Patent Description:
Video coding systems may be used to compress digital video signals. For example, video coding systems may reduce storage space consumed and/or reduce transmission bandwidth consumption associated with video signals. For example, block-based hybrid video coding systems may be used.

Digital video signals may include three color planes. The three color planes may include a luma plane, a blue-difference chroma plane, and a red-difference chroma plane. Pixels of the chroma planes may have smaller dynamic ranges than pixels of the luma plane. For example, the chroma planes of a video image may be smoother and/or have less detail than the luma plane of the video image. A chroma block of a video image may be easier to predict (e.g., accurately predict). For example, prediction of the chroma block may consume fewer resources and/or result in less prediction error.

A high dynamic range (HDR) video may offer a wider dynamic range than a standard dynamic range (SDR) video. The dynamic range of HDR video may be closer to the capacities of the human eye. Chroma artifacts in HDR video may be more visible against a brighter background than chroma artifacts in SDR video. HDR video coding may include preprocessing, coding, decoding, and/or post-processing. Non-patent literature<NPL> (<NUM>-<NUM>-<NUM>) discloses an SEI message carrying post filter coefficients, in order to enhance the quality of the reconstructed chroma planes wherein, on the decoder side, a reconstructed chroma pixel is enhanced by adding an appropriate offset obtained by applying the received post filters, which usually have high-pass characteristics, on the surrounding luma pixels.

Systems, methods, and instrumentalities are disclosed for enhanced chroma coding using cross plane filtering. An indication of a cross-plane filter associated with a current picture may be received. The indication may include one or more filter coefficients associated with the cross-plane filter. The current picture may include an intra-coded video block and a plurality of reference samples. The plurality of reference samples may be used to predict the intra-coded video block. A luma sample region may be determined in the current picture. The luma sample region may include a plurality of luma samples. For example, the luma sample region may be a 3x3 block of luma samples. The plurality of luma samples may include predicted luma samples such that the cross-plane filter is applied to the predicted luma samples prior to reconstruction of the luma samples. The plurality of luma samples may include reconstructed luma samples such that the cross-plane filter may be applied to the reconstructed luma samples after reconstruction. The luma sample region may be determined based on a selected intra prediction mode.

The luma sample region may be determined for enhancing a corresponding chroma sample in the current picture. The corresponding chroma sample may be a predicted chroma sample or a reconstructed chroma sample. When the corresponding chroma sample is a predicted chroma sample in the intra-coded video block, the enhanced chroma sample may be used for prediction. When the corresponding chroma sample is a reconstructed chroma sample before in-loop filtering, the enhanced chroma sample may be used to replace the corresponding chroma sample before in-loop filtering is applied. The corresponding chroma sample may be a reference chroma sample used to predict one or more chroma samples in the intra-coded video block. A determination of whether to apply the cross-plane filter based on a received chroma enhancement indicator. The received chroma enhancement indicator may be received at a block level. The cross-plane filter may be applied to a plurality of luma samples in the luma sample region to determine an offset. The cross-plane filter may be a high pass filter. The offset may be applied to the corresponding chroma sample to determine an enhanced chroma sample. The luma sample region may include an unavailable luma sample. The unavailable luma sample may be replaced with a neighboring available luma sample, for example, prior to applying the cross-plane filter to the plurality of luma samples. The cross-plane filter to apply may be determined based on the selected intra prediction mode.

A plurality of luma sample regions may be determined for a current picture. For example, a first luma sample region and a second luma sample region may be determined. The first luma sample region may include a first plurality of luma samples. The second luma sample region may include a second plurality of luma samples. The first luma sample region may neighbor a first corresponding chroma sample in the current picture. The second luma sample region may neighbor a second corresponding chroma sample in the current picture. The cross-plane filter may be applied to the first plurality of luma samples and the second plurality of luma samples to determine a first offset and a second offset, respectively. The first offset may be applied to the first corresponding chroma sample to determine a first enhanced chroma sample. The second offset may be applied to the second corresponding chroma sample to determine a second enhanced chroma sample.

A detailed description of illustrative embodiments will now be described with reference to the various figures. Although this description provides a detailed example of possible implementations, it should be noted that the details are intended to be exemplary.

Video coding systems may compress digital video signals, for example, to reduce the storage and/or transmission bandwidth of digital video signals. There are a variety of video coding systems, such as block-based, wavelet-based, object-based systems and block-based hybrid video coding systems. Examples of block-based video coding systems are H. <NUM>, (Moving Picture Experts Group) MPEG-<NUM>, MPEG-<NUM>, H. <NUM>/Advanced Video Coding (AVC) and H. <NUM>/ High Efficiency Video Coding (HEVC).

<FIG> shows an example of a block based hybrid video encoder. Spatial prediction (e.g., intra prediction) or temporal prediction (e.g. inter prediction) may be performed, for example, for a (e.g., each) video block, e.g., to reduce spatial and temporal redundancy in video blocks. A prediction block generated from intra or inter prediction may be subtracted from a current video block. A resulting prediction residual may be transformed and quantized. A residual may be reconstructed, for example, by inverse quantizing and inverse transforming quantized residual coefficients. A reconstructed residual may be added to a prediction block, e.g., to form a reconstruction video block. In-loop filtering may be applied to a reconstructed video block. A filtered reconstructed video block, which may be stored in a decoded picture buffer, may be used to code one or more next video blocks.

<FIG> shows an example of a block based hybrid video decoder. The decoder in <FIG> may correspond to the encoder in <FIG>. An encoded video bitstream may be parsed and entropy decoded. A coding mode and associated prediction information may be passed, for example, to spatial prediction or motion compensated prediction, e.g., to form a prediction block. Residual transform coefficients may be inverse quantized and inverse transformed, for example, to reconstruct a residual block. A prediction block and a reconstructed residual block may be added together, e.g., to form a reconstructed block. In-loop filtering may be applied to a reconstructed video block. A filtered reconstructed video block, which may be stored in a reference picture buffer, may be used to predict future video blocks.

Intra coding may be used, for example, to eliminate spatial correlation in some image and video coding techniques, such as Joint Photographic Experts Group (JPEG), <NUM>, MPEG-<NUM>, MPEG-<NUM>, H. <NUM>/AVC and H. <NUM>/HEVC. Directional intra prediction may be used, for example, in H. <NUM>/AVC and H. <NUM>/HEVC, e.g., to improve coding efficiency. Intra prediction modes may utilize a set of reference samples, e.g., from above and to the left of a current block to be predicted. Reference samples may be denoted as Rx, y. In an example, a position (x, y) may have its origin one pixel above and to the left of a block's top-left corner. A predicted sample value at the position (x, y) may be denoted as Px,y.

