook (i.e., laptop) computers, tablet computers, set-top boxes, telephone handsets such as so-called “smart” phones, so-called “smart” pads, televisions, cameras, display devices, digital media players, video gaming consoles, video streaming device, or the like. In some cases, source device12and destination device14may be equipped for wireless communication.

Alternatively, encoded data may be output from output interface22to a storage device17. Similarly, encoded data may be accessed from storage device17by input interface. Storage device17may include any of a variety of distributed or locally accessed data storage media such as a hard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data. In a further example, storage device17may correspond to a file server or another intermediate storage device that may hold the encoded video generated by source device12. Destination device14may access stored video data from storage device17via streaming or download. The file server may be any type of server capable of storing encoded video data and transmitting that encoded video data to the destination device14. Example file servers include a web server (e.g., for a website), an FTP server, network attached storage (NAS) devices, or a local disk drive. Destination device14may access the encoded video data through any standard data connection, including an Internet connection. This may include a wireless channel (e.g., modulated according to a wireless standard such, including a wireless local area network standard such as Wi-Fi or a wireless telecommunication standard such as LTE or another cellular communication standard), a wired connection (e.g., DSL, cable modem, etc.), or a combination of both that is suitable for accessing encoded video data stored on a file server. The transmission of encoded video data from storage device17may be a streaming transmission, a download transmission, or a combination of both.

In the example ofFIG. 1, source device12includes a video source18, video encoder20and an output interface22. In some cases, output interface22may include a modulator/demodulator (modem) and/or a transmitter. In source device12, video source18may include a source such as a video capture device, e.g., a video camera, a video archive containing previously captured video, a video feed interface to receive video from a video content provider, and/or a computer graphics system for generating computer graphics data as the source video, or a combination of such sources. As one example, if video source18is a video camera, source device12and destination device14may form so-called camera phones or video phones. However, the techniques described in this disclosure may be applicable to video coding in general, and may be applied to wireless and/or wired applications.

The captured, pre-captured, or computer-generated video may be encoded by video encoder20. The encoded video data may be transmitted directly to destination device14via output interface22of source device12. The encoded video data may also (or alternatively) be stored onto storage device17for later access by destination device14or other devices, for decoding and/or playback.

Destination device14includes an input interface28, a video decoder30, and a display device32. In some cases, input interface28may include a receiver and/or a modem. Input interface28of destination device14receives the encoded video data over link16. The encoded video data communicated over link16, or provided on storage device17, may include a variety of syntax elements generated by video encoder20for use by a video decoder, such as video decoder30, in decoding the video data. Such syntax elements may be included with the encoded video data transmitted on a communication medium, stored on a storage medium, or stored a file server.

Display device32may be integrated with, or external to, destination device14. In some examples, destination device14may include an integrated display device and also be configured to interface with an external display device. In other examples, destination device14may be a display device. In general, display device32displays the decoded video data to a user, and may comprise any of a variety of display devices such as a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, or another type of display device.

Video encoder20and video decoder30may operate according to a video compression standard, such as HEVC, and may conform to the HEVC Test Model (HM). A working draft of the HEVC standard, referred to as “HEVC Working Draft 10” or “HEVC WD10,” is described in Bross et al., “Editors' proposed corrections to HEVC version 1,” Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, 13thMeeting, Incheon, KR, April 2013. Another HEVC draft specification is available from http://phenix.int-evry.fr/jct/doc_end_user/documents/15_Geneva/wg11/JCTVC-O1003-v2.zip. The techniques described in this disclosure may also operate according to extensions of the HEVC standard that are currently in development.

Alternatively or additionally, video encoder20and video decoder30may operate according to other proprietary or industry standards, such as the ITU-T H.264 standard, alternatively referred to as MPEG-4, Part 10, Advanced Video Coding (AVC), or extensions of such standards. The techniques of this disclosure, however, are not limited to any particular coding standard. Other examples of video compression standards include ITU-T H.261, ISO/IEC MPEG-1 Visual, ITU-T H.262 or ISO/IEC MPEG-2 Visual, ITU-T H.263, ISO/IEC MPEG-4 Visual and ITU-T H.264 (also known as ISO/IEC MPEG-4 AVC), including its Scalable Video Coding (SVC) and Multiview Video Coding (MVC) extensions.

The design of the HEVC has been recently finalized by the JCT-VC of ITU-T Video Coding Experts Group (VCEG) and ISO/IEC Motion Picture Experts Group (MPEG). The Range Extensions to HEVC, referred to as HEVC RExt, are also being developed by the JCT-VC. A recent Working Draft (WD) of Range extensions, referred to as RExt WD7 hereinafter, is available from http://phenix.int-evey.fr/jct/doc_end_user/documents/17_Valencia/wg11/JCTVC-Q1005-v4.zip.

This disclosure will generally refer to the recently finalized HEVC specification text as HEVC version 1 or base HEVC. The range extension specification may become the version 2 of the HEVC. With respect to many coding tools, such as motion vector prediction, HEVC version 1 and the range extension specification are technically similar. Therefore whenever this disclosure describes changes relative to HEVC version 1, the same changes may also apply to the range extension specification, which generally includes the base HEVC specification, plus some additional coding tools. Furthermore, it can generally be assumed that HEVC version 1 modules may also be incorporated into a decoder implementing the HEVC range extension.

New coding tools for screen-content material such as text and graphics with motion are currently in development and being contemplated for inclusion in future video coding standards, including future version of HEVC. These new coding tools potentially improve coding efficiency for screen content. As there is evidence that significant improvements in coding efficiency may be obtained by exploiting the characteristics of screen content with novel dedicated coding tools, a Call for Proposals (CfP) has been issued with the target of possibly developing future extensions of the HEVC standard including specific tools for SCC). Companies and organizations have been invited to submit proposals in response to this Call. The use cases and requirements of this CfP are described in MPEG document N14174. During the 17thJCT-VC meeting, SCC test model (SCM) is established. A recent SCC working draft (WD) is JCTVC-U1005 and is available at http://phenix.int-evry.fr/jct/doc_end_user/documents/21_Warsaw/wg11/JCTVC-U1005-v1.zip.

This disclosure contemplates that video encoder20of source device12may be configured to encode video data according to any of these current or future standards. Similarly, this disclosure also contemplates that video decoder30of destination device14may be configured to decode video data according to any of these current or future standards.

As introduced above, the JCT-VC has recently finalized development of the HEVC standard. The HEVC standardization efforts were based on an evolving model of a video coding device referred to as the HEVC Test Model (HM). The HM presumes several additional capabilities of video coding devices relative to existing devices according to, e.g., ITU-T H.264/AVC. For example, whereas H.264 provides nine intra-prediction encoding modes, the HM may provide as many as thirty-five intra-prediction encoding modes.

In HEVC and other video coding specifications, a video sequence typically includes a series of pictures. Pictures may also be referred to as “frames.” A picture may include three sample arrays, denoted SL, SCb, and SCr. SLis a two-dimensional array (i.e., a block) of luma samples. SCbis a two-dimensional array of Cb chrominance samples. SCris a two-dimensional array of Cr chrominance samples. Chrominance samples may also be referred to herein as “chroma” samples. In other instances, a picture may be monochrome and may only include an array of luma samples.

In order to generate an encoded representation of a picture, video encoder20may generate a set of coding tree units (CTUs). Each of the CTUs may comprise a coding tree block of luma samples, two corresponding coding tree blocks of chroma samples, and syntax structures used to code the samples of the coding tree blocks. In monochrome pictures or pictures having three separate color planes, a CTU may comprise a single coding tree block and syntax structures used to code the samples of the coding tree block. A coding tree block may be an N×N block of samples. A CTU may also be referred to as a “tree block” or a “largest coding unit” (LCU). The CTUs of HEVC may be broadly analogous to the macroblocks of other standards, such as H.264/AVC. However, a CTU is not necessarily limited to a particular size and may include one or more coding units (CUs). A slice may include an integer number of CTUs ordered consecutively in a raster scan order.

To generate a coded CTU, video encoder20may recursively perform quad-tree partitioning on the coding tree blocks of a CTU to divide the coding tree blocks into coding blocks, hence the name “coding tree units.” A coding block may be an N×N block of samples. A CU may comprise a coding block of luma samples and two corresponding coding blocks of chroma samples of a picture that has a luma sample array, a Cb sample array, and a Cr sample array, and syntax structures used to code the samples of the coding blocks. In monochrome pictures or pictures having three separate color planes, a CU may comprise a single coding block and syntax structures used to code the samples of the coding block.

Video encoder20may partition a coding block of a CU into one or more prediction blocks. A prediction block is a rectangular (i.e., square or non-square) block of samples on which the same prediction is applied. A prediction unit (PU) of a CU may comprise a prediction block of luma samples, two corresponding prediction blocks of chroma samples, and syntax structures used to predict the prediction blocks. In monochrome pictures or pictures having three separate color planes, a PU may comprise a single prediction block and syntax structures used to predict the prediction block. Video encoder20may generate predictive luma, Cb, and Cr blocks for luma, Cb, and Cr prediction blocks of each PU of the CU.

Video encoder20may use intra prediction or inter prediction to generate the predictive blocks for a PU. If video encoder20uses intra prediction to generate the predictive blocks of a PU, video encoder20may generate the predictive blocks of the PU based on decoded samples of the picture associated with the PU. If video encoder20uses inter prediction to generate the predictive blocks of a PU, video encoder20may generate the predictive blocks of the PU based on decoded samples of one or more pictures other than the picture associated with the PU.

