Image coding method on basis of secondary transform and device therefor
An image information decoding method performed by means of a decoding device according to the present invention comprises the steps of: decoding a non-separable secondary transform (NSST) index from a bitstream if NSST is applied to a target block; decoding information relating to transform coefficients with respect to the target block from the bitstream on the basis of the decoded NSST index; and deriving the transform coefficients with respect to the target block on the basis of the decoded information relating to the transform coefficients, wherein the NSST index is decoded prior to the information relating to the transform coefficients with respect to the target block.
1. A decoding apparatus for image decoding, the decoding apparatus comprising:
a memory; and
at least one processor connected to the memory, the at least one processor configured to:
decode a secondary transform index from a bitstream;
decode information about transform coefficients for a target block from the bitstream;
generate the transform coefficients for the target block based on the decoded information about the transform coefficients;
derive modified transform coefficients for the target block based on a secondary inverse transform using the transform coefficients and the secondary transform index;
derive residual samples for the target block based on an inverse transform using the modified transform coefficients; and
generate reconstructed samples based on the residual samples,
wherein for the target block of which the secondary transform index is greater than 0, the secondary inverse transform is applied to the transform coefficients in an input target region included in the target block,
wherein for the target block to which the secondary inverse transform is applied, the transform coefficients for the target block in a range of an R+1th to Nth transform coefficients are equal to 0,
wherein the secondary inverse transform is applied based on a reduced inverse transform,
wherein for the reduced inverse transform, a number of transform coefficients in the input target region to which the reduced inverse transform is applied is R, a number of the modified transform coefficients in an output target region is N, and the R is smaller than the N,
wherein the information about the transform coefficients for the target block includes information about a location of a last non-zero transform coefficient, and
wherein for the target block to which the secondary inverse transform is applied, the last non-zero transform coefficient is one of transform coefficients for the target block in the range of the 1th to Rth transform coefficients.
2. The decoding apparatus of claim 1 , wherein the information about the transform coefficients for the target block includes: at least one of group flag information indicating whether a non-zero transform coefficient is included in a subgroup of the target block, and information about the R.
3. The decoding apparatus of claim 2 , wherein the group flag information corresponding to the subgroup of the target block comprising only the transform coefficients for the target block in the range of the R+1 th to N th transform coefficients is not decoded from the bitstream.
4. The decoding apparatus of claim 2 , wherein the group flag information corresponding to a subgroup other than a first subgroup, a last subgroup, and the subgroup comprising only the transform coefficients for the target block in the range of the R+1 th to N th transform coefficients among at least one subgroup of the target block is decoded from the bitstream.
5. The decoding apparatus of claim 4 , wherein when the group flag information decoded from the bitstream indicates 0, all transform coefficients included in the subblock corresponding to the group flag information indicating 0 are each determined as 0.
6. The decoding apparatus of claim 4 , wherein when the group flag information decoded from the bitstream indicates 1, a value of each of the transform coefficients included in the subblock corresponding to the group flag information indicating 1 is decoded.
7. The decoding apparatus of claim 1 , wherein the secondary transform index is decoded prior to the information about the transform coefficients for the target block, and
wherein the location of the last non-zero transform coefficient is determined based on the secondary transform index.
8. The decoding apparatus of claim 7 , wherein based on the secondary transform index being greater than 0 and the last non-zero transform coefficient is a subsequent transform coefficient to the Nth transform coefficient, location information indicating one of locations of the R+1th to Nth transform coefficients are used as the information about the location of the last non-zero transform coefficient which is the subsequent transform coefficient to the Nth transform coefficient.
9. The decoding apparatus of claim 8 , wherein the location information includes x coordinate information and y coordinate information,
wherein based on the location information indicating one of locations of the R+1th to Nth transform coefficients, an x coordinate of the last non-zero transform coefficient which is the subsequent transform coefficient to the Nth transform coefficient is modified as a value of the x coordinate information plus 4 or a y coordinate of the last non-zero transform coefficient which is the subsequent transform coefficient to the Nth transform coefficient is modified as a value of the y coordinate information plus 4.