<FIG> shows an example of reference samples Rx,y used for prediction to obtain predicted samples Px,y for a block size of N×N samples.

<FIG> shows an example of partitioning modes for an intra prediction unit (PU). HEVC intra coding may support multiple types of PU division, e.g., PART_2N×2N and PART NxN, which may split a coding unit (CU) into one or four equal size PUs, respectively. PART_2N×2N may be available when a CU size is a configured minimum CU size.

An 8x8 CU split into four 4x4 PUs may have four luma prediction blocks (PBs), for example, for <NUM>:<NUM>:<NUM> chroma formats. There may be one 4x4 PB per chroma channel for intra coded blocks, for example, to avoid high throughput caused by 2x2 chroma intra prediction blocks.

A CU may be split into multiple transform units (TUs). Intra prediction may be applied sequentially to TUs. For example, as compared to applying intra prediction at PU level. Sequential intra prediction may permit use in intra prediction neighboring reference samples from previous reconstructed TUs that are closer to current TU samples.

<FIG> shows an example of angular intra prediction modes. HEVC may support one or more (e.g. <NUM>) intra prediction modes. The one or more intra prediction modes may include a DC mode, a planar mode, and/or <NUM> directional or 'angular' intra prediction modes.

Angular intra prediction may be used to efficiently model different directional structures in video and image content. The number and angularity of prediction directions may be selected based on a trade-off between encoding complexity and coding efficiency.

A predicted sample Px,y may be obtained, for example, by projecting its location to a reference row or column of pixels, applying a selected prediction direction, and interpolating a predicted value for the sample at <NUM>/<NUM> pixel accuracy. Interpolation may be performed linearly utilizing the two closest reference samples, e.g., Ri,<NUM> and Ri+<NUM>,<NUM> for vertical prediction (e.g. mode <NUM>~<NUM> as shown in <FIG>) and R<NUM>,i and R<NUM>,i+<NUM> for horizontal prediction (e.g. mode <NUM>~<NUM> as shown in <FIG>).

HEVC may support one or more intra prediction modes for luma intra prediction for a variety of (e.g. all) PU sizes. HEVC may define multiple (e.g. three) most probable modes (MPMs) for a (e.g. each) PU, for example, based on the modes of one or more neighboring PUs. A current intra prediction mode may be equal to one of the elements in a set of MPMs. An index in the set may be transmitted to the decoder. A code (e.g. a <NUM>-bit fixed length code) may be used to determine a selected mode outside the set of MPMs.

Reference samples may be smoothed. In an example, a <NUM>-tap smoothing filter may be applied to one or more reference samples. The smoothing may be applied, for example, when an intra_smoothing_disabled_flag is set to <NUM>. Filtering may be controlled, for example, by a given intra prediction mode and/or transform block size. In an example, e.g., for 32x32 blocks, angular modes may use filtered reference samples, for example, except horizontal and vertical angular modes. In another example, e.g., for 16x16 blocks, modes not using filtered reference samples may be extended to four modes (e.g. <NUM>, <NUM><NUM>, <NUM>, <NUM>) closest to horizontal and vertical as shown in <FIG>. In another example, e.g., for 8x8 blocks, diagonal modes (<NUM>, <NUM>, <NUM>) may use filtered reference samples.

Intra prediction may be applied for a chroma component. In an example, an intra prediction mode may be specified, e.g., as planar, DC, horizontal, vertical, 'DM_CHROMA' mode, diagonal mode (<NUM>), for example, for one or more prediction blocks (PBs) associated with chroma.

Table <NUM> shows an example mapping between an intra prediction mode and an intra prediction direction for chroma. A chroma color channel intra prediction mode may be based on a corresponding luma PB intra prediction mode and/or an intra_chroma_pred_mode syntax element.

Table <NUM> shows an example specification of intra prediction mode for <NUM>:<NUM>:<NUM> chroma format, e.g., when a DM_CHROMA mode is selected and a <NUM>:<NUM>:<NUM> chroma format is in use. An intra prediction mode for a chroma PB may be derived, for example, from an intra prediction mode for a corresponding luma PB, e.g., as specified in Table <NUM>.

<FIG> shows an example of an intra boundary filter. An intra-boundary filter may be used, for example, when reconstructing intra-predicted transform blocks (TBs). An intra-boundary filter may be used, for example, to filter predicted luma samples along the left and/or top edges of the TB for PBs using horizontal, vertical, and/or DC intra prediction modes, e.g., as shown in <FIG>.

An intra boundary filter may be defined, for example, based on an array of predicted samples p as an input and/or predSamples as an output.

Intra boundary filtering provided by Eq. (<NUM>) may be used to generate predSamples as an output, for example, for horizontal intra-prediction applied to luma transform blocks of size (nTbS) less than 32x32, disableIntraBoundaryFilter equal to <NUM>, where x = <NUM>. nTbS - <NUM>, y = <NUM>: <MAT>.

Intra boundary filtering provided by Eq. (<NUM>) may be used to generate predSamples as an output, for example, for vertical intra-prediction applied to luma transform blocks of size (nTbS) less than <NUM>×<NUM>, disableIntraBoundaryFilter equal to <NUM>, where x = <NUM>. nTbS - <NUM>, y = <NUM>: <MAT>.

Intra boundary filtering provided by Eq. (<NUM>), Eq. (<NUM>) and Eq. (<NUM>) may be used to generate predSamples as an output, for example, for DC intra-prediction applied to luma transform blocks of size (nTbS) less than 32x32 and a DC predictor dcVal: <MAT> <MAT> <MAT>.

An improvement may be provided by boundary smoothing, e.g. <NUM>% average improvement. An intra boundary filter may be applied on a luma component. An intra boundary filter may not be applied on a chroma component, e.g., because prediction for chroma components tends to be smooth.

HEVC intra mode residual coding may utilize intra mode dependent transforms and/or coefficient scanning to code residual information. A discrete sine transform (DST) may be selected for 4x4 luma blocks. A discrete cosine transform (DCT) may be selected/used for other types of blocks.