Furthermore, video encoder20may use quad-tree partitioning to decompose the luma, Cb, and Cr residual blocks of a CU into one or more luma, Cb, and Cr transform blocks. A transform block is a rectangular (e.g., square or non-square) block of samples on which the same transform is applied. A transform unit (TU) of a CU may comprise a transform block of luma samples, two corresponding transform blocks of chroma samples, and syntax structures used to transform the transform block samples. Thus, each TU of a CU may be associated with a luma transform block, a Cb transform block, and a Cr transform block. The luma transform block associated with the TU may be a sub-block of the CU's luma residual block. The Cb transform block may be a sub-block of the CU's Cb residual block. The Cr transform block may be a sub-block of the CU's Cr residual block. In monochrome pictures or pictures having three separate color planes, a TU may comprise a single transform block and syntax structures used to transform the samples of the transform block.

Video encoder20may apply one or more transforms to a luma transform block of a TU to generate a luma coefficient block for the TU. A coefficient block may be a two-dimensional array of transform coefficients. A transform coefficient may be a scalar quantity. Video encoder20may apply one or more transforms to a Cb transform block of a TU to generate a Cb coefficient block for the TU. Video encoder20may apply one or more transforms to a Cr transform block of a TU to generate a Cr coefficient block for the TU.

After generating a coefficient block (e.g., a luma coefficient block, a Cb coefficient block or a Cr coefficient block), video encoder20may quantize the coefficient block. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the transform coefficients, providing further compression. After video encoder20quantizes a coefficient block, video encoder20may entropy encode syntax elements indicating the quantized transform coefficients. For example, video encoder20may perform Context-Adaptive Binary Arithmetic Coding (CABAC) on the syntax elements indicating the quantized transform coefficients.

Video encoder20may output a bitstream that includes a sequence of bits that forms a representation of coded pictures and associated data. The bitstream may comprise a sequence of NAL units. A NAL unit is a syntax structure containing an indication of the type of data in the NAL unit and bytes containing that data in the form of a RBSP interspersed as necessary with emulation prevention bits. Each of the NAL units includes a NAL unit header and encapsulates a RBSP. The NAL unit header may include a syntax element that indicates a NAL unit type code. The NAL unit type code specified by the NAL unit header of a NAL unit indicates the type of the NAL unit. A RBSP may be a syntax structure containing an integer number of bytes that is encapsulated within a NAL unit. In some instances, an RBSP includes zero bits.

Different types of NAL units may encapsulate different types of RBSPs. For example, a first type of NAL unit may encapsulate an RBSP for a PPS, a second type of NAL unit may encapsulate an RBSP for a coded slice, a third type of NAL unit may encapsulate an RBSP for SEI messages, and so on. NAL units that encapsulate RBSPs for video coding data (as opposed to RBSPs for parameter sets and SEI messages) may be referred to as VCL NAL units.

Video decoder30may receive a bitstream generated by video encoder20. In addition, video decoder30may parse the bitstream to obtain syntax elements from the bitstream. Video decoder30may reconstruct the pictures of the video data based at least in part on the syntax elements obtained from the bitstream. The process to reconstruct the video data may be generally reciprocal to the process performed by video encoder20. In addition, video decoder30may inverse quantize coefficient blocks associated with TUs of a current CU. Video decoder30may perform inverse transforms on the coefficient blocks to reconstruct transform blocks associated with the TUs of the current CU. Video decoder30may reconstruct the coding blocks of the current CU by adding the samples of the predictive blocks for PUs of the current CU to corresponding samples of the transform blocks of the TUs of the current CU. By reconstructing the coding blocks for each CU of a picture, video decoder30may reconstruct the picture.

A video sampling format, which may also be referred to as a chroma format, may define the number of chroma samples included in a CU with respect to the number of luma samples included in a CU. Depending on the video sampling format for the chroma components, the size, in terms of number of samples, of the U and V components may be the same as or different from the size of the Y component. In the HEVC standard, a value called chroma_format_idc is defined to indicate different sampling formats of the chroma components, relative to the luma component. In HEVC, chroma_format_idc is signaled in the SPS. Table 1 illustrates the relationship between values of chroma_format_idc and associated chroma formats.

In Table 1, the variables SubWidthC and SubHeightC can be used to indicate the horizontal and vertical sampling rate ratio between the number of samples for the luma component and the number of samples for each chroma component. In the chroma formats described in Table 1, the two chroma components have the same sampling rate. Thus, in 4:2:0 sampling, each of the two chroma arrays has half the height and half the width of the luma array, while in 4:2:2 sampling, each of the two chroma arrays has the same height and half the width of the luma array. In 4:4:4 sampling, each of the two chroma arrays, may have the same height and width as the luma array, or in some instances, the three color planes may all be separately processed as monochrome sampled pictures.

In the example of Table 1, for the 4:2:0 format, the sampling rate for the luma component is twice that of the chroma components for both the horizontal and vertical directions. As a result, for a coding unit formatted according to the 4:2:0 format, the width and height of an array of samples for the luma component are twice that of each array of samples for the chroma components. Similarly, for a coding unit formatted according to the 4:2:2 format, the width of an array of samples for the luma component is twice that of the width of an array of samples for each chroma component, but the height of the array of samples for the luma component is equal to the height of an array of samples for each chroma component. For a coding unit formatted according to the 4:4:4 format, an array of samples for the luma component has the same width and height as an array of samples for each chroma component. It should be noted that in addition to the YUV color space, video data can be defined according to an RGB space color. In this manner, the chroma formats described herein may apply to either the YUV or RGB color space. RGB chroma formats are typically sampled such that the number of red samples, the number of green samples and the number of blue samples are equal. Thus, the term “4:4:4 chroma format” as used herein may refer to either a YUV color space or an RGB color space wherein the number of samples is equal for all color components.

FIGS. 2A-2Care conceptual diagrams illustrating different sample formats for video data.FIG. 2Ais a conceptual diagram illustrating the 4:2:0 sample format. As illustrated inFIG. 2A, for the 4:2:0 sample format, the chroma components are one quarter of the size of the luma component. Thus, for a CU formatted according to the 4:2:0 sample format, there are four luma samples for every sample of a chroma component.FIG. 2Bis a conceptual diagram illustrating the 4:2:2 sample format. As illustrated inFIG. 2B, for the 4:2:2 sample format, the chroma components are one half of the size of the luma component. Thus, for a CU formatted according to the 4:2:2 sample format, there are two luma samples for every sample of a chroma component.FIG. 2Cis a conceptual diagram illustrating the 4:4:4 sample format. As illustrated inFIG. 2C, for the 4:4:4 sample format, the chroma components are the same size of the luma component. Thus, for a CU formatted according to the 4:4:4 sample format, there is one luma sample for every sample of a chroma component.

FIG. 3is a conceptual diagram illustrating an example of a 16×16 coding unit formatted according to a 4:2:0 sample format.FIG. 3illustrates the relative position of chroma samples with respect to luma samples within a CU. As described above, a CU is typically defined according to the number of horizontal and vertical luma samples. Thus, as illustrated inFIG. 3, a 16×16 CU formatted according to the 4:2:0 sample format includes 16×16 samples of luma components and 8×8 samples for each chroma component. Further, as described above, a CU may be partitioned into smaller CUs. For example, the CU illustrated inFIG. 3may be partitioned into four 8×8 CUs, where each 8×8 CU includes 8×8 samples for the luma component and 4×4 samples for each chroma component.

FIG. 4is a conceptual diagram illustrating an example of a 16×16 coding unit formatted according to a 4:2:2 sample format.FIG. 4illustrates the relative position of chroma samples with respect to luma samples within a CU. As described above, a CU is typically defined according to the number of horizontal and vertical luma samples. Thus, as illustrated inFIG. 4, a 16×16 CU formatted according to the 4:2:2 sample format includes 16×16 samples of luma components and 8×16 samples for each chroma component. Further, as described above, a CU may be partitioned into smaller CUs. For example, the CU illustrated inFIG. 4may be partitioned into four 8×8 CUs, where each CU includes 8×8 samples for the luma component and 4×8 samples for each chroma component.

In accordance with the techniques described in this disclosure, an in-loop color-space transform for residual signals (i.e., residual blocks) is proposed for sequences in 4:4:4 chroma format; however, the techniques are not limited to the 4:4:4 format. The in-loop color-space transform process transforms prediction error signals (i.e., residual signals) in RGB/YUV chroma format into those in a sub-optimal color-space. The in-loop color-space transform can further reduce the correlation among the color components. The transform matrix may be derived from pixel sample values for each CU by a singular-value-decomposition (SVD). The color-space transform may be applied to prediction error of both intra mode and inter mode.

When the color-space transform is applied to inter mode, the residual is firstly converted to a different domain with the derived transform matrix. After the color-space conversion, the coding steps, such as DCT/DST, quantization, and entropy coding are performed, in order.

When the color-space transform is applied to a CU coded using an intra mode, the prediction and current block are firstly converted to a different domain with the derived transform matrix, respectively. After the color-space conversion, the residual between current block and a predictor for the current block is further transformed with DCT/DST, quantized, and entropy coded.

