LUMABASED CHROMA INTRAPREDICTION METHOD THAT UTILIZES DOWNSAMPLED LUMA SAMPLES DERIVED FROM WEIGHTING AND ASSOCIATED LUMABASED CHROMA INTRAPREDICTION APPARATUS

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First Claim
1. A lumabased chroma intraprediction method comprising:
 applying, by a filter circuit with a first weighting table, weighting to reconstructed luma samples to generate a first downsampled luma sample, wherein the reconstructed luma samples are external to a luma block;
computing parameters of a linear model, wherein a pair of the first downsampled luma sample and a reconstructed chroma sample that is external to a chroma block is involved in computing the parameters of the linear model; and
determining a predicted value of a chroma sample included in the chroma block according to the linear model and a second downsampled luma sample that is derived from the luma block.
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Abstract
A lumabased chroma intraprediction method includes: applying, by a filter circuit with a first weighting table, weighting to reconstructed luma samples to generate a first downsampled luma sample, wherein the reconstructed luma samples are external to a luma block; computing parameters of a linear model, wherein a pair of the first downsampled luma sample and a reconstructed chroma sample that is external to a chroma block is involved in computing the parameters of the linear model; and determining a predicted value of a chroma sample included in the chroma block according to the linear model and a second downsampled luma sample that is derived from the luma block.
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18 Claims
 1. A lumabased chroma intraprediction method comprising:
applying, by a filter circuit with a first weighting table, weighting to reconstructed luma samples to generate a first downsampled luma sample, wherein the reconstructed luma samples are external to a luma block; computing parameters of a linear model, wherein a pair of the first downsampled luma sample and a reconstructed chroma sample that is external to a chroma block is involved in computing the parameters of the linear model; and determining a predicted value of a chroma sample included in the chroma block according to the linear model and a second downsampled luma sample that is derived from the luma block.  View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
 12. A lumabased chroma intraprediction method comprising:
applying, by a filter circuit with a weighting table, weighting to reconstructed luma samples included in a luma block to generate a downsampled luma sample; and determining a predicted value of a chroma sample included in a chroma block according to a linear model and the downsampled luma sample.  View Dependent Claims (13)
 14. A lumabased chroma intraprediction method comprising:
computing parameters of a linear model by utilizing only N downsampled luma samples that are derived from reconstructed luma samples external to a 2N×
2N luma block and only N reconstructed chroma samples that are external to an N×
N chroma block, where N is a positive integer;determining predicted values of chroma samples included in the N×
N chroma block according to the linear model and downsampled luma samples that are derived from the 2N×
2N luma block. View Dependent Claims (15, 16)
 17. A lumabased chroma intraprediction apparatus comprising:
a filter circuit, arranged to employ a first weighting table for applying weighting to reconstructed luma samples to generate a first downsampled luma sample, wherein the reconstructed luma samples are external to a luma block; and a parameter derivation circuit, arranged to compute parameters of a linear model, wherein a pair of the first downsampled luma sample and a reconstructed chroma sample that is external to a chroma block is involved in computing the parameters of the linear model; and a chroma intraprediction circuit, arranged to determine a predicted value of a chroma sample included in the chroma block according to the linear model and a second downsampled luma sample that is derived from the luma block.
 18. A lumabased chroma intraprediction apparatus comprising:
a filter circuit, arranged to employ a weighting table for applying weighting to reconstructed luma samples included in a luma block to generate a downsampled luma sample; and a chroma intraprediction circuit, arranged to determine a predicted value of a chroma sample included in a chroma block according to a linear model and the downsampled luma sample.
1 Specification
This application claims the benefit of U.S. provisional application No. 62/750,846, filed on Oct. 26, 2018 and incorporated herein by reference.
The present invention relates to intra prediction of video encoding/decoding, and more particularly, to a lumabased chroma intraprediction method that utilizes downsampled luma samples derived from weighting and an associated lumabased chroma intraprediction apparatus.
The conventional video coding standards generally adopt a block based coding technique to exploit spatial and temporal redundancy. For example, the basic approach is to divide the whole source frame into a plurality of blocks, perform prediction on each block, transform residuals of each block, and perform quantization, scan and entropy encoding. Besides, a reconstructed frame is generated in an internal decoding loop of the video encoder to provide reference pixel data used for coding following blocks. For example, inverse scan, inverse quantization, and inverse transform may be included in the internal decoding loop of the video encoder to recover residuals of each block that will be added to predicted samples of each block for generating a reconstructed frame. The functions of the internal decoding loop of the video encoder are also implemented in a video decoder for recovering residuals of each block and generating a reconstructed frame.
When an intraprediction mode is selected for encoding a block, an intraprediction function of a video encoder is enabled for determining a predicted block. Similarly, when an intraprediction mode is selected for decoding a block, an intraprediction function of a video decoder is enabled for determining a predicted block. The encoding may be performed for each color component forming the pixels of the video data. Although RGB (RedGreenBlue) representation is wellknown, the YUV representation is preferably used for the encoding to reduce the interchannel redundancy. However, interchannel correlation is still observed locally. Thus, there is a need for an innovative intraprediction design which is capable of taking advantage of the interchannel correlation for coding efficiency enhancement.