A linear-model (LM) based chroma intra prediction mode may be used, for example, to predict chroma samples from collocated reconstructed luma samples using a linear model (LM), e.g., in accordance with Eq. (<NUM>): <MAT> where PredC may indicate predicted chroma samples in a block and RecL may indicate corresponding reconstructed luma samples in a block. Parameters α and β may be derived from causal reconstructed luma and chroma samples around a current block.

Linear model chroma intra prediction may improve coding efficiency. As an example, experimental results in a test configuration indicate average Bjøntegaard delta rate (BD-rate) reductions of Y, Cb, Cr components comprising <NUM>%, <NUM>% and <NUM>%, respectively. In an example, a similar level of coding efficiency improvements of chroma components may be provided in a test configuration.

<FIG> shows an example of different partitions for HEVC inter-prediction coding. Inter coding may be used, for example, to remove or reduce temporal redundancy. HEVC inter-prediction coding may support more PB partition shapes than intra-prediction coding (e.g., intra-coding). Intra prediction may support, for example, partitions PART_2Nx2N, PART_2NxN, PART_Nx2N, PART_NxN. Inter-picture prediction may support, for example, partitions PART_2Nx2N, PART_2NxN, PART_Nx2N, PART_NxN and asymmetric motion partitions PART_2NxnU, PART_2NxnD, PART_nLx2N, and PART_nRx2N.

An (e.g. each) inter-predicted PU may have a set of motion parameters comprising one or more motion vectors and one or more reference picture indices. A P slice may, for example, use one reference picture list and a B slice may, for example, use two reference picture lists. Inter-prediction samples of a PB may be determined from one or more samples of a corresponding block region in a reference picture identified by a reference picture index. The reference picture index may be at a position displaced by horizontal and vertical components of a motion vector (MV).

<FIG> shows an example of motion compensated prediction using motion vectors (MVs). Horizontal and vertical motion vectors may be denoted as dx and dy, respectively.

<FIG> shows an example of fractional sample interpolation. Fractional sample interpolation may be used, for example, when a motion vector has a fractional value. Fractional sample interpolation may generate prediction samples for non-integer sample positions. HEVC may support MVs, for example, with units of ¼ of the distance between luma samples. HEVC may support MVs, for example, with units of <NUM>/<NUM> of the distance between chroma samples, e.g., in <NUM>:<NUM>:<NUM> format.

Motion vector prediction may exploit spatial-temporal correlation of a motion vector with neighboring PUs. A merge mode may be used, for example, to reduce motion vector signaling cost. A merge mode merge candidate list may comprise a list of motion vector candidates from neighboring PU positions (e.g. spatial neighbors and/or temporal neighbors) and/or zero vectors. An encoder may select a (e.g. the best) predictor from a merge candidate list and may transmit a corresponding index indicating the predictor chosen from the merge candidate list.

Cross plane filtering may use high frequency information from luma, for example, to improve and enhance chroma quality. High frequency information may be extracted from luma, for example, by applying a high pass filter on the luma component.

Luma and chroma components may have some correlations, such as object contours and edge areas. Cross-plane filtering for chroma enhancement may include applying high pass filtering to a luma component. An output of the high pass filtering may be added to the chroma component to determine an enhanced chroma component. The output of the high pass filtering may be an offset. Eq. <NUM> and Eq. <NUM> indicate an example of a chroma enhancement: <MAT> <MAT> where Y is Luma, C is chroma, cross_plane_filter is a filter applied to the luma signal, Yrec is the reconstructed luma signal, Yoffset is the output of the filtering, Crec is the reconstructed chroma signal, which may be a Cb or Cr component and Cenh is the enhanced chroma signal. The filter may be a 1D or 2D filter. A cross plane filter may be derived, for example, based on original chroma and luma and/or reconstructed chroma and luma, e.g., using a Least Square method.

A luma prediction mode may be utilized in DM CHROMA mode for intra prediction, for example, to derive a chroma prediction mode that reduces a signaling overhead of a chroma prediction mode. The chroma prediction mode may be signaled as DM_CHROMA, for example, when a chroma prediction block (PB) utilizes the same prediction mode as a corresponding luma PB. A linear model (LM) chroma prediction mode may predict one or more chroma samples from collocated reconstructed luma samples, for example, by a linear model.

Reference chroma samples (Rx,y), corresponding chroma prediction samples (Px,y), and reconstructed chroma samples may be processed independently of their corresponding luma component(s). For example, for inter prediction, chroma prediction samples and chroma reconstructed samples may be generated independently from their corresponding luma component(s). Cross-plane filtering may be applied at different stages of the intra coding process.

Cross-plane filtering may be used to enhance one or more neighboring chroma samples. The one or more neighboring chroma samples may neighbor the current video block. The neighboring chroma samples may include reference chroma samples, reconstructed chroma samples, and/or predicted chroma samples. For example, a predicted chroma sample may be used for prediction. Neighboring reference chroma samples, Rx,y, may be used to generate predicted chroma samples Px,y, for intra coding, e.g., as shown in <FIG>. Cross-plane chroma filtering may derive high fidelity information from one or more luma samples that correspond to a chroma sample of the one or more neighboring chroma samples, for example, to improve chroma in coding color space.

A cross plane filter may be applied to one or more neighboring luma samples. The neighboring luma sample(s) may include reference luma samples, reconstructed luma samples, and/or predicted luma samples. Derived high pass information may be used, for example, to enhance the quality of chroma reference samples. Enhanced reference chroma samples may be used to generate predicted chroma samples.

One or more enhanced chroma samples may be determined using cross plane filtering. For example, a cross plane filter may be applied to one or more neighboring luma samples. The cross plane filter may be a high pass filter. The one or more neighboring luma samples may correspond to a chroma sample to be enhanced. The cross plane filter may be applied to available and/or unavailable luma samples. A predicted luma sample and/or a reconstructed luma sample (e.g., before or after loop filtering) may be an available luma sample. A non-reconstructed luma sample and/or a non-predicted luma sample may be an unavailable luma sample.

<FIG> is an example of chroma reference sample enhancement <NUM> with cross plane filtering that does not use reconstructed luma samples from a current video block. The current video block may include a 4x4 block of samples, e.g., as defined by a solid line in <FIG>.