A video encoding device, such as video encoder20, performs a forward operation, where a color-space transform matrix comprising conversion values a, b, c, d, e, f, g, h, and i is applied to three planes G, B, and R to derive values for color components P, Q, and S as follows:

Resulting values may be clipped within the range of the HEVC specification, since values may be enlarged up to √{square root over (3)} times in the worst case. A video decoding device, such as video decoder30, performs an inverse operation, where a color-space transform matrix comprising conversion values at, bt, ct, dt, et, ft, gt, ht, and itis applied to the three color components P′, Q′, and R′ to derive the three planes G′, B′ and R′ as follows,

FIG. 5is a conceptual diagram illustrating an example of a target block and reference sample for an intra 8×8 block, according to one or more techniques of the current disclosure. A transform matrix may be derived using singular-value-decomposition (SVD) from the reference sample values. A video coding device (e.g., video encoder20or video decoder30) may use different reference samples for the intra case and inter case. For the case of an intra coded block, the target blocks and reference samples may be as shown inFIG. 5. InFIG. 5, the target block consists of 8×8 crosshatched samples94, and above reference samples96are shown as striped, and left references samples98are shown as dotted.

For the case of an inter coded block, reference samples for the matrix derivation may be the same as the reference samples for motion compensation. Reference samples in the advanced motion prediction (AMP) block may be sub-sampled such that the number of reference samples is reduced. For example, the number of reference samples in a 12×16 block is reduced by ⅔.

In some of the above examples, the color-space transform process may be always applied. Therefore, there may be no need to signal whether the color-space transform process is invoked or not. In addition, both video encoder20and video decoder30may use the same method to derive the transform matrix in order to avoid the overhead for signaling the transform matrix.

Video encoder20and video decoder30may use various color-space transform matrices. For example, video encoder20and video decoder30may apply different color-space transform matrices for different color spaces. For instance, video encoder20and video decoder30may use a pair of YCbCr transform matrixes to convert sample values from the RGB color space to the YCbCr color space and back. The following equations show one example set of YCbCr transform matrixes:

In another example, video encoder20and video decoder30may use a pair of YCoCg transform matrixes to convert sample values from the RGB color space to the YCoCg color space and back. The following equations show one example set of YCoCg transform matrixes:

Another such matrix may be the YCoCg-R matrix, which is a revisable version of the YCoCg matrix that scales the Co and Cg components by a factor of two. By using a lifting technique, video encoder20and video decoder30may achieve the forward and inverse transform by the following equations:

In the above equations and matrices, the forward transformations may be performed before the encoding process (e.g., by a video encoder). Conversely, the inverse transformations may be performed after the decoding process (e.g., by a video decoder). It should also be noted that video encoder20includes a decoding loop to reconstruct the encoded date for use in predicting other video data. Accordingly, like video decoder30, the decoding loop of video encoder20may also perform the inverse transformations.

The techniques of this disclosure potentially address one or more problems and more specifically, potential problems with the QP derivation when cu_residual_act_flag is enabled. For example, according to existing solutions, when adaptive color transform is enabled, during the scaling and transformation process, a QP offset of −5 is added for luma and Cb chroma component, and −3 is added for Cr chroma component. The resultant value of Qp, however, may underflow the allowed Qp range. For example, in the current test model it is possible that the resultant Qp may underflow to −5 when the range allowed by HEVC is between 0 and 51. This disclosure also describes techniques for signalling adaptive QP offsets when adaptive color transform is enabled.

A portion of the scaling and transformation process is set forth below.

8.6.2 Scaling and Transformation Process

The quantization parameter qP is derived as follows in the current test model, If cIdx is equal to 0,
qP=Qp′Y+(cu_residual_act_flag[xTbY][yTbY]?−5:0)  (8-261)

Otherwise (cIdx is equal to 2),
qP=Qp′Cr+(cu_residual_act_flag[xTbY][yTbY]?−3:0)  (8-263)
where cIdx specifies the colour component of the current block and cu_residual_act_flag specifies whether adaptive colour transform is applied to the residual samples of the current coding unit.

This disclosure describes various techniques that may address the problems introduced above. Each of the following techniques may be implemented separately or jointly with one or more of the others. According to one technique of this disclosure, video decoder30may clip the resultant Qp's from section 8.6.2 equation 8-261, 8-262, 8-263—scaling and transformation process (after offset is added when adaptive color transform is enabled) to HEVC Qp range that is 0, 51+QpBdOffsetY. According to another technique of this disclosure, video encoder20may signal to video decoder30the Qp offset to be applied in section 8.6.2 (scaling and transformation process) when adaptive color transform is enabled. This signaling of Qp offset may be done at various granularity levels like VPS, SPS, PPS, slice header or its extension. The Qp offset may be signalled for all the components (luma+chroma) or only some of the components (e.g. chroma)

According to another technique of this disclosure, video encoder20may signal to video decoder30a flag indicating whether or not QP offset is to be applied in section 8.6.2 (scaling and transformation process) when adaptive color transform is enabled. This signaling of a flag can be done at various granularity levels like VPS, SPS, PPS, slice header or its extension. The signaling of flag can be signalled for all the components (luma+chroma) or only some of the components (e.g. chroma).

Example implementations of the techniques intoduce above will now be described in more detail. According to one technique of this disclosure, video encoder20and video decoder30can be configured to clip the Qp's to within the HEVC Qp's range. In order to keep the allowed Qp range as same that is used in HEVC when adaptive color transform is used, this disclosure describes techniques for clipping the range of the Qp values to that of HEVC Qp range. The proposed changes to the test model are italicized below.

8.6.2 Scaling and Transformation Process

If cIdx is equal to 0,
qP=clip3(0,51+QpBdOffsetY,Qp′Y+(cu_residual_act_flag[xTbY][yTbY]?−5:0))
When ChromaArrayType is not equal to 0,

if cIdx is equal to 1,
qP=clip3(0,51+QpBdOffsetC,Qp′Cb+(cu_residual_act_flag[xTbY][yTbY]?−5:0))  (8-262)

Otherwise (cIdx is equal to 2),
qP=clip3(0,51+QpBdOffsetC,Qp′Cr+(cu_residual_act_flag[xTbY][yTbY]?−3:0)  (8-262)
Flexible Signalling of QP Offset for Adaptive Color Transform

It is proposed to clip the range of the QP values for luma and chroma component.

If cIdx is equal to 0,
qP=Clip3(0,51+QpBdOffsetY,Qp′Y+(cu_residual_act_flag[xTbY][yTbY]?pps_act_y_qp_offset+slice_act_y_qp_offset:0))
When ChromaArrayType is not equal to 0,

if cIdx is equal to 1,
qP=Clip3(0,51+QpBdOffsetC,Qp′Cb+(cu_residual_act_flag[xTbY][yTbY]?pps_act_pps_cb_qp_offset+slice_act_cb_qp_offset:0))  (8-262)

Otherwise (cIdx is equal to 2),
qP=Clip3(0,51+QpBdOffsetC,Qp′Cr+(cu_residual_act_flag[xTbY][yTbY]?pps_act_pps_cr_qp_offset+slice_act_cr_qp_offset:0)  (8-263)pps_act_y_qp_offset, pps_act_cb_qp_offset and pps_act_cr_qp_offset specify offsets to the luma, cb and cr quantization parameter qP derived in section 8.6.2, respectively. The values of pps_act_y_qp_offset, pps_cb_qp_offset and pps_cr_qp_offset shall be in the range of −12 to +12, inclusive. When ChromaArrayType is equal to 0, pps_act_cb_qp_offset and pps_act_cr_qp_offset are not used in the decoding process and decoders shall ignore their value.pps_slice_act_qp_offsets_present_flag equal to 1 specifies that slice_act_y_qp_offset, slice_act_cb_qp_offset, slice_act_cr_qp_offset are present in the slice header. pps_slice_act_qp_offsets_present_flag equal to 0 specifies that slice_act_y_qp_offset, slice_act_cb_qp_offset, slice_act_cr_qp_offset are not present in the slice header. When not present, the value of cu_chroma_qp_offset_enabled_flag is inferred to be equal to 0.slice_act_y_qp_offset, slice_cb_qp_offset and slice_cr_qp_offset specify offsets to the luma, cb and cr quantization parameter qP derived in section 8.6.2, respectively. The values of slice_act_y_qp_offset, slice_cb_qp_offset and slice_cr_qp_offset shall be in the range of −12 to +12, inclusive. When ChromaArrayType is equal to 0, slice_act_cb_qp_offset and slice_act_cr_qp_offset are not used in the decoding process and decoders shall ignore their value.

Techniques for signalling the presence of QP offset for adaptive color transform will now be described. As discussed in technique (1) above, fixed negative QP offset when adaptive color transform is enabled narrows the Qp range at the higher Qp's. For example, with the current definition when adaptive color transform is enabled it is impossible to reach QP's over 46+QpBdOffsetY, which in some scenarios are necessary to meet target bitrate. In the below solution, it is proposed to signal a flag to indicate whether Qp offset shall be added or not. The proposed changes to the test model are highlighted in yellow text.

