Method and apparatus for chroma intra prediction in video coding
Devices and methods of directional intra prediction for chroma component of a picture are provided. The method includes obtaining an initial intra prediction mode of the chroma component, and deriving a chroma intra prediction mode (intraPredModeC) from a look up table (LUT) by using the initial intra prediction mode of the chroma component. The chroma component has different subsampling ratios in horizontal and vertical directions. The method further includes performing wide-angle mapping on the chroma intra prediction mode (intraPredModeC) to obtain a modified intraPredModeC; obtaining an angle parameter for the chroma component based on the modified intraPredModeC; and obtaining predicted samples of the chroma component based on the angle parameter. The method provides the minimum number of entries in the LUT that is used to determine chroma intra prediction mode from the initial chroma intra prediction mode.
1. A method of directional intra prediction for chroma component of a picture, the method comprising:
obtaining an initial intra prediction mode of the chroma component of the picture;
deriving a chroma intra prediction mode (intraPredModeC) from a look up table (LUT) by using the initial intra prediction mode of the chroma component, wherein a horizontal direction of the chroma component (SubWidthC) is different from a vertical direction of the chroma component (SubHeightC);
performing wide-angle mapping on the intraPredModeC to obtain a modified intraPredModeC;
obtaining an angle parameter for the chroma component based on the modified intraPredModeC; and
obtaining predicted samples of the chroma component based on the angle parameter.
2. The method of claim 1 , wherein
the method further comprises: obtaining a value of a luma intra prediction mode (IntraPredModeY) from a bitstream;
wherein obtaining the initial intra prediction mode of the chroma component of the picture comprises: obtaining the initial intra prediction mode of the chroma component based on the value of the IntraPredModeY.
3. The method of claim 1 , wherein performing the wide-angle mapping on the intraPredModeC to obtain the modified intraPredModeC comprises:
performing wide-angle mapping on an original intra prediction mode (predModeIntra) to obtain a modified predModeIntra, wherein a value of the original predModeIntra is equal to a value of the intraPredModeC.
4. The method of claim 3 , wherein obtaining the angle parameter for the chroma component comprises:
obtaining the angle parameter for the chroma component from a mapping table by using the modified predModeIntra.
5. The method of claim 2 , wherein wide-angle mapping is performed for the IntraPredModeY and a resulting intra prediction mode is used to get a value of the angle parameter.
6. The method of claim 1 , wherein the chroma intra prediction mode IntraPredModeC[xCb][yCb] is derived by using intra_chromapred_mode[xCb][yCb] and IntraPredModeY[xCb+cbWidth/2][yCb+cbHeight/2] according to Table 8-2 and Table 8-3.
7. The method of claim 1 , wherein the LUT has 67 entries, an index for the LUT is a value of 0 to 66.
8. The method of claim 1 , wherein the LUT includes:
X
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Y
6
8
10
12
13
14
16
18
20
22
23
24
26
28
30
wherein mode X is the initial intra prediction mode of the chroma component, mode Y is the intraPredModeC derived from the LUT.
9. The method of claim 3 , wherein performing the wide-angle mapping on the original predModeIntra to obtain the modified predModeIntra comprises:
for non-square blocks,
setting the modified predModeIntra to (the original predModeIntra+65) when all of the following are true:
a variable nW is greater than a variable nH,
the original predModeIntra is greater than or equal to 2, and
the original predModeIntra is less than (a variable whRatio>1)?(8+2*the variable whRatio):8; or
setting the modified predModeIntra to (the original predModeIntra−67) when all of the following are true:
the variable nH is greater than the variable nW,
the original predModeIntra is less than or equal to 66, and
the original predModeIntra is greater than (the variable whRatio>1)?(60−2*the variable whRatio):60;
wherein the variable whRatio is set equal to Abs(Log 2(the variable nW/the variable nH)).
10. The method of claim 1 , wherein a chroma format of the chroma component is defined as YUV 4:2:2.
11. A device, comprising:
one or more processors; and
a non-transitory computer-readable storage medium coupled to the one or more processors and storing programming instructions, which when executed by the one or more processors, cause the device to:
obtain an initial intra prediction mode of a chroma component of a picture;
derive a chroma intra prediction mode (intraPredModeC) from a look up table (LUT) by using the initial intra prediction mode of the chroma component, wherein a horizontal direction of the chroma component (SubWidthC) is different from a vertical direction of the chroma component (SubHeightC);
perform wide-angle mapping on the intraPredModeC to obtain a modified intraPredModeC;
obtain an angle parameter for the chroma component based on the modified intraPredModeC; and
obtain predicted samples of the chroma component based on the angle parameter.
12. The device of claim 11 , wherein the programming instructions, which when executed by the one or more processors, further cause the device to:
obtain a value of a luma intra prediction mode (IntraPredModeY) from a bitstream;
wherein to obtain the initial intra prediction mode of the chroma component, the programming instructions, which when executed by the one or more processors, cause the device to obtain the initial intra prediction mode of the chroma component based on the value of the IntraPredModeY.
13. The device of claim 11 , wherein to perform the wide-angle mapping on the intraPredModeC to obtain the modified intraPredModeC, the programming instructions, which when executed by the one or more processors, cause the device to:
perform wide-angle mapping on an original intra prediction mode (predModeIntra) to obtain a modified predModeIntra, wherein a value of the original predModeIntra is equal to a value of the chroma intra prediction mode (intraPredModeC).
14. The device of claim 13 , wherein to obtain the angle parameter for the chroma component, the programming instructions, which when executed by the one or more processors, cause the device to:
obtain the angle parameter for the chroma component from a mapping table by using the modified predModeIntra.
15. The device of claim 13 , wherein the angle parameter is left or right shifted to compensate for an effect of chroma subsampling.
16. The device of claim 11 , wherein to derive the intraPredModeC from the LUT, the programming instructions, which when executed by the one or more processors, cause the device to:
derive the chroma intra prediction mode IntraPredModeC[xCb][yCb] by using intra_chromapred_mode[xCb][yCb] and IntraPredModeY[xCb+cbWidth/2][yCb+cbHeight/2] according to Table 8-2 and Table 8-3.
17. The device of claim 11 , wherein the LUT includes:
X
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Y
6
8
10
12
13
14
16
18
20
22
23
24
26
28
30
wherein mode X is the initial intra prediction mode of the chroma component, mode Y is the intraPredModeC derived from the LUT.
18. The device of claim 13 , wherein to perform the wide-angle mapping on the original predModeIntra to obtain the modified predModeIntra, the programming instructions, which when executed by the one or more processors, cause the device to:
set the modified predModeIntra to (the original predModeIntra+65) when all of the following are true:
a variable nW is greater than a variable nH,
the original predModeIntra is greater than or equal to 2, and
the original predModeIntra is less than (a variable whRatio>1)?(8+2*the variable whRatio):8; or
set the modified predModeIntra to (the original predModeIntra−67) when all of the following are true:
the variable nH is greater than the variable nW,
the original predModeIntra is less than or equal to 66, and
the original predModeIntra is greater than (the variable whRatio>1)?(60−2*the variable whRatio):60;
wherein the variable whRatio is set equal to Abs(Log 2(the variable nW/the variable nH)).
19. The device of claim 11 , wherein a chroma format of the chroma component is defined as YUV 4:2:2.
20. The device of claim 11 , wherein the device is an encoder or a decoder.