One or more luma sample regions such as 902A, 902B may be determined for a current picture. As shown, a luma sample region such as 902A, 902B may include a plurality of luma samples that neighbor a corresponding chroma sample and/or a luma sample that is collocated to the corresponding chroma sample. A sample may neighbor another sample if it is above, below, to the left of, to the right of, and/or diagonal to the other sample. For example, a neighboring sample may be next to the corresponding sample. Collocated samples may include a luma sample at the same location as a chroma sample. The luma sample region 902A may be determined to include one or more neighboring luma samples and/or a collocated luma sample. The luma sample region 902A may be determined such that a chroma sample for enhancement (e.g., enhanced chroma sample 912A) is at the center of each of the one or more luma sample regions 902A, 902B, respectively. For example, the chroma sample that is located at the center of a luma sample region may be enhanced using the cross plane filtering.

The luma samples in a luma sample region such as 902A, may include one or more luma sample(s) 904A reference luma sample(s) 906A, and/or predicted luma sample(s) 908A. The reference luma samples 906A may be reconstructed luma samples used to replace predicted luma sample(s) 908A. For example, each of the one or more predicted luma samples 908A, 908B may be replaced by (e.g., padded from) a respective neighboring reconstructed luma sample of the one or more reconstructed luma samples <NUM>. For example, a luma sample region 902A, 902B may be an MxN window of luma samples, e.g., such as the 3x3 window highlighted by a dashed box in <FIG>. The luma samples in the MxN window may correspond to a chroma sample location. A cross plane filter may be applied to the plurality of luma samples of the luma sample region 902A. An offset may be determined as an output of applying the cross plane filter to the plurality of luma samples of the luma sample region 902A, 902B. The offset may be applied 910A, 910B (e.g., added) to the corresponding chroma sample, for example, to determine an enhanced chroma sample, such as 912A, 912B. The cross plane filter may be applied to a plurality of luma sample regions, such as 902A, 902B in the current picture to determine a plurality of enhanced chroma samples 912A, 912B. An enhanced chroma sample, such as enhanced chroma samples 912A, 912B may be used as reference samples for intra-prediction of the current video block.

In an example, the plurality of luma samples in an MxN luma sample region, e.g., a 3x3 window, may include (e.g. only) reconstructed luma samples before or after in-loop filtering. For example, the plurality of luma samples in the M×N luma sample region may be available. In another example, one or more luma samples in an M×N luma sample region may not have been reconstructed, e.g., as shown in <FIG>. Luma samples that have not been reconstructed may be unavailable luma samples. The unavailable luma samples may be replaced by (e.g., padded using) a neighboring available (e.g., reconstructed) luma sample. Prediction and reconstruction of luma and chroma samples in the current block may be performed in parallel.

<FIG> is an example of chroma reference sample enhancement <NUM> with cross plane filtering using reconstructed luma samples from a current block. In an example, the luma samples in a current block may be reconstructed before the respective corresponding chroma samples, for example, when higher latency between different color channels may be tolerated. Reconstructed luma samples in an MxN (e.g. 3x3) luma sample region 1002A, 1002B may be available, e.g. without padding, when the cross plane filter is applied. The luma sample region 1002A, 1002B may be an MxN (e.g., 3x3) window.

For example, a luma sample region such as 1002A, 1002B may be determined for a current picture. A luma sample region such as 1002A, 1002B may include a plurality of luma samples that neighbor a corresponding chroma sample and/or a luma sample that is collocated with the corresponding chroma sample. The plurality of luma samples in each of the luma sample regions 1002A, 1002B may include one or more reconstructed luma samples 1004A, 1004B that are outside the current block, and/or one or more reconstructed luma samples 1008A, <NUM> that are within the current block.

Cross plane filtering may be applied to the plurality of luma samples in the one or more luma sample regions 1002A, 1002B. The luma samples in the one or more luma sample regions 1002A, 1002B may include some luma samples within a current block (e.g., as shown in <FIG>). The luma samples in the current block may include one or more reconstructed luma samples, e.g., before or after loop filtering.

One or more enhanced reference chroma samples 1012A, 1012B may be generated, for example, in accordance with Eq. <NUM>: <MAT> where RC[x][y] may be reconstructed reference chroma samples before enhancement, RC_enh [x][y] may be enhanced reference chroma samples, SL(xL, yL) may be an array of reconstructed luma samples centering at position (xL, yL). The one or more enhance reference chroma samples 1012A, <NUM> may correspond to a center of the one or more luma sample regions 1002A, 1002B.

A luma sample position (xL, yL) of a corresponding chroma sample position (x, y) may be calculated based on chroma format, for example, in accordance with Eq. <NUM>: <MAT> where scaleX and scaleY may be, for example, (<NUM>,<NUM>), (<NUM>,<NUM>) (<NUM>,<NUM>), respectively, for chroma format <NUM>:<NUM>:<NUM>, <NUM>:<NUM>:<NUM>, and <NUM>:<NUM>:<NUM>. Enhanced chroma reference samples RC_enh [x][y] may be used in the intra prediction process, for example, using a directional/DC/planar intra prediction mode, e.g., as shown in <FIG>.

A two-dimensional (<NUM>-D) cross plane filter may be applied to a luma sample region (e.g., an NxN region of luma samples as shown in <FIG> and <FIG>). A one-dimensional (<NUM>-D) cross plane filter may be applied to one or more luma samples in a <NUM>×N or N×<NUM> luma sample region. As an example, N horizontal luma reference samples above a current block may be filtered, for example, when an N×<NUM> luma sample region is used in cross plane filtering. As an example, N vertical luma reference samples to the left of a current block may be filtered, for example, when a <NUM>×N luma sample region is used in cross plane filtering.

In an example, a luma sample region may be adaptively selected, for example, based on an intra prediction mode. The intra prediction mode may be a directional (e.g., vertical, horizontal, etc.) intra prediction mode, a DC intra prediction mode, a planar intra prediction mode, or any other intra prediction mode. The cross plane filter to apply may be determined based on which intra prediction mode is selected. A different set of cross-plane filters may be selected, for example, to match the edge and/or boundary characteristics of various different modes. As an example, a vertical prediction mode may use the top row of reference samples. When the vertical prediction mode is selected, one or more chroma reference samples above the current video block may be enhanced. For example, a luma sample region may be selected such that it includes one or more luma samples that neighbor a corresponding chroma reference sample above the current video block. The luma sample region may include a luma sample that is collocated with the corresponding chroma reference sample above the current video block. As another example, when horizontal prediction mode is selected, one or more chroma reference samples to the left or right of the current video block may be enhanced. A luma sample region may be selected such that it includes one or more luma samples that neighbor a corresponding chroma reference samples to the left or right of the current video block. The luma sample region may include a luma sample that is collocated with the corresponding chroma reference sample to the left or right of the current video block. An edge may occur vertically, for example, when a vertical prediction mode is selected.

