Complexity reduction for 32-p and 64-p LGT

A method of decoding a coded bitstream composed of compressed video/image data performed by at least one processor, the method comprising:

• receiving the coded bitstream composed of the compressed video/image data;
• generating a dequantized block by a process including entropy decoding the coded bitstream, and dequantizing a block;
• determining whether at least one of a height (N) or a width (M) of the dequantized block is greater than or equal to a pre-defined threshold;
• obtaining a reduced-complexity dequantized block based on the dequantized block;
• and
• based on determining that the at least one of the height or the width of the dequantized block is greater than or equal to the pre-defined threshold:
• performing transform coding of the reduced-complexity dequantized block using a line graph transform (LGT) core to perform direct matrix multiplications for each of the horizontal and vertical dimensions of the reduced-complexity dequantized block,
• wherein the obtaining the reduced-complexity dequantized block includes:
• based on a secondary transform being enabled for the M×N dequantized block with zero-out also enabled and an input to the secondary transform being first K coefficients along a scanning order, zeroing-out all coefficients of the dequantized block outside of an m×n subblock of the dequantized block.

A method of decoding a coded bitstream composed of compressed video/image data performed by at least one processor, the method comprising:

• receiving the coded bitstream composed of the compressed video/image data;
• generating a dequantized block by a process including entropy decoding the coded bitstream and dequantizing a block;
• determining whether at least one of a height or a width of the dequantized block is greater than or equal to a pre-defined threshold;
• obtaining a reduced-complexity dequantized block based on the dequantized block;
• and
• based on determining that the at least one of the height or the width of the dequantized block is greater than or equal to the pre-defined threshold:
• performing transform coding of the reduced-complexity dequantized block using a line graph transform (LGT) core to perform direct matrix multiplications for each of the horizontal and vertical dimensions of the reduced-complexity dequantized block;
• and
• storing, in a memory, in advance, a pre-defined table that associates dequantized blocks of different block width and block height sizes (M×N) with subblocks of different subblock width and subblock height sizes (m×n), where all coefficients outside of the m×n region are zeroed-out.

An 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:
• receiving code configured to cause the at least one processor to receive a coded bitstream composed of compressed video/image data;
• generating code configured to cause the at least one processor to generate a dequantized block by a process including entropy decoding the coded bitstream and dequantizing a block;
• determining code configured to cause the at least one processor to determine whether at least one of a height or a width of the dequantized block is greater than or equal to a pre-defined threshold;
• and
• reduced-complexity code configured to cause the at least one processor to:
• based on determining that the at least one of the height or the width of the dequantized block is greater than or equal to the pre-defined threshold:
• obtain a reduced-complexity dequantized block based on the dequantized block and perform transform coding of the reduced-complexity dequantized block using a line graph transform (LGT) core to perform direct matrix multiplications for each of the horizontal and vertical dimensions of the reduced-complexity dequantized block,
• wherein the at least one memory stores, in advance, a pre-defined table that associates dequantized blocks of different block width and block height (M×N) sizes with subblocks of different width and subblock height (m×n) sizes.

A method of coding image data performed by at least one processor, the method comprising:

• receiving information regarding a data block of an image;
• determining whether at least one of a height or a width of a residual coding block corresponding to the data block of the image is greater than or equal to a pre-defined threshold;
• and
• based on determining that the at least one of the height or the width of the residual coding block is greater than or equal to the pre-defined threshold:
• identifying or generating a reduced-complexity residual coding block by reducing the number of non-zero coefficients in the residual coding block;
• and
• performing transform coding of the reduced-complexity residual coding block using a line graph transform (LGT) core to perform direct matrix multiplications for each of the horizontal and vertical dimensions of the reduced-complexity residual coding block,
• wherein the performing the transform coding includes determining an order of performing horizontal and vertical transforms to the reduced-complexity residual coding block when zero-out is enabled on LGT, depending on a ratio of width to height (M/N) of the residual coding block.