De-scriptorslice_segment_header( ) {first_slice_segment_in_pic_flagu(1).................slice_qp_deltase(v)if( pps_slice_chroma_qp_offsets_present_flag ) {slice_cb_qp_offsetse(v)slice_cr_qp_offsetse(v)}if( chroma_qp_offset_list_enabled_flag )cu_chroma_qp_offset_enabled_flagu(1)if(residual—adaptive—colour—transform—enabled—flag)slice—act—qp—offset—present—flagu(1)......
slice_act_qp_offset_present_flag equal to 1 specifies that a Qp offset is applied for the coding units with cu_residual_act_flag equal to 1. slice_act_qp_offset_present_flag equal to 0 specifies that a Qp offset is not applied for the coding units with cu_residual_act_flag equal to 1. When not present, the value of cu_chroma_qp_offset_enabled_flag is inferred to be equal to 0.
If cIdx is equal to 0,
qP=Clip3(0,51+QpBdOffsetY,Qp′Y+(cu_residual_act_flag[xTbY][yTbY] && slice_act_qp_offset_present_flag?−5:0))
When ChromaArrayType is not equal to 0,

Another example implementation of QP offset for adaptive color transform will now be described. This disclosure proposes the following:a) Signal the adaptive color transform enabled flag in the picture parameter set instead of sequence parameter set. This potentially benefits from being able to adapt at picture level the usage of adaptive color transform.b) A bitstream restriction is proposed to disable adaptive color transform when chroma format is not 4:4:4. In one example, this restriction is proposed to be applied on the adaptive color transform enable flag (residual_adaptive_colour_transform_enabled_flag)

Below an example syntax and semantics are detailed.

residual_adaptive_colour_transform_enabled_flag equal to 1 specifies that an adaptive colour transform may be applied to the residual in the decoding process for the pictures referring to the PPS. residual_adaptive_colour_transform_enabled_flag equal to 0 specifies that adaptive colour transform is not applied to the residual for the pictures referring to the PPS. When not present, the value of residual_adaptive_colour_transform_enabled_flag is inferred to be equal to 0.

When chroma_format_idc is not equal to 3, residual_adaptive_colour_transform_enabled_flag shall be equal to 0.

pps_slice_act_qp_offsets_present_flag equal to 1 specifies that slice_act_y_qp_offset, slice_act_cb_qp_offset, slice_act_cr_qp_offset are present in the slice header. pps_slice_act_qp_offsets_present_flag equal to 0 specifies that slice_act_y_qp_offset, slice_act_cb_qp_offset, slice_act_cr_qp_offset are not present in the slice header. When not present, the value of cu_chroma_qp_offset_enabled_flag is inferred to be equal to 0.

slice_act_y_qp_offset, slice_cb_qp_offset and slice_cr_qp_offset specify offsets to the luma, cb and cr quantization parameter qP derived in section 8.6.2, respectively. The values of slice_act_y_qp_offset, slice_cb_qp_offset and slice_cr_qp_offset shall be in the range of −12 to +12, inclusive. When ChromaArrayType is equal to 0, slice_act_cb_qp_offset and slice_act_cr_qp_offset are not used in the decoding process and decoders shall ignore their value.

FIG. 6is a block diagram illustrating an example video encoder20that may implement the techniques described in this disclosure. Video encoder20may be configured to output video to post-processing entity27. Post-processing entity27is intended to represent an example of a video entity, such as a media aware network element (MANE) or a splicing/editing device, that may process encoded video data from video encoder20. In some instances, post-processing entity27may be an example of a network entity, such as a MANE, but in other instances post-processing entity27may be considered part of encoder20. For example, in some video encoding systems, post-processing entity27and video encoder20may be parts of separate devices, while in other instances, the functionality described with respect to post-processing entity27may be performed by the same device that comprises video encoder20. In still other examples, post-processing entity27may be implemented as part of storage device17ofFIG. 1

Video encoder20may perform intra-, inter-, and IMC coding of video blocks within video slices. Intra-coding relies on spatial prediction to reduce or remove spatial redundancy in video within a given video frame or picture. Inter-coding relies on temporal prediction to reduce or remove temporal redundancy in video within adjacent frames or pictures of a video sequence. Intra-mode (I mode) may refer to any of several spatial based compression modes. Inter-modes, such as uni-directional prediction (P mode) or bi-prediction (B mode), may refer to any of several temporal-based compression modes. IMC coding modes, as described above, may remove spatial redundancy from a frame of video data, but unlike tradition intra modes, IMC coding codes may be used to locate predictive blocks in a larger search area within the frame and refer to the predictive blocks with offset vectors, rather than relying on intra-prediction coding modes.

In the example ofFIG. 6, video encoder20includes video data memory33, partitioning unit35, prediction processing unit41, filter unit63, decoded picture buffer64, summer50, transform processing unit52, quantization unit54, and entropy encoding unit56. Prediction processing unit41includes motion estimation unit42, motion compensation unit44, and intra-prediction processing unit46. For video block reconstruction, video encoder20also includes inverse quantization unit58, inverse transform processing unit60, and summer62. Filter unit63is intended to represent one or more loop filters such as a deblocking filter, an adaptive loop filter (ALF), and a sample adaptive offset (SAO) filter. Although filter unit63is shown inFIG. 6as being an in loop filter, in other configurations, filter unit63may be implemented as a post loop filter.

Video data memory33may store video data to be encoded by the components of video encoder20. The video data stored in video data memory33may be obtained, for example, from video source18. Decoded picture buffer64may be a reference picture memory that stores reference video data for use in encoding video data by video encoder20, e.g., in intra-, inter-, or IMC coding modes. Video data memory33and decoded picture buffer64may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. Video data memory33and decoded picture buffer64may be provided by the same memory device or separate memory devices. In various examples, video data memory33may be on-chip with other components of video encoder20, or off-chip relative to those components.

As shown inFIG. 6, video encoder20receives video data and stores the video data in video data memory33. Partitioning unit35partitions the data into video blocks. This partitioning may also include partitioning into slices, tiles, or other larger units, as wells as video block partitioning, e.g., according to a quadtree structure of LCUs and CUs. Video encoder20generally illustrates the components that encode video blocks within a video slice to be encoded. The slice may be divided into multiple video blocks (and possibly into sets of video blocks referred to as tiles). Prediction processing unit41may select one of a plurality of possible coding modes, such as one of a plurality of intra coding modes, one of a plurality of inter coding modes, or one of a plurality of IMC coding modes, for the current video block based on error results (e.g., coding rate and the level of distortion). Prediction processing unit41may provide the resulting intra-, inter-, or IMC coded block to summer50to generate residual block data and to summer62to reconstruct the encoded block for use as a reference picture.

Intra-prediction processing unit46within prediction processing unit41may perform intra-predictive coding of the current video block relative to one or more neighboring blocks in the same frame or slice as the current block to be coded to provide spatial compression. Motion estimation unit42and motion compensation unit44within prediction processing unit41may perform inter-predictive coding of the current video block relative to one or more predictive blocks in one or more reference pictures to provide temporal compression. Motion estimation unit42and motion compensation unit44within prediction processing unit41may also perform IMC coding of the current video block relative to one or more predictive blocks in the same picture to provide spatial compression.

Motion estimation unit42may be configured to determine the inter-prediction mode or IMC mode for a video slice according to a predetermined pattern for a video sequence. The predetermined pattern may designate video slices in the sequence as P slices, B slices or GPB slices. Motion estimation unit42and motion compensation unit44may be highly integrated, but are illustrated separately for conceptual purposes. Motion estimation, performed by motion estimation unit42, is the process of generating motion vectors, which estimate motion for video blocks. A motion vector, for example, may indicate the displacement of a PU of a video block within a current video frame or picture relative to a predictive block within a reference picture. In the case of IMC coding, a motion vector, which may be referred to as an offset vector in IMC, may indicate the displacement of a PU of a video block within a current video frame or picture relative to a predictive block within the current video frame.

According to some techniques of this disclosure, when coding a video block using an IMC mode, motion estimation unit42may determine a motion vector, or offset vector, for a luma component of the video block, and determine an offset vector for a chroma component of the video block based on the offset vector for the luma component. In another example, when coding a video block using an IMC mode, motion estimation unit42may determine a motion vector, or offset vector, for a chroma component of the video block, and determine an offset vector for a luma component of the video block based on the offset vector for the chroma component. Thus, video encoder20may signal in the bitstream only one offset vector, from which offset vectors for both chroma and luma components of the video block may be determined.

Motion compensation, performed by motion compensation unit44, may involve fetching or generating the predictive block based on the motion vector determined by motion estimation, possibly performing interpolations to sub-pixel precision. Interpolation filtering may generate additional pixel samples from known pixel samples, thus potentially increasing the number of candidate predictive blocks that may be used to code a video block. Upon receiving the motion vector for the PU of the current video block, motion compensation unit44may locate the predictive block to which the motion vector points in one of the reference picture lists, or in the case of the IMC coding, within the picture being coded. Video encoder20forms a residual video block by subtracting pixel values of the predictive block from the pixel values of the current video block being coded, forming pixel difference values. The pixel difference values form residual data for the block, and may include both luma and chroma difference components. Summer50represents the component or components that perform this subtraction operation. Motion compensation unit44may also generate syntax elements associated with the video blocks and the video slice for use by video decoder30in decoding the video blocks of the video slice.

Intra-prediction processing unit46may intra-predict a current block, as an alternative to the inter-prediction and IMC performed by motion estimation unit42and motion compensation unit44, as described above. In particular, intra-prediction processing unit46may determine an intra-prediction mode to use to encode a current block. In some examples, intra-prediction processing unit46may encode a current block using various intra-prediction modes, e.g., during separate encoding passes, and intra-prediction processing unit46(or mode select unit40, in some examples) may select an appropriate intra-prediction mode to use from the tested modes. For example, intra-prediction processing unit46may calculate rate-distortion values using a rate-distortion analysis for the various tested intra-prediction modes, and select the intra-prediction mode having the best rate-distortion characteristics among the tested modes. Rate-distortion analysis generally determines an amount of distortion (or error) between an encoded block and an original, unencoded block that was encoded to produce the encoded block, as well as a bit rate (that is, a number of bits) used to produce the encoded block. Intra-prediction processing unit46may calculate ratios from the distortions and rates for the various encoded blocks to determine which intra-prediction mode exhibits the best rate-distortion value for the block.