A luma sample region may be selected with a horizontal rectangular shape, for example, in comparison to a square <NUM>-D luma sample region depicted in examples shown in <FIG> and <FIG>. For example, a <NUM>-D horizontal luma sample region may be selected and/or applied for a vertical selection mode. <NUM>-D horizontal filtering (e.g., using the top neighboring luma samples) may be used, for example, to retrieve the vertical high pass edge information more effectively and/or reduce filtering complexity. In an example of adaptive filtering for a horizontal prediction mode, a left column of reference samples may be used. An edge may occur horizontally, for example, when horizontal prediction mode is selected. A luma sample region with a vertical rectangular shape may be used. For example, a <NUM> -D vertical luma sample region may be selected and/or applied. A 1D vertical filtering (e.g. using the left neighboring luma samples) may retrieve the horizontal high pass edge information more effectively and/or reduce filtering complexity.

A luma sample region may be selected with an 'L' shape such that the luma sample region corresponds to the chroma samples in the current video block. For example, when DC intra-prediction mode is selected, the mean of the one or more reference samples to the left and above the current video block may be used to predict the one or more samples in the current video block. As another example, when planar intra-prediction mode is selected, a linear function of the one or more reference samples to the left and above the current video block may be used to predict the one or more samples in the current video block. When DC intra-prediction and/or planar intra-prediction mode is selected, the luma sample region may be selected such that the one or more reference chroma samples to the left and above the current video block are enhanced.

A chroma enhancement indicator may be signaled, for example, to indicate whether to enable or disable chroma enhancement. The chroma enhancement indicator may be signaled, for example, at slice level, picture level, group of picture level, or sequence level.

A chroma enhancement indicator may be signaled at a block level, for example, to indicate whether chroma enhancement processing is applied to a current coding block. The chroma enhancement indicator may be signaled, for example, when a current block is an intra coded block.

One or more cross plane filter coefficients may be derived at an encoder and transmitted to a decoder. A filter coefficient training, e.g., for enhancement filters applied to intra coded blocks to improve reference samples, may use intra coded samples. One or more cross plane filter coefficients may be transmitted, for example, at slice level, picture level, group of picture level, or sequence level. The one or more cross plane filter coefficients may be transmitted at a block level.

<FIG> is an example of a block based hybrid video coding device <NUM> (e.g., an encoder or a decoder) with predicted chroma sample enhancement. A prediction (e.g., intra prediction <NUM> and/or inter prediction <NUM>) may be performed on an input video <NUM>. A prediction residual <NUM> may be determined, for example by subtracting a prediction block <NUM> from an original block of the input video <NUM>. The prediction residual <NUM> may be transformed, for example, using DCT and/or DST block transforms, and/or quantized <NUM>. A prediction residual <NUM> may be transformed for luma and/or chroma components of the input video <NUM>. Improving the accuracy of predicted chroma samples may improve coding efficiency. Chroma enhancement <NUM> may be implemented, for example, by using a cross plane (e.g., high pass) filter to enhance one or more predicted chroma samples. Chroma samples may be enhanced, for example, for object contours and/or edge areas.

A cross plane filter may be used on one or more (e.g. all) predicted chroma samples of a current block, for example, using reconstructed luma samples of the same and neighboring blocks. For example, the one or more predicted chroma samples of a current block may be enhanced by applying a cross plane filter to a plurality of luma samples that correspond to the one or more predicted chroma samples. The current block may be an intra-coded video block. An enhanced predicted chroma sample may be generated, for example, as indicated in Eq. <NUM>: <MAT> where PC[x][y] may be a predicted chroma sample generated at chroma position (x,y). A predicted chroma sample PC[x][y] may be predicted using intra prediction or inter prediction. Although not shown in equation (<NUM>), a cross plane filter used in chroma enhancement may vary, for example, depending on the coding mode (e.g. intra or inter) for a current block. In an example, a plurality of (e.g. two) sets of cross plane filters may be trained separately. A first set of cross plane filters may be applied to one or more intra predicted chroma samples. A second set of cross plane filters may be applied to one or more inter predicted chroma samples.

SL(xL, yL) may be reconstructed luma samples at position (xL, yL), where (xL, yL) may be calculated based on chroma format, for example, as indicated in Eq. <NUM>: <MAT> where (scaleX, scaleY) may be (<NUM>,<NUM>), (<NUM>,<NUM>) and (<NUM>,<NUM>), respectively, for chroma format <NUM>:<NUM>:<NUM>, <NUM>:<NUM>:<NUM> and <NUM>:<NUM>:<NUM>.

A cross plane filter may be applied to one or more neighboring corresponding reference luma samples, for example, to improve the accuracy of one or more predicted chroma samples.

The cross plane filter may enhance the quality of the one or more predicted chroma samples. Prediction residual information may be smaller, leading to improved coding performance, for example, when enhanced predicted chroma samples are used to generate a prediction residual.

Cross plane filtering may be used to derive one or more enhanced chroma samples. For example, a cross plane filter may be applied to a neighborhood of luma samples (e.g., a luma sample region) that correspond to a current chroma sample location.

<FIG> is an example of enhancing predicted chroma samples with cross plane filtering. A plurality of luma sample regions 1202A, 1202B, 1202C may be determined. Each of the plurality of luma sample regions 1202A, 1202B, 1202C may correspond to a predicted chroma sample. A cross plane filter may be applied to a plurality of luma samples within a luma sample region 1202A, 1202B, 1202C (e.g. a 3x3 window of luma samples in the dashed box shown in <FIG>). The cross plane filter may be applied to reconstructed luma samples within a current block (e.g. as indicated by vertical striped circles). The current block may be an intra-coded video block. An output may be determined. The output of applying the cross plane filter may be an offset, for example given by Eq. <NUM>. An enhanced predicted chroma sample 1206A, 1206B, 1206C may be determined, for example, by applying (e.g., adding) the offset 1204A, 1204B, 1204C to a corresponding chroma sample to determine an enhanced chroma sample 1206A, 1206B, 1206C. Availability of corresponding reconstructed luma samples of the current block to be filtered for use in enhancing predicted chroma samples may introduce a coding latency between luma and chroma components on a particular block.

