Refined entropy coding for level maps

An apparatus for coding a transform block using level maps, the apparatus comprising:

• a memory;
• and
• a processor, wherein the memory includes instructions executable by the processor to:
• code a scan position, in a first scan direction, corresponding to an end-of-block;
• code, in a second scan direction that is different from the first scan direction, an up to whether a transform coefficient of the transform block is greater than a maximum map level of the level maps, wherein the coding starts at a position that is based on the scan position, wherein to code the up to whether the transform coefficient of the transform block is greater than the maximum map level of the level maps comprises to:
• select, based on a transform class used to generate the transform block, a template for determining a coding context to code the up to whether the transform coefficient is greater than the maximum map level, wherein the transform class is one of a vertical transform class, a horizontal transform class, or a two-dimensional transform class;
• and
• code a coefficient residual map, each residual coefficient of the coefficient residual map corresponding to a respective non-zero transform coefficient of the transform block having an absolute value exceeding the maximum map level.

A method for decoding a transform block using level maps, comprising:

• decoding a scan position, in a first scan direction, corresponding to an end-of-block;
• decoding, in a second scan direction that is different from the first scan direction, an up to whether a transform coefficient of the transform block is greater than a maximum map level of the level maps, wherein the decoding starts at a position that is based on the scan position, the decoding the up to whether the transform coefficient of the transform block is greater than the maximum map level of the level maps comprising:
• selecting a template based on a transform class used for the transform block, wherein the transform class is at least one of a vertical transform class, a horizontal transform class, or a two-dimensional transform class;
• selecting, using the template, a context to code the up to whether the transform coefficient is greater than the maximum map level, wherein selecting the context comprises:
• determining a context index by summing values corresponding to positions of the template, each value corresponding to a position of the template is determined by summing respective values of at least some of the level maps;
• and
• upper-bounding the context index by a predetermined number;
• and
• decoding a coefficient residual map, each residual coefficient of the coefficient residual map corresponding to a respective non-zero transform coefficient of the transform block having an absolute value exceeding the maximum map level.