After prediction processing unit41generates the predictive block for the current video block (e.g., via inter-prediction, intra-prediction, or IMC) video encoder20forms a residual video block by subtracting the predictive block from the current video block. The residual video data in the residual block may be included in one or more TUs and applied to transform processing unit52. Transform processing unit52transforms the residual video data into residual transform coefficients using a transform, such as a discrete cosine transform (DCT) or a conceptually similar transform. Transform processing unit52may convert the residual video data from a pixel domain to a transform domain, such as a frequency domain.

Following quantization, entropy encoding unit56entropy encodes the quantized transform coefficients. For example, entropy encoding unit56may perform context adaptive variable length coding (CAVLC), context adaptive binary arithmetic coding (CABAC), syntax-based context-adaptive binary arithmetic coding (SBAC), probability interval partitioning entropy (PIPE) coding or another entropy encoding methodology or technique. Following the entropy encoding by entropy encoding unit56, the encoded bitstream may be transmitted to video decoder30, or archived for later transmission or retrieval by video decoder30. Entropy encoding unit56may also entropy encode the motion vectors and the other syntax elements for the current video slice being coded.

Inverse quantization unit58and inverse transform processing unit60apply inverse quantization and inverse transformation, respectively, to reconstruct the residual block in the pixel domain for later use as a reference block of a reference picture. Motion compensation unit44may calculate a reference block by adding the residual block to a predictive block of one of the reference pictures within one of the reference picture lists. Motion compensation unit44may also apply one or more interpolation filters to the reconstructed residual block to calculate sub-integer pixel values for use in motion estimation. Interpolation filtering may generate additional pixel samples from known pixel samples, thus potentially increasing the number of candidate predictive blocks that may be used to code a video block. Summer62adds the reconstructed residual block to the motion compensated prediction block produced by motion compensation unit44to produce a reference block for storage in decoded picture buffer64. The reference block may be used by motion estimation unit42and motion compensation unit44as a reference block to inter-predict a block in a subsequent video frame or picture.

FIG. 7is a block diagram illustrating an example video decoder30that may implement the techniques described in this disclosure. In the example ofFIG. 7, video decoder30includes a video data memory78, entropy decoding unit80, prediction processing unit81, inverse quantization unit86, inverse transform processing unit88, summer90, filter unit91, and decoded picture buffer92. Prediction processing unit81includes motion compensation unit82and intra-prediction processing unit84. Video decoder30may, in some examples, perform a decoding pass generally reciprocal to the encoding pass described with respect to video encoder20fromFIG. 6.

During the decoding process, video decoder30receives video data, e.g. an encoded video bitstream that represents video blocks of an encoded video slice and associated syntax elements, from video encoder20. Video decoder30may receive the video data from network entity29and store the video data in video data memory78. Video data memory78may store video data, such as an encoded video bitstream, to be decoded by the components of video decoder30. The video data stored in video data memory78may be obtained, for example, from storage device17, e.g., from a local video source, such as a camera, via wired or wireless network communication of video data, or by accessing physical data storage media. Video data memory78may form a coded picture buffer that stores encoded video data from an encoded video bitstream. Thus, although shown separately inFIG. 7, video data memory78and decoded picture buffer92may be provided by the same memory device or separate memory devices. Video data memory78and decoded picture buffer92may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. In various examples, video data memory78may be on-chip with other components of video decoder30, or off-chip relative to those components.

Network entity29, for example, may comprise a server, a MANE, a video editor/splicer, or other such device configured to implement one or more of the techniques described above. Network entity29may or may not include a video encoder, such as video encoder20. Some of the techniques described in this disclosure may be implemented by network entity29prior to network entity29transmitting the encoded video bitstream to video decoder30. In some video decoding systems, network entity29and video decoder30may be parts of separate devices, while in other instances, the functionality described with respect to network entity29may be performed by the same device that comprises video decoder30. Network entity29may be an example of storage device17ofFIG. 1in some cases.

Entropy decoding unit80of video decoder30entropy decodes the bitstream to generate quantized coefficients, motion vectors, and other syntax elements. Entropy decoding unit80forwards the motion vectors and other syntax elements to prediction processing unit81. Video decoder30may receive the syntax elements at the video slice level and/or the video block level.

When the video slice is coded as an intra-coded (I) slice, intra-prediction processing unit84of prediction processing unit81may generate prediction data for a video block of the current video slice based on a signaled intra prediction mode and data from previously decoded blocks of the current frame or picture. When the video frame is coded as an inter-coded (i.e., B, P or GPB) slice or when a block is IMC coded, motion compensation unit82of prediction processing unit81produces predictive blocks for a video block of the current video slice based on the motion vectors and other syntax elements received from entropy decoding unit80. For inter prediction, the predictive blocks may be produced from one of the reference pictures within one of the reference picture lists. Video decoder30may construct the reference frame lists, List 0 and List 1, using default construction techniques based on reference pictures stored in decoded picture buffer92. For IMC coding, the predictive blocks may be produced from the same picture as the block being predicted.

Motion compensation unit82may also perform interpolation based on interpolation filters. Motion compensation unit82may use interpolation filters as used by video encoder20during encoding of the video blocks to calculate interpolated values for sub-integer pixels of reference blocks. In this case, motion compensation unit82may determine the interpolation filters used by video encoder20from the received syntax elements and use the interpolation filters to produce predictive blocks.

According to some techniques of this disclosure, when coding a video block using an IMC mode, motion compensation unit82may determine a motion vector, or offset vector, for a luma component of the video block, and determine a motion vector for a chroma component of the video block based on the motion vector for the luma component. In another example, when coding a video block using an IMC mode, motion compensation unit82may determine a motion vector, or offset vector, for a chroma component of the video block, and determine a motion vector for a luma component of the video block based on the motion vector for the chroma component. Thus, video decoder30may receive in the bitstream only one offset vector, from which offset vectors for both chroma and luma components of the video block may be determined.

When decoding a video block using IMC mode, motion compensation unit82may, for example, modify a motion vector, referred to as an offset vector for IMC mode, for a luma component to determine an offset vector for a chroma component. Motion compensation unit82may, for example, modify one or both of an x-component and y-component of the offset vector of the luma block based on a sampling format for the video block and based on a precision of a sub-pixel position to which the offset vector points. For example, if the video block is coded using the 4:2:2 sampling format, then motion compensation unit82may only modify the x-component, not the y-component, of the luma offset vector to determine the offset vector for the chroma component. As can be seen fromFIG. 4, in the 4:2:2 sampling format, chroma blocks and luma blocks have the same number of samples in the vertical direction, thus making modification of the y-component potentially unneeded. Motion compensation unit82may only modify the luma offset vector, if when used for locating a chroma predictive block, the luma offset vector points to a position without a chroma sample (e.g., at a sub-pixel position in the chroma sample of the current picture that includes the current block). If the luma offset vector, when used to locate a chroma predictive block, points to a position where a chroma sample is present, then motion compensation unit82may not modify the luma offset vector.

In other examples, if the video block is coded using the 4:2:0 sampling format, then motion compensation unit82may modify either or both of the x-component and the y-component of the luma offset vector to determine the offset vector for the chroma component. As can be seen fromFIG. 3, in the 4:2:0 sampling format, chroma blocks and luma blocks have a different number of samples in both the vertical direction and the horizontal direction. Motion compensation unit82may only modify the luma offset vector, if when used for locating a chroma predictive block, the luma offset vector points to a position without a chroma sample (e.g., at a sub-pixel position in the chroma sample of the current picture that includes the current block). If the luma offset vector, when used to locate a chroma predictive block, points to a position where a chroma sample is present, then motion compensation unit82may not modify the luma offset vector.

Motion compensation unit82may modify a luma offset vector to generate a modified motion vector, also referred to as a modified offset vector. Motion compensation unit82may modify a luma offset vector that, when used to locate a chroma predictive block, points to a sub-pixel position such that the modified offset vector, used for the chroma block, points to a lower resolution sub-pixel position or to an integer pixel position. As one example, a luma offset vector that points to a ⅛ pixel position may be modified to point to a ¼ pixel position, a luma offset vector that points to a ¼ pixel position may be modified to point to a ½ pixel position, etc. In other examples, motion compensation unit82may modify the luma offset vector such that the modified offset vector always points to an integer pixel position for locating the chroma reference block. Modifying the luma offset vector to point to a lower resolution sub-pixel position or to an integer pixel position may eliminate the need for some interpolation filtering and/or reduce the complexity of any needed interpolation filtering.

Referring toFIGS. 3 and 4and assuming the top left sample is located at position (0, 0), a video block has luma samples at both odd and even x positions and both odd and even y positions. In a 4:4:4 sampling format, a video block also has chroma samples at both odd and even x positions and both odd and even y positions. Thus, for a 4:4:4 sampling format, motion compensation unit may use the same offset vector for locating both a luma predictive block and a chroma predictive block. For a 4:2:2 sampling format, as shown inFIG. 4, a video block has chroma samples at both odd and even y positions but only at even x positions. Thus, for the 4:2:2 sampling format, if a luma offset vector points to an odd x position, motion compensation unit82may modify the x-component of the luma offset vector to generate a modified offset vector that points to an even x position so that the modified offset vector can be used for locating the reference chroma block for the chroma block of the current block without needing interpolation. Motion compensation unit82may modify the x-component, for example, by either rounding up or rounding down to the nearest even x position, i.e. changing the x-component such that it points to either the nearest left x position or nearest right x position. If the luma offset vector already points to an even x position, then no modification may be necessary.