A cross plane filter may be applied on luma samples that are not yet reconstructed <NUM>. Luma samples that are not yet reconstructed <NUM> may be padded, for example, using reconstructed luma samples that neighbor (e.g., to the left or top of) the not yet reconstructed luma samples <NUM>. For example, an unavailable luma sample may be replaced by a neighboring reconstructed luma sample.

Separate (e.g., different) filters may be applied, for example, depending on a prediction mode (e.g. intra or inter prediction). Other techniques (e.g. procedures) may be used to classify and apply different cross plane filters. As an example, a cross plane filter may be further classified or subclassified for applicability, for example, depending on whether integer or fractional (e.g. half or quarter) pixel motion vectors are used. A cross plane filter may be classified and/or subclassified for applicability for example, depending on which reference picture is used in inter prediction. The cross plane filter may be adaptively selected and/or applied these and other coding parameters.

<FIG> is an example of a block based hybrid video coding device <NUM> (e.g., an encoder or a decoder) with chroma enhancement on reconstructed chroma samples. For example, chroma enhancement may be applied after prediction and reconstruction. A reconstructed block <NUM> (e.g. before in-loop filtering) of an input video <NUM> may be generated, for example, by adding a reconstructed residual block from the inverse quantization/inverse transform <NUM> to a prediction block <NUM>. Enhancement of reconstructed chroma samples may improve the overall picture quality and/or may improve coding efficiency of the following blocks or pictures. Chroma enhancement <NUM> may be implemented, for example, by applying a cross plane filter. Applying the cross plane filter may enhance one or more reconstructed chroma samples, e.g., at object contours and/or edge areas. The one or more reconstructed chroma samples may be enhanced before or after in-loop filtering.

In an example, a cross plane filter may be applied to reconstructed luma samples, St_[x][y], for example, to enhance the reconstructed chroma samples of a current block. Enhanced reconstructed chroma samples may be calculated, for example, in accordance with Eq. <NUM>: <MAT> where SC[x][y] may be reconstructed chroma samples and SC_enh[x][y] may be enhanced reconstructed chroma samples.

<FIG> is an example of enhancement of one or more chroma reconstructed samples with cross plane filtering.

A plurality of luma sample regions <NUM>, <NUM> may be determined. A first luma sample region <NUM> may include a plurality of reconstructed luma samples from a current block <NUM>. A second luma sample region <NUM> may include a plurality of reconstructed luma samples from the current block <NUM> and a plurality of reconstructed luma samples from one or more previous blocks.

A cross plane filter may be applied to one or more reconstructed luma samples SL (xL, yL). One or more reconstructed chroma samples SC[x][y] of a current block <NUM> may be enhanced, for example, by applying <NUM>, <NUM> (e.g., adding) the output of the selected and applied cross plane filter to a corresponding reconstructed chroma sample to generate one or more enhanced reconstructed chroma samples <NUM>, <NUM>, SC_enh[x][y]. For example, a luma sample region such as luma sample region <NUM> may include one or more reconstructed luma samples from a current block <NUM> and one or more reconstructed luma samples from one or more previous blocks.

Cross plane filter classification, adaptive selection, and application, e.g., as described herein, may be applicable to enhancement of reconstructed chroma samples. A cross plane filter classification may depend, for example, on a block prediction mode (e.g. intra or inter), a motion vector precision, and/or a reference picture, etc..

Signaling of filter coefficients may be performed, for example, as described herein. One or more sets of cross plane filters may be signaled in the bitstream. The one or more sets of cross plane filters may be signaled based on the filter classification methods utilized. A number of filter sets to be signaled may be denoted as N. The filter coefficients of N sets of cross plane filters may be transmitted over slice level, picture level, group of picture level, or sequence level. A decoder may select one or more appropriate cross plane filters based on the coding mode, motion vector precision, and/or a reference picture of the current block. The decoder may apply one or more appropriate cross plane filters, for example, based on the coding mode, motion vector precision, and/or reference picture of the current block.

As shown in <FIG>, the communications system <NUM> may include wireless transmit/receive units (WTRUs) 102a, 102b, 102c, and/or 102d (which generally or collectively may be referred to as WTRU <NUM>), a radio access network (RAN) <NUM>/<NUM>/<NUM>, a core network <NUM>/<NUM>/<NUM>, a public switched telephone network (PSTN) <NUM>, the Internet <NUM>, and other networks <NUM>, though it will be appreciated that the disclosed embodiments contemplate any number of WTRUs, base stations, networks, and/or network elements. By way of example, the WTRUs 102a, 102b, 102c, 102d may be configured to transmit and/or receive wireless signals and may include user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a smartphone, a laptop, a netbook, a personal computer, a wireless sensor, consumer electronics, and the like.

The communications systems <NUM> may also include abase station 114a and a base station 114b. Each of the base stations 114a, 114b may be any type of device configured to wirelessly interface with at least one of the WTRUs 102a, 102b, 102c, 102d to facilitate access to one or more communication networks, such as the core network <NUM>/<NUM>/<NUM>, the Internet <NUM>, and/or the networks <NUM>. By way of example, the base stations 114a, 114b may be a base transceiver station (BTS), a Node-B, an eNode B, a Home Node B, a Home eNode B, a site controller, an access point (AP), a wireless router, and the like.

The base station 114a may be part of the RAN <NUM>/<NUM>/<NUM>, which may also include other base stations and/or network elements (not shown), such as a base station controller (BSC), a radio network controller (RNC), relay nodes, etc. The base station 114a and/or the base station 114b may be configured to transmit and/or receive wireless signals within a particular geographic region, which may be referred to as a cell (not shown). Thus, in one embodiment, the base station 114a may include three transceivers, e.g., one for each sector of the cell. In another embodiment, the base station 114a may employ multiple-input multiple output (MIMO) technology and, therefore, may utilize multiple transceivers for each sector of the cell.

The base stations 114a, 114b may communicate with one or more of the WTRUs 102a, 102b, 102c, 102d over an air interface <NUM>/<NUM>/<NUM>, which may be any suitable wireless communication link (e.g., radio frequency (RF), microwave, infrared (IR), ultraviolet (UV), visible light, etc.). The air interface <NUM>/<NUM>/<NUM> may be established using any suitable radio access technology (RAT).

