Coding method, decoding method, encoder, and decoder

A decoding method comprising:

• performing inverse quantization of data corresponding to a current block of an image;
• performing an inverse secondary transform, using one candidate of the inverse secondary transform selected from candidates of the inverse secondary transform, on a result of the inverse quantization to generate second transform coefficients, the candidates of the inverse secondary transform including transform bases for the inverse secondary transform and a skip of the inverse secondary transform;
• and
• performing an inverse primary transform, using one candidate of the inverse primary transform selected from candidates of the inverse primary transform, on the second transform coefficients or the result of the inverse quantization to generate first transform coefficients, the candidates of the inverse primary transform including transform bases for the inverse primary transform,
• wherein a fixed first candidate is always used for the inverse primary transform in response to a case of a selected candidate of the inverse secondary transform being other than a fixed second candidate, and
• wherein the inverse primary transform is a separable transform.

An encoder comprising:

• a processor;
• and
• a memory,
• wherein the processor, using the memory:
• performs a primary transform, using one candidate of the primary transform selected from candidates of the primary transform, on residual signals of a current block to generate first transform coefficients, the candidates of the primary transform including transform bases for the primary transform;
• performs a secondary transform, using one candidate of the secondary transform selected from candidates of the secondary transform, on the first transform coefficients to generate second transform coefficients, the candidates of the secondary transform including transform bases for the secondary transform and a skip of the secondary transform;
• and
• quantizes the first transform coefficients or the second transform coefficients to generate quantized coefficients,
• wherein a fixed second candidate is always used for the secondary transform in response to a case of a selected candidate of the primary transform being other than a fixed first candidate, and
• wherein the primary transform is a separable transform.

An encoding method comprising:

• performing a primary transform, using one candidate of the primary transform selected from candidates of the primary transform, on residual signals of a current block to generate first transform coefficients, the candidates of the primary transform including transform bases for the primary transform;
• performing a secondary transform, using one candidate of the secondary transform selected from candidates of the secondary transform, on the first transform coefficients to generate second transform coefficients, the candidates of the secondary transform including transform bases for the secondary transform and a skip of the secondary transform;
• and
• quantizing the first transform coefficients or the second transform coefficients to generate quantized coefficients,
• wherein a fixed second candidate is always used for the secondary transform in response to a case of a selected candidate of the primary transform being other than a fixed first candidate, and
• wherein the primary transform is a separable transform.

A non-transitory computer readable medium storing a bitstream, the bitstream comprising:

• information according to which a decoder selects a skip of an inverse secondary transform from candidates of inverse secondary transform;
• and
• a picture including a current block on which a decoding process is performed,
• wherein in the decoding process:
• inverse quantization is performed on data corresponding to the current block of an image;
• an inverse secondary transform is performed, using one candidate of the inverse secondary transform selected from the candidates of the inverse secondary transform, on a result of the inverse quantization to generate second transform coefficients, the candidates of the inverse secondary transform including transform bases for the inverse secondary transform and the skip of the inverse secondary transform;
• and
• an inverse primary transform is performed, using one candidate of the inverse primary transform selected from candidates of the inverse primary transform, on the second transform coefficients or the result of the inverse quantization to generate first transform coefficients, the candidates of the inverse primary transform including transform bases for the inverse primary transform,
• wherein a fixed first candidate is always used for the inverse primary transform in response to a case of a selected candidate of the inverse secondary transform being other than a fixed second candidate, and
• wherein the inverse primary transform is a separable transform.