Encoder, decoder, encoding method, and decoding method utilizing transform and inverse transform bases selected based on block size

A decoder comprising:

• circuitry;
• and
• memory,
• wherein the circuitry, using the memory:
• determines a first group of candidates and performs an inverse-transform on a current block using an inverse transform basis which is one of the candidates included in the first group of candidates determined, when the current block has a size of 16 or less;
• and
• determines one second candidate outside of the first group of candidates and performs an inverse-transform on the current block using an inverse transform basis which is the second candidate determined, when the current block has a size larger than 16, and
• wherein the inverse-transform is a separable transform.

A method of transmitting a bitstream comprising:

• determining a first group of candidates and performing a transform on a current block using a transform basis which is one of the candidates included in the first group of candidates determined, when the current block has a size of 16 or less;
• determining one second candidate outside of the first group of candidates and performing a transform on the current block using a transform basis which is the second candidate determined, when the current block has a size larger than 16;
• quantizing a result of the transform to generate quantized coefficients;
• encoding the quantized coefficients to generate a bitstream;
• and
• transmitting the bitstream,
• wherein the transform is a separable transform.