For example, the base station 114a in the RAN <NUM>/<NUM>/<NUM> and the WTRUs 102a, 102b, 102c may implement a radio technology such as Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (UTRA), which may establish the air interface <NUM>/<NUM>/<NUM> using wideband CDMA (WCDMA).

In another embodiment, the base station 114a and the WTRUs 102a, 102b, 102c may implement a radio technology such as Evolved UMTS Terrestrial Radio Access (E-UTRA), which may establish the air interface <NUM>/<NUM>/<NUM> using Long Term Evolution (LTE) and/or LTE-Advanced (LTE-A).

In other embodiments, the base station 114a and the WTRUs 102a, 102b, 102c may implement radio technologies such as IEEE <NUM> (e.g., Worldwide Interoperability for Microwave Access (WiMAX)), CDMA2000, CDMA2000 1X, CDMA2000 EV-DO, Interim Standard <NUM> (IS-<NUM>), Interim Standard <NUM> (IS-<NUM>), Interim Standard <NUM> (IS-<NUM>), Global System for Mobile communications (GSM), Enhanced Data rates for GSM Evolution (EDGE), GSM EDGE (GERAN), and the like.

The base station 114b in <FIG> may be a wireless router, Home Node <NUM>, Home eNode B, or access point, for example, and may utilize any suitable RAT for facilitating wireless connectivity in a localized area, such as a place of business, a home, a vehicle, a campus, and the like. Thus, the base station 114b may not be used to access the Internet <NUM> via the core network <NUM>/<NUM>/<NUM>.

The RAN <NUM>/<NUM>/<NUM> may be in communication with the core network <NUM>/<NUM>/<NUM>, which may be any type of network configured to provide voice, data, applications, and/or voice over internet protocol (VoIP) services to one or more of the WTRUs 102a, 102b, 102c, 102d. For example, the core network <NUM>/<NUM>/<NUM> may provide call control, billing services, mobile location-based services, pre-paid calling, Internet connectivity, video distribution, etc., and/or perform high-level security functions, such as user authentication. Although not shown in <FIG>, it will be appreciated that the RAN <NUM>/<NUM>/<NUM> and/or the core network <NUM>/<NUM>/<NUM> may be in direct or indirect communication with other RANs that employ the same RAT as the RAN <NUM>/<NUM>/<NUM> or a different RAT. For example, in addition to being connected to the RAN <NUM>/<NUM>/<NUM>, which may be utilizing an E-UTRA radio technology, the core network <NUM>/<NUM>/<NUM> may also be in communication with another RAN (not shown) employing a GSM radio technology.

The core network <NUM>/<NUM>/<NUM> may also serve as a gateway for the WTRUs 102a, 102b, 102c, 102d to access the PSTN <NUM>, the Internet <NUM>, and/or other networks <NUM>. For example, the networks <NUM> may include another core network connected to one or more RANs, which may employ the same RAT as the RAN <NUM>/<NUM>/<NUM> or a different RAT.

One or more of the WTRUs 102a, 102b, 102c, 102d in the communications system <NUM> may include multi-mode capabilities, e.g., the WTRUs 102a, 102b, 102c, 102d may include multiple transceivers for communicating with different wireless networks over different wireless links.

<FIG> is a system diagram of an example WTRU <NUM>. As shown in <FIG>, the WTRU <NUM> may include a processor <NUM>, a transceiver <NUM>, a transmit/receive element <NUM>, a speaker/microphone <NUM>, a keypad <NUM>, a display/touchpad <NUM>, non-removable memory <NUM>, removable memory <NUM>, a power source <NUM>, a global positioning system (GPS) chipset <NUM>, and other peripherals <NUM>. Also, embodiments contemplate that the base stations <NUM>14a and <NUM>14b, and/or the nodes that base stations 114a and 114b may represent, such as but not limited to transceiver station (BTS), a Node-B, a site controller, an access point (AP), a home node-B, an evolved home node-B (eNodeB), a home evolved node-B (HeNB), a home evolved node-B gateway, and proxy nodes, among others, may include one or more of the elements depicted in <FIG> and described herein.

The transmit/receive element <NUM> may be configured to transmit signals to, or receive signals from, a base station (e.g., the base station 114a) over the air interface <NUM>/<NUM>/<NUM>.

Thus, in one embodiment, the WTRU <NUM> may include two or more transmit/receive elements <NUM> (e.g., multiple antennas) for transmitting and receiving wireless signals over the air interface <NUM>/<NUM>/<NUM>.

In other embodiments, the processor <NUM> may access information from, and store data in, memory that is not physically-located on the WTRU <NUM>, such as on a server or a home computer (not shown).

As noted above, the RAN <NUM> may employ a UTRA radio technology to communicate with the WTRUs 102a, 102b, 102c over the air interface <NUM>. As shown in <FIG>, the RAN <NUM> may include Node-Bs 140a, 140b, 140c, which may each include one or more transceivers for communicating with the WTRUs 102a, 102b, 102c over the air interface <NUM>. The Node-Bs 140a, 140b, 140c may each be associated with a particular cell (not shown) within the RAN <NUM>. The RAN <NUM> may also include RNCs 142a, 142b. It will be appreciated that the RAN <NUM> may include any number of Node-Bs and RNCs while remaining consistent with an embodiment.

As shown in <FIG>, the Node-Bs 140a, 140b may be in communication with the RNC 142a. Additionally, the Node-B 140c may be in communication with the RNC142b. The Node-Bs 140a, 140b, 140c may communicate with the respective RNCs 142a, 142b via an lub interface. The RNCs 142a, 142b may be in communication with one another via an Iur interface. Each of the RNCs 142a, 142b may be configured to control the respective Node-Bs 140a, 140b, 140c to which it is connected. In addition, each of the RNCs 142a, 142b may be configured to carry out or support other functionality, such as outer loop power control, load control, admission control, packet scheduling, handover control, macrodiversity, security functions, data encryption, and the like.

The MSC <NUM> and the MGW <NUM> may provide the WTRUs 102a, 102b, 102c with access to circuit-switched networks, such as the PSTN <NUM>, to facilitate communications between the WTRUs 102a, 102b, 102c and land-line communications devices.

Thus, the eNode-B 160a, for example, may use multiple antennas to transmit wireless signals to, and receive wireless signals from, the WTRU 102a.

Each of the eNode-Bs 160a, 160b, 160c may be associated with a particular cell (not shown) and may be configured to handle radio resource management decisions, handover decisions, scheduling of users in the uplink and/or downlink, and the like.

