Method and apparatus for improved compound orthonormal transform

An apparatus for controlling residual coding for decoding or encoding of a video sequence, the apparatus comprising:

• at least one memory configured to store computer program code;
• and
• at least one processor configured to access the at least one memory and operate according to the computer program code, the computer program code comprising:
• first determining code configured to cause the at least one processor to determine a transform size of a primary transform to be used for the residual coding of a coded block of the video sequence;
• performing code configured to cause the at least one processor to:
• in response to the transform size of the primary transform being determined to be of 8-point or smaller, automatically perform the residual coding of the coded block, using, as the primary transform, a first transform set consisting both discrete sine transform (DST)-4 and discrete cosine transform (DCT)-4;
• and
• in response to a transform size of the primary transform being determined to be of 16-point or larger, automatically perform the residual coding of the coded block, using, as the primary transform, a second transform set consisting both DST-7 and DCT-8;
• and
• second determining code configured to cause the at least one processor to, in response to the residual coding of the coded block being automatically performed, automatically determine whether a secondary transform is to be used for the residual coding of the coded block of which the residual coding is performed using the primary transform,
• wherein the first determining code is further configured to cause the at least one processor to, in response to the secondary transform being determined to be used, automatically determine the transform size of the secondary transform to be used for the residual coding of the coded block, and
• wherein the performing code is further configured to cause the at least one processor to:
• in response to the transform size of the secondary transform being determined to be of 8-point or smaller, automatically perform the residual coding of the coded block of which the residual coding is automatically performed using the primary transform, using, as the secondary transform, the first transform set consisting both the DST-4 and the DCT-4;
• and
• in response to the transform size of the secondary transform being determined to be of 16-point or larger, automatically perform the residual coding of the coded block of which the residual coding is automatically performed using the primary transform, using, as the secondary transform, the second transform set consisting both the DST-7 and the DCT-8.

A non-transitory computer-readable storage medium storing instructions that cause at least one processor to:

• determine a transform size of a primary transform to be used for residual coding of a coded block of a video sequence;
• in response to the transform size of the primary transform being determined to be of 8-point or smaller, automatically perform the residual coding of the coded block, using, as the primary transform, a first transform set consisting both discrete sine transform (DST)-4 and discrete cosine transform (DCT)-4;
• in response to a transform size of the primary transform being determined to be of 16-point or larger, automatically perform the residual coding of the coded block, using, as the primary transform, a second transform set consisting both DST-7 and DCT-8;
• in response to the residual coding of the coded block being automatically performed, automatically determine whether a secondary transform is to be used for the residual coding of the coded block of which the residual coding is automatically performed using the primary transform;
• in response to the secondary transform being determined to be used, automatically determine the transform size of the secondary transform to be used for the residual coding of the coded block;
• in response to the transform size of the secondary transform being determined to be of 8-point or smaller, automatically perform the residual coding of the coded block of which the residual coding is automatically performed using the primary transform, using, as the secondary transform, the first transform set consisting both the DST-4 and the DCT-4;
• and
• in response to the transform size of the secondary transform being determined to be of 16-point or larger, automatically perform the residual coding of the coded block of which the residual coding is automatically performed using the primary transform, using, as the secondary transform, the second transform set consisting both the DST-7 and the DCT-8.