10. A encoding apparatus for image encoding, the encoding apparatus comprising:
a memory; and
at least one processor connected to the memory, the at least one processor configured to:
derive residual samples for a target block;
generate a secondary transform index;
generate modified transform coefficients for the target block based on a transform using the residual samples;
generate transform coefficients for the target block based on a secondary transform using the secondary transform index and the modified transform coefficients for the target block;
generate information about the transform coefficients for the target block; and
encode the information about the transform coefficients and the secondary transform index,
wherein the secondary transform index represents whether the secondary transform is applied,
wherein for a case that the secondary transform is applied to the modified transform coefficients in an input target region included in the target block, the secondary transform index is greater than 0,
wherein for the target block to which the secondary transform is applied, the transform coefficients for the target block in a range of an R+1th to Nth transform coefficients are equal to 0,
wherein the secondary transform is applied based on a reduced transform,
wherein for the reduced transform, a number of modified transform coefficients in the input target region to which the reduced transform is applied is N, a number of the transform coefficients in an output target region is R, and the R is smaller than the N,
wherein the information about the transform coefficients for the target block includes information about a location of a last non-zero transform coefficient, and
wherein for the target block to which the secondary transform is applied, the last non-zero transform coefficient is one of transform coefficients for the target block in the range of the 1 th to Rth transform coefficients.
11. The encoding apparatus of claim 10 , wherein the information about the transform coefficients for the target block includes: at least one of group flag information indicating whether a non-zero transform coefficient is included in a subgroup of the target block, and information about the R.
12. The encoding apparatus of claim 11 , wherein the group flag information corresponding to the subgroup of the target block comprising only the transform coefficients for the target block in the range of the R+1th to Nth transform coefficients is not encoded.
13. The encoding apparatus of claim 10 , wherein the secondary transform index is encoded prior to the information about the transform coefficients for the target block, and
wherein the location of the last non-zero transform coefficient is determined based on the secondary transform index.
14. The encoding apparatus of claim 13 , wherein based on the secondary transform index being greater than 0 and the last non-zero transform coefficient is a subsequent transform coefficient to the Nth transform coefficient, location information indicating one of locations of the R+1th to Nth transform coefficients are used as the information about the location of the last non-zero transform coefficient which is the subsequent transform coefficient to the Nth transform coefficient.
15. The encoding apparatus of claim 14 , wherein the location information includes x coordinate information and y coordinate information,
wherein based on the location information indicating one of locations of the R+1th to Nth transform coefficients, an x coordinate of the last non-zero transform coefficient which is the subsequent transform coefficient to the Nth transform coefficient is modified as a value of the x coordinate information plus 4 or a y coordinate of the last non-zero transform coefficient which is the subsequent transform coefficient to the Nth transform coefficient is modified as a value of the y coordinate information plus 4.
16. A non-transitory computer-readable storage medium storing a bitstream generated by the apparatus of claim 10 .
17. A transmission apparatus of data for image, the transmission apparatus comprising:
at least one processor configured to obtain bitstream generated based on deriving residual samples for a target block, generating a secondary transform index, generating modified transform coefficients for the target block based on a transform using the residual samples, generating transform coefficients for the target block based on a secondary transform using the secondary transform index and the modified transform coefficients for the target block, generating information about the transform coefficients for the target block, and encoding the information about the transform coefficients and the secondary transform index; and
a transmitter configured to transmit the data comprising the bitstream,
wherein the secondary transform index represents whether the secondary transform is applied,
wherein for a case that the secondary transform is applied to the modified transform coefficients in an input target region included in the target block, the secondary transform index is greater than 0,
wherein for the target block to which the secondary transform is applied, the transform coefficients for the target block in a range of an R+1th to Nth transform coefficients are equal to 0,
wherein the secondary transform is applied based on a reduced transform,
wherein for the reduced transform, a number of modified transform coefficients in the input target region to which the reduced transform is applied is N, a number of the transform coefficients in an output target region is R, and the R is smaller than the N,
wherein the information about the transform coefficients for the target block includes information about a location of a last non-zero transform coefficient, and
wherein for the target block to which the secondary transform is applied, the last non-zero transform coefficient is one of transform coefficients for the target block in the range of the 1 th to Rth transform coefficients.