The MME <NUM> may be connected to each of the eNode-Bs 160a, 160b, 160c in the RAN <NUM> via an S1 interface and may serve as a control node. The MME <NUM> may also provide a control plane function for switching between the RAN <NUM> and other RANs (not shown) that employ other radio technologies, such as GSM or WCDMA.

The serving gateway <NUM> may be connected to each of the eNode-Bs 160a, 160b, 160c in the RAN <NUM> via the S1 interface. The serving gateway <NUM> may generally route and forward user data packets to/from the WTRUs 102a, 102b, 102c. The serving gateway <NUM> may also perform other functions, such as anchoring user planes during inter-eNode B handovers, triggering paging when downlink data is available for the WTRUs 102a, 102b, 102c, managing and storing contexts of the WTRUs 102a, 102b, 102c, and the like.

For example, the core network <NUM> may provide the WTRUs 102a, 102b, 102c with access to circuit-switched networks, such as the PSTN <NUM>, to facilitate communications between the WTRUs 102a, 102b, 102c and land-line communications devices.

The RAN <NUM> may be an access service network (ASN) that employs IEEE <NUM> radio technology to communicate with the WTRUs 102a, 102b, 102c over the air interface <NUM>. As will be further discussed below, the communication links between the different functional entities of the WTRUs 102a, 102b, 102c, the RAN <NUM>, and the core network <NUM> may be defined as reference points.

As shown in <FIG>, the RAN <NUM> may include base stations 180a, 180b, 180c, and an ASN gateway <NUM>, though it will be appreciated that the RAN <NUM> may include any number of base stations and ASN gateways while remaining consistent with an embodiment. The base stations 180a, 180b, 180c may each be associated with a particular cell (not shown) in the RAN <NUM> and may each include one or more transceivers for communicating with the WTRUs 102a, 102b, 102c over the air interface <NUM>. In one embodiment, the base stations 180a, 180b, 180c may implement MIMO technology. Thus, the base station 180a, for example, may use multiple antennas to transmit wireless signals to, and receive wireless signals from, the WTRU 102a. The base stations 180a, 180b, 180c may also provide mobility management functions, such as handoff triggering, tunnel establishment, radio resource management, traffic classification, quality of service (QoS) policy enforcement, and the like. The ASN gateway <NUM> may serve as a traffic aggregation point and may be responsible for paging, caching of subscriber profiles, routing to the core network <NUM>, and the like.

The air interface <NUM> between the WTRUs 102a, 102b, 102c and the RAN <NUM> may be defined as an R1 reference point that implements the IEEE <NUM> specification. In addition, each of the WTRUs 102a, 102b, 102c may establish a logical interface (not shown) with the core network <NUM>. The logical interface between the WTRUs 102a, 102b, 102c and the core network <NUM> may be defined as an R2 reference point, which may be used for authentication, authorization, IP host configuration management, and/or mobility management.

The communication link between each of the base stations 180a, 180b, 180c may be defined as an R8 reference point that includes protocols for facilitating WTRU handovers and the transfer of data between base stations. The communication link between the base stations 180a, 180b, 180c and the ASN gateway <NUM> may be defined as an R6 reference point. The R6 reference point may include protocols for facilitating mobility management based on mobility events associated with each of the WTRUs 102a, 102b, 102c.

The MIP-HA may be responsible for IP address management, and may enable the WTRUs 102a, 102b, 102c to roam between different ASNs and/or different core networks. The MIP-HA <NUM> may provide the WTRUs 102a, 102b, 102c with access to packet-switched networks, such as the Internet <NUM>, to facilitate communications between the WTRUs 102a, 102b, 102c and IP-enabled devices. The AAA server <NUM> may be responsible for user authentication and for supporting user services. The gateway <NUM> may facilitate interworking with other networks. For example, the gateway <NUM> may provide the WTRUs 102a, 102b, 102c with access to circuit-switched networks, such as the PSTN <NUM>, to facilitate communications between the WTRUs 102a, 102b, 102c and land-line communications devices. In addition, the gateway <NUM> may provide the WTRUs 102a, 102b, 102c with access to the networks <NUM>, which may include other wired or wireless networks that are owned and/or operated by other service providers.

Systems, methods, and instrumentalities have been disclosed for enhanced chroma coding using cross plane filtering. A reference, predicted and/or reconstructed chroma sample may be enhanced, for example, using information derived from cross plane filtering of a <NUM>-D or <NUM>-D MxN window of luma samples (a filter support region) corresponding to the reference, predicted or reconstructed chroma sample, respectively. Luma samples may be reconstructed or padded. A filter support region may be adaptively selected, for example, based on a directional intra prediction mode. Cross plane filters may be classified, e.g., for applicability, and may be adaptively selected, for example, based on a filter support region, whether intra or inter prediction mode is used for a current block, whether integer or fractional pixel motion vectors are used and/or whether a reference picture is used in inter prediction. Signaling may be provided to a decoder, for example, to indicate at least one of: whether chroma enhancement is enabled, whether chroma enhancement is applied to a current block, a cross plane filter type, a cross plane filter (e.g. set of filters) and corresponding cross plane filter coefficients. A decoder may select a cross plane filter to apply to a filter support region based on received signaling.

Claim 1:
A method of video processing, the method comprising:
obtaining an indication of a cross-plane filter associated with a current picture, wherein the current picture comprises a video block;
obtaining a plurality of reference samples used to predict the video block, wherein the plurality of reference samples comprises a reference chroma sample;
determining a luma sample region in the current picture for enhancing the reference chroma sample in the plurality of reference samples, wherein the luma sample region comprises a plurality of luma samples that neighbors the reference chroma sample, wherein the luma sample region encompasses at least one of an available luma sample and an unavailable luma sample, wherein the available luma sample is a reconstructed luma sample, and wherein the unavailable luma sample is replaced with padding the available luma sample prior to reconstruction of the unavailable luma sample;
applying the cross-plane filter to the plurality of luma samples in the luma sample region to determine an offset, wherein the cross-plane filter is applied to the luma sample region encompassing the unavailable luma sample that is replaced with padding the available luma sample prior to the reconstruction of the unavailable luma sample, and wherein the cross-plane filter comprises a high pass filter;
applying the offset to the reference chroma sample to determine an enhanced reference chroma sample; and
predicting a chroma sample in the video block using the enhanced reference chroma sample.