Context modeling for intra-prediction modes

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

determining a left intra-mode of a left neighbor block;

determining an above intra-mode of an above neighbor block;

on condition that the left intra-mode and the above intra-mode are a same mode, using that same mode to determine a probability distribution for coding the intra-prediction mode;

on condition that at least one of the left intra-mode or the above intra-mode is a smooth intra-prediction mode, using the other of the left intra-mode and the above intra-mode to determine the probability distribution for coding the intra-prediction mode; and

coding the intra-prediction mode using the probability distribution.

2. The method of claim 1 , wherein the smooth intra-prediction mode is one of a SMOOTH_PRED intra-prediction mode, a SMOOTH_H_PRED intra-prediction mode, and a SMOOTH_V_PRED intra-prediction mode.

3. The method of claim 1 , wherein the left intra-mode and the above intra-mode are each selected from a set comprising DC_PRED, V_PRED, H_PRED, D 45 _PRED, D 135 _PRED, D 117 _PRED, D 153 _PRED, D 207 _PRED, D 63 _PRED, SMOOTH_PRED, SMOOTH_V_PRED, and SMOOTH_H_PRED, and PAETH_PRED intra-prediction modes.

4. The method of claim 1 , wherein determining the left intra-mode of the left neighbor block, comprises:

in a case where the left neighbor block is not available, selecting a default left intra-prediction mode for the left intra-mode.

5. The method of claim 4 , wherein the default left intra-prediction mode is a DC_PRED intra-prediction mode.

6. The method of claim 1 , wherein determining the above intra-mode of the above neighbor block, comprises:

in a case where the above neighbor block is not available, selecting a default above intra-prediction mode for the above intra-mode.

7. The method of claim 1 , wherein using the other of the left intra-mode and the above intra-mode to determine the probability distribution for coding the intra-prediction mode comprises:

on condition that the left intra-mode and the above intra-mode are the smooth intra-prediction mode, using the left intra-mode to determine the probability distribution for coding the intra-prediction mode.

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

a memory;

a processor, the memory includes instructions executable by the processor to:

determine a first intra-prediction class of a first intra-prediction mode used for decoding a first neighboring block of the current block;

determine a second intra-prediction class of a second intra-prediction mode used for decoding a second neighboring block of the current block,

wherein the first intra-prediction class and the second intra-prediction class are each selected from a set comprising a DC class, a horizontal class, a vertical class, a smooth class, and a directional class, and

wherein the smooth class comprises a SMOOTH_PRED intra-prediction mode, a SMOOTH_V_PRED intra-prediction mode, and a SMOOTH_H_PRED intra-prediction mode; and

code the intra-prediction mode using the first intra-prediction class and the second intra-prediction class, wherein to code the intra-prediction mode comprises to:

code a first symbol indicative of a class of the intra-prediction mode; and

on condition that the intra-prediction mode is one of a third intra-prediction mode or a fourth intra-prediction mode, code a second symbol indicative of the intra-prediction mode, wherein to code the second symbol comprises to:

on condition that the intra-prediction mode is the third intra-prediction mode, select a single available context model for coding the second symbol.

9. The apparatus of claim 8 , wherein the first neighboring block is above the current block and the second neighboring block is left of the current block in a raster scan order.

10. The apparatus of claim 8 , wherein the set further comprises DC_PRED, V_PRED, H_PRED, D 45 _PRED, D 135 _PRED, D 117 _PRED, D 153 _PRED, D 207 _PRED, D 63 _PRED, SMOOTH_PRED, SMOOTH_V_PRED, and SMOOTH_H_PRED, and PAETH_PRED intra-prediction modes.

11. The apparatus of claim 8 , wherein the set further comprising a paeth class.

12. The apparatus of claim 11 , wherein the directional class comprises D 45 _PRED, D 135 _PRED, D 117 _PRED, D 153 _PRED, D 207 _PRED, D 63 _PRED intra-prediction modes.

13. The apparatus of claim 8 , wherein the third intra-prediction mode is one of a smooth intra-prediction mode and the fourth intra-prediction mode is a directional intra-prediction mode.

14. The apparatus of claim 13 , wherein the instructions to code the second symbol indicative of the intra-prediction mode comprise instructions to:

on condition that the intra-prediction mode is the directional intra-prediction mode,

on condition that the first intra-prediction class and the second intra-prediction class are a same class, select a first context model for coding the second symbol, and

on condition that the first intra-prediction class and the second intra-prediction class are different classes, select a second context model for coding the second symbol.

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

determining a first intra-prediction class of a first intra-prediction mode used for decoding a first neighboring block of the current block;

determining a second intra-prediction class of a second intra-prediction mode used for decoding a second neighboring block of the current block,

wherein the first intra-prediction class and the second intra-prediction class are ordinal values, and

wherein the first intra-prediction class is greater than or equal to the second intra-prediction class;

using the first intra-prediction class to select an index into a list of available context models; and

using the second intra-prediction class as an offset from the index into the list of available context models to select a context model for decoding the intra-prediction mode.

16. The method of claim 15 , wherein the first intra-prediction class and the second intra-prediction class are selected from a set of classes comprising a north-east intra-prediction class, a north-west intra-prediction class, and a south-west intra-prediction class.

17. The method of claim 15 , wherein the first intra-prediction class and the second intra-prediction class are selected from a set of classes comprising a smooth intra prediction class, a DC prediction class, a horizontal prediction class, a vertical prediction class, and a paeth prediction class.