Binarizing secondary transform index

A device for decoding video data, the device comprising:

• a memory configured to store video data;
• and
• one or more processors implemented in circuitry and configured to:
• determine a maximum possible value for a secondary transform syntax element for a block of video data;
• determine a first context model for an ordinal first bit of the secondary transform syntax element regardless of whether the block is a luminance block or a chrominance block;
• entropy decode a value for the secondary transform syntax element of the block to form a binarized value representative of the secondary transform for the block, wherein to entropy decode the value for the secondary transform syntax element, the one or more processors are configured to:
• entropy decode the ordinal first bit using the first context model;
• when the block is the luminance block, entropy decode a second number of bits of the secondary transform syntax element other than the ordinal first bit using a first set of contexts;
• and
• when the block is the chrominance block, entropy decode the second number of bits of the secondary transform syntax element other than the ordinal first bit using a second set of contexts that is different than the first set of contexts;
• reverse binarize the value for the secondary transform syntax element using a common binarization scheme regardless of the maximum possible value to determine the secondary transform for the block;
• and
• inverse-transform transform coefficients of the block using a primary transform and the determined secondary transform.