EP-4742675-A2 - METHOD AND APPARATUS FOR RESTRICTED LINEAR MODEL PARAMETER DERIVATION IN VIDEO CODING

EP4742675A2EP 4742675 A2EP4742675 A2EP 4742675A2EP-4742675-A2

Abstract

Methods and apparatus for cross-color LM (Linear Mode) Intra prediction are disclosed. According to one method, when a current top area or a current left area of the current block is across a restricted boundary, a current LM parameter set is derived based on reduced neighboring reconstructed areas. The reduced neighboring reconstructed areas comprise a reduced top area having less than N top lines or less than N left lines if the current top area or the current left area is across the restricted boundary. According to another method, if the current top area is across the restricted boundary, expanded left area is used for deriving LM parameter set and if the current left area is across the restricted boundary, expanded top area is used for deriving LM parameter set. A method for multi-LM mode is also disclosed for restricted boundaries.

Inventors

TSAI, CHIA-MING
HSU, CHIH-WEI
CHEN, CHING-YEH
CHUANG, TZU-DER
HUANG, YU-WEN

Assignees

HFI Innovation Inc.

Dates

Publication Date: 20260513
Application Date: 20190423

Claims (6)

A method of cross-color Intra prediction, wherein an LM coded block uses neighboring reconstructed areas comprising a top area and a left area of the LM coded block to derive an LM parameter set of the LM coded block associated with a linear model relating luma pixels and chroma pixels of the LM coded block, the method comprising: receiving (910) input data at an encoder side or a decoder side, wherein the input data comprise pixel data associated with a current block to be encoded at the encoder side or compressed data associated with the current block to be decoded at the decoder side; and when a current top area or a current left area of the current block is across a restricted boundary (920): deriving (930) a current LM parameter set for the current block based on the luma pixels and the chroma pixels in a modified neighboring reconstructed region, wherein the modified neighboring reconstructed region comprises only left neighboring reconstructed pixels of the current block if the current top area is across the restricted boundary or only top neighboring reconstructed pixels of the current block if the current left area is across the restricted boundary; generating (940) a cross-color Intra predictor for the chroma pixels of the current block according to the luma pixels of the current block and the current LM parameters; and encoding (950) or decoding the chroma pixels of the current block using the cross-color Intra predictor, wherein said only left neighboring reconstructed pixels of the current block corresponds to the left area of the current block; or said only top neighboring reconstructed pixels of the current block corresponds to the top area of the current block, wherein additional top neighboring reconstruction samples from the only top area of the current block are used for the LM parameter derivation of access restricted LM model, wherein additional left neighboring reconstruction samples from the only left area of the current block are used for the LM parameter derivation of access restricted LM model.
The method of Claim 1, wherein if said only left neighboring reconstructed pixels of the current block or said only top neighboring reconstructed pixels of the current block comprise multiple lines, a syntax is signaled to indicate which line or which lines of said only left neighboring reconstructed pixels of the current block or said only top neighboring reconstructed pixels of the current block are used for said deriving the current LM parameter set for the current block.
The method of Claim 1, wherein the restricted boundary corresponds to a coding tree unit, CTU, boundary, slice boundary, tile boundary, tile group boundary, or picture boundary.
An apparatus for cross-color Intra prediction based on reconstructed pixels of another color component in a video encoder or decoder, wherein an LM coded block uses neighboring reconstructed areas comprising a top area and a left area of the LM coded block to derive an LM parameter set of the LM coded block associated with a linear model relating luma pixels and chroma pixels of the LM coded block, the apparatus comprising one or more electronics or processors arranged to: receive (910) input data at an encoder side or a decoder side, wherein the input data comprise pixel data associated with a current block to be encoded at the encoder side or compressed data associated with the current block to be decoded at the decoder side; and when a current top area or a current left area of the current block is across a restricted boundary (920): derive (930) a current LM parameter set for the current block based on the luma pixels and the chroma pixels in a modified neighboring reconstructed region, wherein the modified neighboring reconstructed region comprises only left neighboring reconstructed pixels of the current block if the current top area is across the restricted boundary or only top neighboring reconstructed pixels of the current block if the current left area is across the restricted boundary; generate (940) a cross-color Intra predictor for the chroma pixels of the current block according to the luma pixels of the current block and the current LM parameters; and encode or decode (950) the chroma pixels of the current block using the cross-color Intra predictor, wherein said only left neighboring reconstructed pixels of the current block corresponds to the left area of the current block; or said only top neighboring reconstructed pixels of the current block corresponds to the top area of the current block, wherein additional top neighboring reconstruction samples from the only top area of the current block are used for the LM parameter derivation of access restricted LM model, wherein additional left neighboring reconstruction samples from the only left area of the current block are used for the LM parameter derivation of access restricted LM model.
A method of cross-color Intra prediction based on reconstructed pixels using multi-LM modes, wherein the multi-LM modes comprise a Left-only mode using only left neighboring reconstructed pixels of a multi-LM modes coded block, a Top-only mode using only top neighboring reconstructed pixels of the multi-LM modes coded block, and a Left+Top mode using both the left neighboring reconstructed pixels and the top neighboring reconstructed pixels of the multi-LM modes coded block, the method comprising: receiving (1010) input data at an encoder side or a decoder side, wherein the input data comprise pixel data associated with a current block to be encoded at the encoder side or compressed data associated with the current block to be decoded at the decoder side; when current left neighboring reconstructed pixels or current top neighboring reconstructed pixels of the current block are across a restricted boundary (1020): deriving (1030) a set of reduced multi-LM modes by excluding the Left-Only mode if the current left neighboring reconstructed pixels of the current block are across the restricted boundary or excluding the Top-Only mode if the current top neighboring reconstructed pixels of the current block are across the restricted boundary; wherein said excluding the Left-Only mode or said excluding the Top-Only mode is indicated implicitly without signaling an indicator, when left or top neighboring reconstructed samples are not accessible for the current block, additional neighboring reconstructed samples from the accessible above-side or accessible left-side of the current block are used in LM parameter derivation, wherein the Top-only mode uses additional top neighboring reconstructed samples than the Left+Top mode in LM parameter derivation; and wherein the Left-only mode uses additional left neighboring reconstructed samples than the Left+Top mode in the LM parameter derivation; and encoding or decoding (1040) chroma pixels of the current block using the set of reduced multi-LM modes, wherein the additional top neighboring reconstructed samples using additional neighboring reconstructed samples from only above, both above and above-left, or only above-left area of the current block, wherein the additional left neighboring reconstructed samples using additional neighboring reconstructed samples from only left, both left and left-bottom, or only left-bottom area of the current block.
The method of Claim 5, wherein the set of reduced multi-LM modes further excludes the Top-Only mode if the current left neighboring reconstructed pixels of the current block are across the restricted boundary and excludes the Left-Only mode if the current top neighboring reconstructed pixels of the current block are across the restricted boundary.