One of the objectives of the claimed invention is to provide a lumabased chroma intraprediction method that utilizes downsampled luma samples derived from weighting and an associated lumabased chroma intraprediction apparatus.
According to a first aspect of the present invention, an exemplary lumabased chroma intraprediction method is disclosed. The exemplary lumabased chroma intraprediction method includes: applying, by a filter circuit with a first weighting table, weighting to reconstructed luma samples to generate a first downsampled luma sample, wherein the reconstructed luma samples are external to a luma block; computing parameters of a linear model, wherein a pair of the first downsampled luma sample and a reconstructed chroma sample that is external to a chroma block is involved in computing the parameters of the linear model; and determining a predicted value of a chroma sample included in the chroma block according to the linear model and a second downsampled luma sample that is derived from the luma block.
According to a second aspect of the present invention, an exemplary lumabased chroma intraprediction method is disclosed. The exemplary lumabased chroma intraprediction method includes: applying, by a filter circuit with a weighting table, weighting to reconstructed luma samples included in a luma block to generate a downsampled luma sample; and determining a predicted value of a chroma sample included in a chroma block according to a linear model and the downsampled luma sample.
According to a third aspect of the present invention, an exemplary lumabased chroma intraprediction method is disclosed. The exemplary lumabased chroma intraprediction method includes: computing parameters of a linear model by utilizing only N downsampled luma samples that are derived from reconstructed luma samples external to a 2N×2N luma block and only N reconstructed chroma samples that are external to an N×N chroma block, where N is a positive integer; and determining predicted values of chroma samples included in the N×N chroma block according to the linear model and downsampled luma samples that are derived from the 2N×2N luma block.
According to a fourth aspect of the present invention, an exemplary lumabased chroma intraprediction apparatus is disclosed. The exemplary lumabased chroma intraprediction apparatus includes a filter circuit and a parameter derivation circuit. The filter circuit is arranged to employ a first weighting table for applying weighting to reconstructed luma samples to generate a first downsampled luma sample, wherein the reconstructed luma samples are external to a luma block. The parameter derivation circuit is arranged to compute parameters of a linear model, wherein a pair of the first downsampled luma sample and a reconstructed chroma sample that is external to a chroma block is involved in computing the parameters of the linear model. The chroma intraprediction circuit is arranged to determine a predicted value of a chroma sample included in the chroma block according to the linear model and a second downsampled luma sample that is derived from the luma block.
According to a fifth aspect of the present invention, an exemplary lumabased chroma intraprediction apparatus is disclosed. The exemplary lumabased chroma intraprediction apparatus includes a filter circuit and a chroma intraprediction circuit. The filter circuit is arranged to employ a weighting table for applying weighting to reconstructed luma samples included in a luma block to generate a downsampled luma sample. The chroma intraprediction circuit is arranged to determine a predicted value of a chroma sample included in a chroma block according to a linear model and the downsampled luma sample.
According to a sixth aspect of the present invention, an exemplary lumabased chroma intraprediction method is disclosed. The exemplary lumabased chroma intraprediction method includes a parameter derivation circuit and a chroma intraprediction circuit. The parameter derivation circuit is arranged to compute parameters of a linear model by utilizing only N downsampled luma samples that are derived from reconstructed luma samples external to a 2N×2N luma block and only N reconstructed chroma samples that are external to an N×N chroma block, where N is a positive integer. The chroma intraprediction circuit is arranged to determine predicted values of chroma samples included in the N×N chroma block according to the linear model and downsampled luma samples that are derived from the 2N×2N luma block.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
Certain terms are used throughout the following description and claims, which refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not in function. In the following description and in the claims, the terms “include” and “comprise” are used in an openended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.
C(x,y)=α×L(x,y)+β
The parameters (α, β) of the linear model are determined on the basis of sample pairs, each including one reconstructed chroma sample and one downsampled reconstructed luma sample.
After the slope parameter (α) and the offset parameter (β) of the linear model are determined, the chroma intraprediction circuit 106 is arranged to determine a predicted value of a chroma sample included in a chroma block (e.g., N×N chroma block BK_C) according to the linear model and a downsampled luma sample that is derived from a luma block (e.g., 2N×2N luma block BK_L). The same weighting concept may be employed for generating a downsampled luma sample that is used to determine a predicted value of a chroma sample in a chroma block. For example, the filter circuit 102 is arranged to employ a weighting table for applying weighting to reconstructed luma samples included in a luma block (e.g., 2N×2N luma block BK_L) to generate a downsampled luma sample, and the chroma intraprediction circuit 106 is arranged to determine a predicted value of a chroma sample included in a chroma block (e.g., N×N chroma block BK_C) according to a linear model and the downsampled luma sample.
As mentioned above, a weighting function is employed for generating a downsampled luma sample that is involved in computing parameters (α, β) of a linear model and/or a predicted value of a chroma sample included in a chroma block.
