Decoder, encoder and methods comprising a coding for intra subpartitions

A method for decoding a picture, the method comprising:

• decoding, from a data stream, a secondary transform index for an intra-predicted block partitioned into partitions, the secondary transform index indicating whether a secondary transform is to be applied to all partitions of the intra-predicted block;
• for each partition of the intra-predicted block, determining a partition-specific transform domain residual;
• determining whether, for all partitions, no non-zero transform coefficient of the partition-specific transform domain residual lies outside a predetermined area;
• if it is determined that, for all partitions, no non-zero transform coefficient lies outside the predetermined area, applying the secondary transform indicated by the secondary transform index to all partitions;
• and
• reconstructing each partition of the intra-predicted block using the partition-specific transform domain residuals, wherein the partition-specific transform domain residuals are either transformed or not transformed according to the secondary transform index.

A video encoder configured to:

• determine, for each partition of an intra-predicted block partitioned into partitions, a partition-specific transform domain residual;
• determine whether, for all partitions, no non-zero transform coefficient of the partition-specific transform domain residual lies outside a predetermined area;
• if it is determined that, for all partitions, no non-zero transform coefficient lies outside the predetermined area, encode, into a data stream, a secondary transform index for the intra-predicted block, the secondary transform index indicating whether a secondary transform is to be applied to all partitions of the intra-predicted block;
• apply the secondary transform indicated by the secondary transform index to all partitions of the intra-predicted block;
• and
• encode, into the data stream, the partition-specific transform domain residuals, wherein the partition-specific transform domain residuals are either transformed or not transformed according to the secondary transform index.

A method for encoding a picture, the method comprising:

• for each partition of an intra-predicted block partitioned into partitions, determining a partition-specific transform domain residual;
• determining whether, for all partitions, no non-zero transform coefficient of the partition-specific transform domain residual lies outside a predetermined area;
• if it is determined that, for all partitions, no non-zero transform coefficient lies outside the predetermined area, encoding, into a data stream, a secondary transform index for the intra-predicted block, the secondary transform index indicating whether a secondary transform is to be applied to all partitions of the intra-predicted block;
• applying the secondary transform indicated by the secondary transform index to all partitions of the intra-predicted block;
• and
• encoding, into the data stream, the partition-specific transform domain residuals, wherein the partition-specific transform domain residuals are either transformed or not transformed according to the secondary transform index.