Coding of intra-prediction modes

1. A method for coding a current block using an intra-prediction mode, comprising:

receiving a block of quantized transform coefficients;

determining a context for coding the intra-prediction mode using the quantized transform coefficients, wherein the context is used to select a probability context model for entropy coding the intra-prediction mode, and wherein determining the context comprising:

determining the context for coding the intra-prediction mode based on a result of a comparison of a first number of non-zero coefficients in a first portion of a row of the block of quantized transform coefficients to a second number of non-zero coefficients in a second portion of a column of the block of quantized transform coefficients; and

coding the intra-prediction mode using the probability context model.

2. The method of claim 1 , wherein coding the intra-prediction mode using the context comprises:

decoding, from an encoded bitstream, the intra-prediction mode using the context.

3. The method of claim 1 , wherein coding the intra-prediction mode using the context comprises:

encoding, in an encoded bitstream, the intra-prediction mode using the context.

4. The method of claim 3 , further comprising:

determining an intra-prediction mode predictor using the quantized transform coefficients; and

encoding, in the encoded bitstream, whether the intra-prediction mode predictor and the intra-prediction mode are the same.

5. The method of claim 1 , wherein the row is a top-most row of the block of quantized transform coefficients, and the column is a left-most column of the block of quantized transform coefficients.

6. The method of claim 1 , wherein determining the context comprises:

determining a category for the intra-prediction mode using the quantized transform coefficients, the category having a value selected from a set comprising a first category value and a second category value.

7. The method of claim 6 , wherein determining the context further comprises:

determining a first context for coding the intra-prediction mode; and

determining the context using the first context and the category.

8. The method of claim 6 , wherein determining the category for the intra-prediction mode comprises:

determining a difference between the first number of non-zero coefficients in the row of the block of quantized transform coefficients and the second number of non-zero coefficients in the column of the block of quantized transform coefficients;

on condition that the difference is greater than a first threshold, setting the category to the first category value; and

on condition that the difference is less than a second threshold, setting the category to the second category value.

9. The method of claim 8 , wherein determining the category for the intra-prediction mode further comprises:

on condition that the difference is not greater than the first threshold and is not less than the second threshold, determining the category based on a comparison of a third number of non-zero coefficients in another row of the block of quantized transform coefficients to a fourth number of non-zero coefficient in another column of the block of quantized transform coefficients.

10. An apparatus for decoding a current block using an intra-prediction mode, comprising:

a memory; and

a processor configured to execute instructions stored in the memory to:

receive, in an encoded bitstream, a block of quantized transform coefficients for a residual block of the current block;

determine a context for decoding the intra-prediction mode using the quantized transform coefficients, wherein the context is used to select a probability context model for entropy coding the intra-prediction mode, and wherein to determine the context comprises to:

determine the context for decoding the intra-prediction mode based on a comparison of a first number of non-zero coefficients in at least some rows of the block of quantized transform coefficients to a second number of non-zero coefficient in at least some columns of the block of quantized transform coefficients; and

decode, from the encoded bitstream, the intra-prediction mode using the probability context model.

11. The apparatus of claim 10 , wherein the at least some rows include a row that is a top-most row of the block of quantized transform coefficients, and the at least some columns include a column that is a left-most column of the block of quantized transform coefficients.

12. The apparatus of claim 10 , wherein to determine the context comprises to:

determine a category for the intra-prediction mode using the quantized transform coefficients.

13. The apparatus of claim 12 , wherein to determine the category for the intra-prediction mode comprises to:

determine a difference between the first number of non-zero coefficients in the at least some rows of the block of quantized transform coefficients and the second number of non-zero coefficient in the at least some columns of the block of quantized transform coefficients; and

set the category to a category value based on the difference.

14. The apparatus of claim 13 , wherein to determine the context for the intra-prediction mode comprises to:

determine a first context for coding the intra-prediction mode; and

determine the context using the first context and the category.

15. An apparatus for decoding a current block using an intra-prediction mode, comprising:

a memory; and

a processor configured to execute instructions stored in the memory to:

decode, from an encoded bitstream, a block of quantized transform coefficients;

identify a direction of the intra-prediction mode based on a distribution of non-zero quantized transform coefficients in at least a part of the block of quantized transform coefficients, the direction being one of horizontal, vertical, or diagonal;

determine a first context for decoding the intra-prediction mode;

determine a second context using the direction and the first context; and

decode the intra-prediction mode using the second context.

16. The apparatus of claim 15 , wherein the instructions further comprise instructions to:

decode, from the encoded bitstream, whether an intra-prediction mode predictor is the same as the intra-prediction mode, wherein the intra-prediction mode predictor is a prediction of the intra-prediction mode and is determined by an encoder using the quantized transform coefficients.