The filter circuit 102 may be implemented using a 4tap interpolator, where tap coefficients are set on the basis of the weighting table {w_{1}, w_{2}, w_{3}, w_{4}}. A downsampled luma sample for a 2×2 block is generated by applying a first weighting value w_{1 }to a top left reconstructed luma sample, applying a second weighting value w_{2 }to a top right reconstructed luma sample, applying a third weighting value w_{3 }to a bottom left reconstructed luma sample, and applying a fourth weighting value w_{4 }to a bottom right reconstructed luma sample. Hence, the 4×4 luma block 704 can be converted into a downsampled luma block 706 that matches the chroma resolution. The downsampled luma block 706 includes downsampled luma samples L′ (0, 0), L′ (1, 0), L′ (0, 1), L′ (1, 1). In addition, downsampled luma samples L′ (−1, −1), L′ (−1, 0), L′ (−1, 1), L′ (0, −1), L′ (1, −1) around the downsampled luma block 706 are generated.
In a case where an Lshaped sample set is used for deriving parameters (α, β) of a linear model needed by lumabased chroma intraprediction of the 2×2 chroma block 702, the sample pairs {L′ (−1, 1), C(−1, 1)}, {L′ (−1, 0), C(−1, 0)}, {L′ (−1, −1), C(−1, −1)}, {L′ (0, −1), C(0, −1)}, {L′ (1, −1), C(1, −1)} are used by the parameter derivation circuit 104 to compute the parameters (α, β). After parameters (α, β) of the linear model are determined, the chroma intraprediction circuit 106 determines a predicted value of the chroma sample C(0, 1) according to the downsampled luma sample L′(0, 1) and the linear model with parameters (α, β), determines a predicted value of the chroma sample C(0, 0) according to the downsampled luma sample L′ (0, 0) and the linear model with parameters (α, β), determines a predicted value of the chroma sample C(1, 1) according to the downsampled luma sample L′ (1, 1) and the linear model with parameters (α, β), and determines a predicted value of the chroma sample C(1, 0) according to the downsampled luma sample L′ (1, 0) and the linear model with parameters (α, β). The chroma intraprediction can be expressed using the formula C(x, y)=α×L′(x, y)+β, (x, y≥0).
In another case where a different sample set is used for deriving parameters (α, β) of a linear model needed by lumabased chroma intraprediction of the 2×2 chroma block 702, proper downsampled luma samples are generated and selected. For example, only the downsampled luma samples L′ (0, −1) and L′ (1, −1) located directly above the top boundary of the 4×4 luma block 704 (particularly, the top boundary of the downsampled luma block 706) and only the reconstructed chroma samples C(0, −1) and C(1, −1) located directly above the top boundary of the 2×2 chroma block 702 are involved in deriving parameters (α, β) of the linear model. In other words, only two sample pairs {L′ (0, −1), C(0, −1)} and {L′ (1, −1), C(1, −1)} are used by the parameter derivation circuit 104 to compute the parameters (α, β). For another example, only the downsampled luma samples L′ (−1, 0) and L′ (−1, 1) located directly left to the left boundary of the 4×4 luma block 704 (particularly, the left boundary of the downsampled luma block 706) and only the reconstructed chroma samples C(−1, 0) and C(−1, 1) located directly left to the left boundary of the 2×2 chroma block 702 are involved in deriving parameters (α, β) of the linear model. In other words, only two sample pairs {L′ (−1, 0), C(−1, 0)} and {L′ (−1, 1), C(−1, 1)} are used by the parameter derivation circuit 104 to compute the parameters (α, β).
In some embodiments of the present invention, a weighting table {w_{1}, w_{2}, w_{3}, w_{4}} used by the filter circuit 102 to generate downsampled luma samples (which are derived from reconstructed luma samples external to a luma block and are used for linear model parameter derivation) may be the same as a weighting table {w_{1}, w_{2}, w_{3}, w_{4}} used by the filter circuit 102 to generate other downsampled luma samples (which are derived from reconstructed luma samples included in the luma block and are used for chroma intraprediction).
In some embodiments of the present invention, the boundaries of coding tree units (CTUs) may be further referenced for controlling settings of different weighting tables {w_{1}, w_{2}, w_{3}, w_{4}} that are used in different cases. For example, the setting of the weighting table {w_{1}, w_{2}, w_{3}, w_{4}} may depend on a location of a 2N×2N luma block in a frame and a location of a CTU defined in the frame.
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Alternatively, a weighting table {w_{1}, w_{2}, w_{3}, w_{4}} used by the filter circuit 102 to generate downsampled luma samples (which are derived from reconstructed luma samples external to a luma block and are used for linear model parameter derivation) may be different from a weighting table {w_{1}, w_{2}, w_{3}, w_{4}} used by the filter circuit 102 to generate other downsampled luma samples (which are derived from reconstructed luma samples included in the luma block and are used for chroma intraprediction). In some embodiments of the present invention, the boundaries of coding tree units (CTUs) may be referenced for controlling settings of different weighting tables {w_{1}, w_{2}, w_{3}, w_{4}} used in different cases.
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Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.