For a 4:2:0 sampling format, as shown inFIG. 3, a video block has chroma samples only at even y positions and only at even x positions. Thus, for the 4:2:0 sampling format, if a luma offset vector points to an odd x position or odd y position, motion compensation unit82may modify the x-component or y-component of the luma offset vector to generate a modified offset vector that points to an even x position so that the modified offset vector can be used for locating the reference chroma block for the chroma block of the current block without needing interpolation. Motion compensation unit82may modify the x-component, for example, by either rounding up or rounding down to the nearest even x position, i.e. changing the x-component such that it points to either the nearest left x position or nearest right x position. Motion compensation unit82may modify the y-component, for example, by either rounding up or rounding down to the nearest even y position, i.e. changing the y-component such that it points to either the nearest above y position or nearest below y position. If the luma offset vector already points to an even x position and an even y position, then no modification may be necessary.

Inverse quantization unit86inverse quantizes, i.e., de-quantizes, the quantized transform coefficients provided in the bitstream and decoded by entropy decoding unit80. The inverse quantization process may include use of a quantization parameter calculated by video encoder20for each video block in the video slice to determine a degree of quantization and, likewise, a degree of inverse quantization that should be applied. Inverse transform processing unit88applies an inverse transform, e.g., an inverse DCT, an inverse integer transform, or a conceptually similar inverse transform process, to the transform coefficients in order to produce residual blocks in the pixel domain.

After motion compensation unit82generates the predictive block for the current video block based on the motion vectors and other syntax elements, video decoder30forms a decoded video block by summing the residual blocks from inverse transform processing unit88with the corresponding predictive blocks generated by motion compensation unit82. Summer90represents the component or components that perform this summation operation. If desired, loop filters (either in the coding loop or after the coding loop) may also be used to smooth pixel transitions, or otherwise improve the video quality. Filter unit91is intended to represent one or more loop filters such as a deblocking filter, an adaptive loop filter (ALF), and a sample adaptive offset (SAO) filter. Although filter unit91is shown inFIG. 7as being an in loop filter, in other configurations, filter unit91may be implemented as a post loop filter. The decoded video blocks in a given frame or picture are then stored in decoded picture buffer92, which stores reference pictures used for subsequent motion compensation. Decoded picture buffer92may be part of a memory that also stores decoded video for later presentation on a display device, such as display device32ofFIG. 1, or may be separate from such a memory.

FIG. 8is a block diagram illustrating another example video encoder21that may utilize techniques for transforming video data having an RGB color space to blocks of video data having a second color space using a color transform in accordance with one or more aspects of this disclosure.

FIG. 8illustrates a more detailed version of video encoder20. Video encoder21may be an example of video encoder20(FIG. 2) or video encoder20(FIG. 1). The example ofFIG. 8illustrates two possible examples for implementing the techniques of this disclosure. In the first implementation, video encoder21adaptively transforms a first block of an input video signal having a first color space to a second block having a second color space using a color transform of one or more color transform. The second illustrated example performs the same techniques, but performs the color transformation on blocks of residual video data, rather than on an input signal.

In the example ofFIG. 8, video encoder21is shown as performing color transforms on predictive and residual blocks of video data based on the states of switches101,105,113,121. If switches101,105,113, and121are switched the alternative position, video encoder21is configured to perform color transforms on blocks of video data of an original signal having an RGB color space to blocks of video data having a second color space before performing motion estimation, and motion prediction, rather than transforming blocks of predictive and/or residual video data.

One example process of performing color transforms on blocks of residual video data as illustrated inFIG. 8is now described in greater detail. In the example ofFIG. 8, an original signal100is passed to prediction processing unit104(following the path of switch101). Prediction processing unit104may receive data from one or more reference pictures from reference picture memory122. Prediction processing unit104generates a predictive block of video data, and combines the predictive block of video data from the original signal100to generate residual signal124. In this example, adaptive color transformer106transforms the predictive block and the residual block of video data from an RGB color space to a second predictive block and a second residual block of video having a second color space. In some examples, video encoder21may select the second color space and the color transform based on a cost function.

Transform/quantization unit108may perform a transform (e.g., a discrete cosine transformation (DCT) or another type of transform) on the second video block having the second color space. In addition, transform/quantization unit108may quantize the second video block (i.e., the transformed residual video block). Entropy encoder110may entropy encode the quantized residual video block. Entropy encoder110may output a bitstream that includes the quantized residual video block for decoding by a video decoder, e.g. video decoder30.

Dequantization/inverse transform unit112may also receive the quantized, transformed coefficient and/or residual video blocks, and may inversely transform and dequantize the transformed coefficient and residual video blocks. The dequantized, inversely transformed video blocks may still have the second color space at this point. The result of the dequantization/inverse transform is reconstructed residual signal126. Inverse adaptive color transformer114may inversely color transform the reconstructed residual signal based on the inverse color transform associated with the transform performed by adaptive color transformer106. The resulting inversely adaptive color transformed coefficient and/or residual video blocks may have an RGB color space at this point.

Following application of an inverse color transformation to a residual video block, prediction compensator116may add back in a predictive block to the residual video block. Deblock filter118may deblock the resulting block. SAO filter120may perform SAO filtering. Reference picture memory122may then store the resulting reconstructed signal128for future use.

To color transform a video block of an input signal (i.e., unencoded video data), rather than a block of residual video data, switch101is flipped to the alternate position, and adaptive transformer102color transforms the input video block from a video block having an RGB color space to a second color space using a color transform of the one or more color transforms. Prediction with prediction processing unit104proceeds as described above, but the result may be fed to transform/quantization unit108directly because switch105is in the alternate position (as compared to the position illustrated inFIG. 8), rather than being color transformed by adaptive color transformer106.

Transform/quantization unit108, entropy encoder110, and dequantization/inverse transform unit112may each operate as described above with respect to color transforming a residual video block, and reconstructed signal126is generated, and is also in the second color space. Reconstructed signal126is fed to prediction compensator116via switch113. Switch113is in the alternate position to the position illustrated inFIG. 8, and inverse adaptive color transformer114is bypassed. Prediction compensator116, deblock filter118, and SAO filter120may operate as described above with respect to color transforming a residual video block to produce reconstructed signal128. However, unlike reconstructed signal128described above, in this example, a block of reconstructed signal128may still have the second color space, rather than the RGB color space.

Reconstructed signal128may be fed to inverse adaptive color transformer130via switch121, which is in the alternate position to that illustrated inFIG. 8. Inverse adaptive color transformer130may inversely color transform blocks of reconstructed signal128to blocks having an RGB color space, and reference picture memory122may store the blocks as blocks of a reference picture for future reference.

As described above, video encoder21may select a transform of the one or more color spaces to transform a first block of the video data having an RGB color space, to a second color space. In some examples, video encoder21selects the color transform adaptively by calculating rate-distortion costs associated with each of the color transforms. For instance, video encoder21may select the color transform of the plurality of color transforms that has the lowest associated distortion cost for a CU or block of a CU. Video encoder21may signal an index syntax element or other syntax data that indicates the selected color transform that has the lowest associated distortion cost.

In some examples, video encoder21may utilize a Lagrangian cost function that accounts for the tradeoff between the bitrate (e.g. the compression achieved) by the color transform, as well as the distortion (e.g., the loss of fidelity) associated with the color transform. In some examples, the Lagrangian cost corresponds to L=D+λR, where L is the Lagrangian cost, D is the distortion, λ is a Lagrange multiplier, and R is the bitrate. In some examples, video encoder21may signal an index syntax element that indicates the color transform of the plurality of color transforms that minimizes the Lagrangian cost.

In some high performance or high fidelity video coding applications or configurations, distortion should be minimized above minimizing bitrate. In such cases, when transforming video data from an RGB color space to a second color space, video encoder21may select the color transform, and the color space that results in the least distortion. Video encoder21may signal an index syntax element that indicates the selected color transform or color space that results in the least distortion.

In some other cases, video encoder21may calculate a cost of transforming blocks of an RGB color space to a second color space based on the correlation between each of the color components of the block of RGB video data and the color components of the block of the second color space. The color transform having the lowest associated cost may be the color transform that has color components that are most closely correlated with the RGB color components of the input signal. Video encoder21may signal an index syntax element that indicates the selected color transform that has the highest correlation between its color components and RGB color components.

It should be recognized that in some cases, video encoder21may select different color transforms for different CUs, LCUs, CTUs, or other units of video data. That is, for a single picture, video encoder21may select different color transforms associated with different color spaces. Selecting multiple different color transforms may better optimize coding efficiency and reduce rate distortion. To indicate which transform of the multiple transforms that video encoder21has selected for the current block, video encoder21may signal an index value corresponding to the selected color transform. Video encoder21may signal the index value at one or more of the first block of video a CTU, CU, PU, and a TU.

