Image encoding/decoding method and device, and recording medium in which bitstream is stored
An image decoding method and device according to the present disclosure may determine a transform type for inverse transform of a current block from one or more MTS candidates available to the current block, and perform inverse transform on transform coefficients of the current block on the basis of the determined transform type, so as to acquire residual samples of the current block. Here, the number of the one or more MTS candidates available to the current block may be determined on the basis of at least one among a sum of the transform coefficients in the current block, the number of the transform coefficients in the current block, sizes of the one or more transform coefficients in the current block, a location of a last significant coefficient in the current block, and the number of one or more non-zero coefficients in the current block.
1 . An image decoding method, comprising:
determining a transform type for an inverse transform of a current block from one or more multi transform selection (MTS) candidates available to the current block; and
performing an inverse transform on transform coefficients of the current block based on the determined transform type to acquire residual samples of the current block,
wherein a number of the one or more MTS candidates available to the current block is determined based on at least one of a sum of the transform coefficients in the current block, a number of the transform coefficients in the current block, a size of one or more transform coefficients in the current block, a position of a last significant coefficient in the current block, or a number of one or more non-zero coefficients in the current block.
2 . The method of claim 1 , wherein the number of the one or more MTS candidates available to the current block is determined based on a comparison between the sum of the transform coefficients belonging to the current block and a predetermined threshold value.
3 . The method of claim 2 , wherein the predetermined threshold value includes a first threshold value and a second threshold value,
wherein when the sum of the transform coefficients belonging to the current block is less than or equal to the first threshold value, the number of the one or more MTS candidates is determined as N 1 ,
wherein when the sum of the transform coefficients belonging to the current block is greater than the first threshold value and is less than or equal to the second threshold value, the number of the one or more MTS candidates is determined as N 2 , and
wherein when the sum of the transform coefficients belonging to the current block is greater than the second threshold value, the number of the one or more MTS candidates is determined as N 3 .
4 . The method of claim 2 , wherein the predetermined threshold value is determined based on a predetermined constant factor, and
wherein the predetermined constant factor is determined based on at least one of a slice type, a quantization parameter of the current block, a size of the current block, a shape of the current block, a ratio of a width and a height of the current block, or information signaled through a bitstream.
5 . The method of claim 1 , wherein the number of the one or more MTS candidates available to the current block is determined based on at least one of the position of the last significant coefficient in the current block, a scaling factor, a size of the current block, or a shape of the current block.
6 . The method of claim 1 , wherein the number of the one or more MTS candidates available to the current block is determined based on a comparison between a number of non-zero coefficients among the transform coefficients of the current block and a third threshold value.
7 . The method of claim 1 , wherein when the number of the one or more MTS candidates available to the current block is two or more, the transform type of the current block is determined based on an MTS index signaled through a bitstream.
8 . The method of claim 7 , wherein a maximum value (cMax) for a binarization of the MTS index is determined based on at least one of the sum of the transform coefficients belonging to the current block, the number of the transform coefficients in the current block, the size of the one or more transform coefficients in the current block, the position of the last significant coefficient in the current block, or the number of the non-zero coefficients among the transform coefficients of the current block.
9 . An image encoding method, comprising:
determining a transform type of a current block; and
performing a transform on residual samples of the current block based on the transform type of the current block to derive transform coefficients of the current block,
wherein the transform type of the current block is selected from one or more multi transform selection (MTS) candidates belonging to any one of a plurality of pre-defined candidate groups, and
wherein based on a candidate group where the transform type of the current block is selected, at least one of a sum of the transform coefficients of the current block, a number of the transform coefficients in the current block, a size of one or more transform coefficients in the current block, a position of a last significant coefficient in the current block, or a number of non-zero coefficients in the current block is required to belong to a predetermined range.
10 . A method for transmitting data for an image, comprising:
acquiring a bitstream for the image, wherein the bitstream is generated by determining a transform type of a current block, performing a transform on residual samples of the current block based on the transform type of the current block to derive transform coefficients of the current block, and encoding the derived transform coefficients; and
transmitting the data including the bitstream,
wherein the transform type of the current block is selected from one or more multi transform selection (MTS) candidates belonging to any one of a plurality of pre-defined candidate groups, and
wherein based on a candidate group where the transform type of the current block is selected, at least one of a size of one or more transform coefficients of the current block, a sum of transform coefficients of the current block, a number of the transform coefficients in the current block, the size of the one or more transform coefficients in the current block, a position of a last significant coefficient in the current block, or a number of non-zero coefficients in the current block is required to belong to a predetermined range.