Description

FIELD OF THE INVENTION The present invention relates to video coding. In particular, the present invention relates to coding techniques associated with Intra prediction using linear mode with restricted data. BACKGROUND AND RELATED ART Motion compensated inter-frame coding has been widely adopted in various coding standards, such as MPEG-1/2/4 and H.261/H.263/H.264/AVC. While motion-compensated inter-frame coding can effectively reduce bitrate for compressed video, Intra coding is required to compress the regions with high motion or scene changes. Besides, Intra coding is also used to process an initial picture or to periodically insert I-pictures or I-blocks for random access or for alleviation of error propagation. Intra prediction exploits the spatial correlation within a picture or within a picture region. In practice, a picture or a picture region is divided into blocks and the Intra prediction is performed on a block basis. Intra prediction for a current block can rely on pixels in neighboring blocks that have been processed. For example, if blocks in a picture or picture region are processed row by row first from left to right and then from top to bottom, neighboring blocks on the top and neighboring blocks on the left of the current block can be used to form Intra prediction for pixels in the current block. While any pixels in the processed neighboring blocks can be used for Intra predictor of pixels in the current block, very often only pixels of the neighboring blocks that are adjacent to the current block boundaries on the top and on the left are used. The Intra predictor is usually designed to exploit spatial features in the picture such as smooth area (DC mode), vertical line or edge, horizontal line or edge and diagonal line or edge. Furthermore, spatial correlation often exists between the luminance (luma) and chrominance (chroma) components. Therefore, reconstructed luma pixels can be used to derive the Intra chroma prediction. In the emerging High Efficiency Video Coding (HEVC), a chroma Intra prediction mode based on the reconstructed luminance signal has been considered. This type of chroma Intra prediction is termed as Linear Model (LM) prediction. Fig. 1 illustrates the Intra prediction derivation for LM mode. First, the neighboring reconstructed pixels (indicated by circles) of a collocated luma block(i.e., Y block) and the neighboring reconstructed pixels (indicated by circles) of a chroma block (i.e., U or V block)in Fig. 1 are used to derive the linear model parameters between the blocks. The predicted pixels of the chroma block are generated using the parameters and the reconstructed pixels of the luma block. In the parameters derivation, the top reconstructed pixel row adjacent to the top block boundary of the current luma block and the left reconstructed pixel column adjacent to the left block boundary of the current luma block are used. It is noted that the second left reconstructed pixel column from the left boundary is used instead of the left column immediately adjacent to the left boundary in order to match the sampling locations of the chroma pixels. The specific row and column of the luma block are used in order to match the 4:2:0 sampling format of the chroma components. While Fig. 1 illustrates the example of LM chroma mode for the 4:2:0 sampling format, the LM chroma mode for other chroma sampling format may also derived similarly. According to the LM prediction mode, the chroma values are predicted from reconstructed luma values of a collocated block. The chroma components may have lower spatial resolution than the luma component. In order to use the luma signal for chroma Intra prediction, the resolution of the luma signal may have to be reduced to match with that of the chroma components. For example, for the 4:2:0 sampling format, the U and V components only have half of the number of samples in vertical and horizontal directions as the luma component. Therefore, 2:1 resolution reduction in vertical and horizontal directions has to be applied to the reconstructed luma samples. The resolution reduction can be achieved by down-sampling process or sub-sampling process. In LM chroma mode, for a to-be-predicted chroma sample V with its collocated reconstructed luma sample Vcol, the linear model to generate LM predictor P is formulated as follows: P=a⋅Vcol+b In the above equation, a and b are referred as LM parameters. The LM parameters can be derived from the neighboring reconstructed luma and chroma samples around the current block so that the parameters do not need to be coded in the bitstream. After deriving the LM parameters, chroma predictors can be generated from the collocated reconstructed luma samples in the current block according to the linear model. For example, if the video format is YUV420, then there are one 8x8 luma block and two 4x4 chroma blocks for each 8x8 coding unit, as shown in Fig. 1.In Fig. 1, each small square corresponds to one pixel in the cur