However, in some cases, video encoder21may determine a single color transform that is to be applied to one or a plurality of blocks, or a sequence of coded pictures, referred to as a CVS. In the case that only one color transform is selected, for each block, video encoder21may signal a flag syntax element. One value of the flag syntax element may indicate that video encoder21has applied the single transform to the current block or to all of the pictures in the CVS. The other value of the flag syntax element indicates that no transform has been applied to the current block. Video encoder21may determine whether or not to apply the color transform to each of the blocks of the picture on an individual basis, e.g. using the cost-based criteria described above.

In some examples, video encoder21determine whether to apply a pre-defined color transform of the plurality of inverse color transforms to each one of the plurality of blocks. For example, video encoder21and video decoder31may utilize a default pre-defined color transform/inverse color transform. Responsive to determining to apply the pre-defined color transform to each one of the plurality of blocks, video encoder21may transform each of the plurality of blocks using the pre-defined color transform without decoding data indicating that the pre-defined color transform has been applied to each one of the plurality blocks of video data.

In a reciprocal manner, video decoder31may be configured to determine whether to apply a pre-defined inverse color transform of the plurality of inverse color transforms to each one of the plurality of blocks. Responsive to determining to apply the pre-defined inverse color transform to each one of the plurality of blocks, video decoder31may inversely transform each of the plurality of blocks using the pre-defined color transform without decoding data indicating that the pre-defined color transform has been applied to each one of the plurality blocks of video data

The color transforms of this disclosure may include, but are not necessarily limited to, an identity transform, a differential transform, a weighted differential transform, a DCT, a YCbCr transform, a YCgCo transform, and a YCgCo-R transform to the block of video data. A video coder configured in accordance with the techniques of this disclosure, such as video encoder21, may apply one or more of these transforms and/or their inverses as well as other transforms, such as transforms to/from Adobe RGB, sRGB, scRGB, Rec. 709, Rec. 2020, Adobe Wide Gamut RGB, ProPhoto RGB, CMYK, Pantone, YIQ, YDbDr, YPbPr, xvYCC, ITU BT.601, ITU BT.709, HSV, and other color spaces, color spaces, and/or chroma subsampling formats not specifically described herein.

To apply a color transform to a block of video data having an RGB color space, video encoder21may multiply a 3×1 matrix comprising the Red, Green, and Blue color components of an RGB pixel with a color transform matrix. The result of the multiplication is a pixel having a second color space. The video coder may apply the color transform matrix to each pixel of the video block to produce a second block of pixels in a second color space. Various color transforms are now described in greater detail.

In some examples, video encoder21may apply an identity transform matrix or inverse identity transform matrix. The identity transform matrix comprises:

[100010001],
and the inverse transform matrix, which video decoder30may apply, comprises:

[100010001].
When a video coder applies the identity transform, the resulting pixel value is identical to the input pixel value, i.e. applying the identity transform is equivalent to not applying a color transform at all. Video encoder21may select the identity transform when maintaining the RGB color space of the video blocks is required.

In another example, video encoder21may apply a differential transform matrix. The differential transform matrix comprises:

In another example, video encoder21may be configured apply a weighted differential transform or inverse weighted differential transform. The weighted differential transform matrix comprises:

In the weighted differential transforms, α1and α2are parameters that a video coder may adjust. In some examples, video encoder20may calculate the parameters α1and α2according to the following equations:
α1=cov(G,B)/var(G), and
α2=cov(G,R)/var(G).
Video encoder21may signal the values of α1and α2in the coded video bitstream in various examples.

In these equations, R corresponds to a red color channel, G corresponds to a green color channel, and B corresponds to a blue color channel of the RGB color space. In the differential transform equations, “cov( )” is the covariance function, and “var( )” is the variance function.

To determine the values of R, G, and B, an encoder or decoder may utilize a set of reference pixels in order to ensure that the covariance and variance functions have the same result or weight when calculated by the encoder or by the decoder. In some examples, the particular reference pixels may be signaled in the coded video bitstream (e.g. as syntax elements in a coded video bitstream). In other examples, the encoder and decoder may be preprogrammed to use certain reference pixels.

In some examples, video encoder21may restrict or constrain the values of α1and α2when transforming blocks using the differential transform. The video coder may constrain the values of α1and α2to a set of integers or dyadic numbers, e.g. ½, ¼, ⅛, etc. . . . . In other examples, a video coder may restrict α1and α2to values of a fraction having a dyadic number, e.g. ⅛, 2/8, ⅜, . . . , 8/8. A dyadic number or dyadic fraction is a rational number having a denominator that is a power of two, and where the numerator is an integer. Restricting the values of α1and α2may improve the bitstream efficiency of coding α1and α2.

In other examples, video encoder21may be configured to transform a block having an RGB color space to generate a second block, using a DCT transform. The DCT transforms samples of a block to express the samples as a sum of sinusoids of different frequencies and amplitudes. A DCT transform or inverse transform may transform pixel to and from a finite sequence of data points in terms of a sum of cosine functions. The DCT transform matrix corresponds to:

[0.57740.57740.57740.70710-0.70710.4082-0.81560.4082].
In a reciprocal manner, video decoder31may be configured to apply an inverse transform to blocks transformed using the DCT revert the blocks back to the original samples. The inverse DCT transform matrix corresponds to:

Video encoder21may also apply a YCbCr transform to a block having an RGB color space to produce a block having a YCbCr color space. As described above, the YCbCr color space includes a luma (Y) component, as well as blue chrominance (Cb) and red chrominance (Cr) components. The YCbCr transform matrix may correspond to:

Video decoder31may be configured to apply an inverse YCbCr transform to convert a block having a YCbCbr color space to a block having an RGB color space. The inverse YCbCr transform matrix may correspond to:

Video encoder21may also apply a YCgCo transform to a block having an RGB color space to produce a block having a YCgCo color space. A YCgCo color space includes a luma (Y) component, as well as green chrominance (Cg) and orange chrominance (Co) components. The YCgCo transform matrix may correspond to:

Video decoder31may be configured to apply an inverse YCgCo transform to convert a block having a YCgCo color space to a block having an RGB color space. The inverse YCgCo transform matrix may correspond to:

Video encoder21may also be configured to apply a YCgCo-R transform to a block having an RGB color space to produce a block having a YCgCo-R color space. The YCgCo-R color space includes a luma (Y) component, as well as green chrominance (Cg) and orange chrominance (Co) components. Unlike the YCgCo transform described above, however, the YCgCg-R transform is reversible, e.g. the YCgCo-R transform may not produce any distortion, for example due to rounding errors.

The YCbCr transform matrix may correspond to:
Co=R−B
t=B+└Co/2┘
Cg=G−t.
Y=t+└Cg/2┘
Video decoder31may be configured to apply an inverse YCgCo-R transform. The YCgCo-R inverse transform inversely transforms blocks having a YCgCo-R color space to blocks having an RGB color space. The inverse YCgCo-R transform matrix may correspond to:
t=Y−└Cg/2┘
G=Cg+t
B=t−└Co/2┘.
R=B+Co

In order to apply any of the color transforms described herein, video encoder21may implement a lifting scheme that has flexible parameters. A lifting scheme is a technique of decomposing a discrete wavelet transform into a finite sequence of simple filtering steps, referred to as lifting steps or as ladder structures. Video encoder21may signal the parameters in the coded video bitstream, or video encoder21may derive the parameters may be derive the parameters the same way. One example of a lifting scheme is as follows:
R′=R+└aB┘
B′=B+└bR′┘
G′=G+└cB′┘,
R″=R′+└dG′┘
where a, b, c, and d are parameters as described above. In this lifting scheme, R, G, and B are red, green, and blue color channels or samples, respectively. As with the a parameters described above with respect to the weighted differential transform, the values of a, b, c, and d may be restricted or limited, e.g. so the signs can only be positive or negative. In some cases, there may be additional steps in the lifting scheme, such as:
R′″=└eR″+f┘
B″=└gB′+h┘,
G″=└iG′+j┘
where f, g, h, i, and j are parameters. When using the lifting scheme, as well as in other examples, the video encoder20and video decoder30can normalize the output depth of the three components, R′″, B″, and G″ can be normalized within a pre-determined bit depth, which may not necessarily be the same for each component.

FIG. 9is a block diagram illustrating another example video decoder31that may utilize techniques for inversely transforming video data having a first color space to video data having a second, RGB color space using an inverse color transform in accordance with one or more aspects of this disclosure.

FIG. 9illustrates a more detailed version of video decoder31relative to video decoder30ofFIG. 1andFIG. 7. Indeed, in some examples video decoder31may be considered a more specific example of video decoder30(FIG. 7) and/or video decoder30(FIG. 1). The example ofFIG. 9illustrates two possible examples for implementing the techniques of this disclosure. In the first implementation, video decoder31adaptively inversely transforms a block of an input video signal from a first color space (e.g., a non-RGB color space) to a second block having a second, RGB color space using an inverse color transform of a plurality of inverse color transforms. The second illustrated example performs the same techniques, but performs the inverse color transformation on blocks of residual video data, rather than on an input signal.

In the example ofFIG. 9, video decoder31is shown as performing inverse color transforms on blocks of residual video data example because of the way switches145, and156are currently switched. If switches145and156are switched the alternative position, video decoder31is configured to inversely color transform blocks of input video data having a first representation to a blocks of video data having a second, RGB color space, rather than inversely transforming blocks of residual video data.

The process of performing inverse color transforms on blocks of residual video data as illustrated inFIG. 9is now described in detail. In the example ofFIG. 9, an encoded input bitstream140(also referred to as an input signal) is passed to entropy decoding unit142. Entropy decoding unit142may entropy decode bitstream140to produce a quantized block of residual video data having a first color space. For instance, entropy decoding unit142may entropy decode particular syntax elements included in bitstream140. Dequantization/inverse transform unit144may dequantize a transform coefficient block. Additionally, dequantization/inverse transform unit144may apply an inverse transform to the transform coefficient block to determine a transform block comprising residual video data. Thus, dequantization/inverse transform unit144may dequantize and inversely transform blocks of entropy decoded video data of bitstream140. When video decoder31is configured to inversely color transform blocks of residual data, switch148feeds a block of residual video data having a first color space to inverse adaptive color transformer150. In this way, inverse adaptive color transformer150may receive a transform block of a TU.

Inverse adaptive color transformer150may adaptively inversely transform a block of video data having the first color space to a second block of video data having a second, RGB color space. For example, inverse adaptive color transformer150may select an inverse transform to apply to a transform block of a TU. In this example, inverse adaptive color transformer150may apply the selected inverse transform to the transform block in order to transform the transform block from the first color space to the RGB color space. Prediction compensation unit152may combine a reference picture from reference picture memory154. For example, prediction compensation unit152may receive a transform block of a TU of a CU. In this example, prediction compensation unit152may determine a coding block for the CU. In this example, each sample of the coding block of the CU may be equal to a sum of a sample in the transform block and a corresponding sample in a prediction block for a PU of the CU. Deblock filter157may deblock the combined, reconstructed image. SAO filter unit158may perform additional SAO filtering if applicable.

The output of SAO filter unit158is reconstructed signal160. If video decoder31is configured to inversely color transform blocks of residual video data, switch162feeds reconstructed signal160to reference picture memory154for future use as a reference picture. Video decoder31may also output reconstructed signal160as image/video164.

In examples where video decoder31is configured to inversely color transform blocks of the original input signal as opposed to blocks of residual video data, entropy decoding unit142and dequantization/inverse transform unit144operate in the manner previously described. Switch148is in the alternate position and feeds reconstructed residual signal directly to prediction compensation unit152. At this point, the residual block provided to prediction compensation unit152is still in the first color space, rather than the RGB color space.

Prediction compensation unit152may reconstruct a block of the original image and may combine the residual block with one or more blocks of pictures from reference picture memory154. Deblock filter157and SAO filter unit158may operate as described above with respect to inversely transforming residual blocks of video data. The output of SAO filter unit158is reconstructed signal160, the blocks of which are still in the first color space, and may not be have the RGB color space (e.g., the blocks may still have the RGB color space if the identity transform was used).

Reconstructed signal160may be fed to inverse adaptive color transformer166via switch162, which is in the alternate position as compared to the position illustrated inFIG. 9. Inverse adaptive color transformer166may inversely color transform a block of reconstructed signal having a first color space to a second block of video data having a second, RGB color space using an inverse color transform of one or more inverse color transforms. In some examples, the particular inverse transform that decoder31uses may be signaled in bitstream140. Inverse adaptive color transformer166may feed the second block having the second color space for output as image/video164, as well as to reference picture memory154for future storage and usage as a reference picture.

FIG. 10shows an example of a method of decoding video data in accordance with the techniques of this disclosure. The techniques ofFIG. 10will be described with respect to a generic video decoder. The generic video decoder may, for example, correspond to video decoder30ofFIG. 7or video decoder31ofFIG. 9, although the techniques ofFIG. 10are not limited to any particular type of video decoder. As video encoders typically perform video decoding as part of the encoding process, the techniques ofFIG. 10may also be performed by a video encoder, such as video encoder20ofFIG. 6and video encoder21ofFIG. 8. Video encoder20, for example, includes inverse quantization unit58and inverse transform processing unit60, which form part of a decoding loop, in which the techniques ofFIG. 10may be implemented. Thus, while the techniques ofFIG. 10will be explained with reference to a video decoder, it should be understood that this video decoder may be part of a video encoder.

In the example ofFIG. 10, the video decoder determines for one or more blocks of the video data that adaptive color transform is enabled (210). In some examples, the video decoder may determine for the one or more blocks of the video data that adaptive color transform is enabled by receiving a syntax element that indicates if adaptive color transform is enabled. The syntax element may, for example, be received in the PPS or at another level. By parsing the received syntax element, the video decoder can determine if adaptive color transform is enabled or disabled. In other examples, the video decoder may determine for the one or more blocks of the video data that adaptive color transform is enabled by determining a chroma format for the video data. For example, in response to determining a chroma format for the video data is 4:4:4, the video decoder may determine that adaptive color transform is enabled. In response to determining a chroma format for the video data is other than 4:4:4, the video decoder may determine that adaptive color transform is disabled.

For video data with adaptive color transform enabled, the video decoder may determine a quantization parameter for the one or more blocks (212). In response to a value of the quantization parameter being below a threshold, the video decoder may modify the quantization parameter to determine a modified quantization parameter (214). The threshold may, for example, be zero, and a value of the modified quantization parameter may be greater than or equal to zero. The modified quantization parameter may be less than or eqaul to 51 plus an offset value. To modify the quantization parameter, the video decoder may add an offset value to the quantization parameter. The video decoder may receive a flag to indicate if the offset value is to be added to the quantization parameter.

The video decoder may, for example, receive the offset value as a syntax element. The offset value may be an offset to the quantization parameter (when adaptive color transform is enabled for the block). The video decoder may dequantize transform coefficients based on the modified quantization parameter (216).

FIG. 11shows an example of a method of encoding video data in accordance with the techniques of this disclosure. The techniques ofFIG. 11will be described with respect to a generic video encoder. The generic video encoder may, for example, correspond to video encoder20ofFIG. 6or video encoder21ofFIG. 8, although the techniques ofFIG. 11are not limited to any particular type of video encoder. The video encoder selects a chroma sampling format for the video data (220). In response to the chroma sampling format being a first chroma sampling format, the video encoder generates a syntax element to indicate if adaptive color transform is enabled (222). In response to the chroma sampling format being other than the first chroma sampling format, the video encoder encodes the video data without adaptive color transform (224). The first chroma sampling format may, for example, be a 4:4:4 chroma sampling format.

FIG. 12shows an example of a method of decoding video data in accordance with the techniques of this disclosure. The techniques ofFIG. 12will be described with respect to a generic video decoder. The generic video decoder may, for example, correspond to video decoder30ofFIG. 7or video decoder31ofFIG. 9, although the techniques ofFIG. 12are not limited to any particular type of video decoder. The generic video decoder may also correspond to a decoding loop of a video encoder, in some examples.

In the example ofFIG. 12, the video decoder determines, based on a chroma sampling format for the video data, that adaptive color transform is enabled for one or more blocks of the video data (230). The video decoder may, for example, determine that adaptive color transform is enabled for one or more blocks of the video data by determining the chroma sampling format is a 4:4:4 sampling format. The video decoder may determine a quantization parameter for the one or more blocks based on determining that the adaptive color transform is enabled (232) and dequantize transform coefficients based on the determined quantization parameter (234).

The video decoder may also, for example, determine for one or more second blocks of the video data that a chroma sampling format for the video blocks is a chroma sampling format other than 4:4:4 and based on the chroma sampling format being other than 4:4:4, determining that adaptive color transform is disabled for the second one or more blocks. The video decoder may, for example, determine that adaptive color transform is disabled for the second one or more blocks without receiving a syntax element other than the indication of the chroma sampling format, to indicate if adaptive color transform is disabled.

FIG. 13shows an example of a method of encoding video data in accordance with the techniques of this disclosure. The techniques ofFIG. 13will be described with respect to a generic video encoder. The generic video encoder may, for example, correspond to video encoder20ofFIG. 6or video encoder21ofFIG. 8, although the techniques ofFIG. 13are not limited to any particular type of video encoder. The video encoder determines for one or more blocks of the video data that adaptive color transform is used to encode the blocks (240). The video encoder determines a quantization parameter for a first color component of a first color space of the video data (242). The video encoder quantizes transform coefficients based on the quantization parameter (244). The video encoder generates for inclusion in a picture parameter set, one or more offset values that represent a difference between a quantization parameter for a first color component of a second color space of the video data and the quantization parameter for the first color component of the second color space of the video data (246).

FIG. 14shows an example of a method of decoding video data in accordance with the techniques of this disclosure. The techniques ofFIG. 14will be described with respect to a generic video decoder. The generic video decoder may, for example, correspond to video decoder30ofFIG. 7or video decoder31ofFIG. 9, although the techniques ofFIG. 14are not limited to any particular type of video decoder. In the example ofFIG. 14, the video decoder determines for one or more blocks of the video data that adaptive color transform is enabled (250). In response to adaptive color transform being enabled, the video decoder receives in a picture parameter set, one or more offset values (252). The video decoder determines a quantization parameter for a first color component of a first color space based on a first of the one or more offset values (254) and dequantizes transform coefficients based on the modified quantization parameter (256). The one or more offset values may include an offset value for the first color component, an offset value for a second color component, and an offset value for a third color component.

The video decoder may determine a quantization parameter for a first color component of a second color space. To determine the quantization parameter for the first color component of the first color space based on the first of the one or more offset values, the video decoder may convert the quantization parameter for the first color component of the second color space to the quantization parameter for the first color component of the first color space by adding the first of the one or more offset values to the quantization parameter for the first color component of the second color space.