Coding images using intra prediction modes

1. A method for decoding at least a portion of an image, comprising steps of:

determining a current prediction mode based on a combination of a prediction mode residue and a function of an index of at least one previous prediction mode wherein the prediction mode residue represents a difference between a result of the function and an index of the current prediction mode; and

decoding the portion of the image using the current prediction mode, wherein the steps of the method are performed by a processor.

2. The method of claim 1 , further comprising:

receiving the prediction mode residue as a part of encoded bit-stream; and

decoding and parsing the prediction mode residue from the bit-stream.

3. The method of claim 1 , wherein the combination includes a summation of the prediction mode residue and a result of the function of at least one previous prediction mode, and wherein the prediction mode residue includes a difference between a result of the function and the current prediction mode.

4. The method of claim 1 , wherein the previous prediction mode is angular intra prediction mode, and the result of the function is a prediction mode adjacent to the angular intra prediction mode.

5. The method of claim 1 , wherein the portion of the image includes a current prediction unit (PU), and the function of at least one previous prediction mode determines an average value of a left prediction mode of a left PU and a top prediction mode of a top PU.

6. The method of claim 1 , wherein the portion of the image includes a current prediction unit (PU), and the function of at least one previous prediction mode determines a median value of a left prediction mode of a left PU and a top prediction mode of a top PU.

7. The method of claim 1 , wherein the determining further comprises:

combining the prediction mode residue with a result of the function of at least one previous prediction mode to produce an intermediate prediction mode; and

selecting the current prediction mode from a set of valid prediction modes using the intermediate prediction mode.

8. The method of claim 7 , wherein the selecting includes:

rounding the intermediate prediction mode to a nearest valid prediction mode.

9. The method of claim 8 , wherein the rounding is performed in a direction of most frequent prediction mode.

10. The method of claim 1 , wherein the function of at least one previous prediction mode is a function of at least two previous prediction modes.

11. The method of claim 10 , further comprising:

selecting the two previous prediction modes from a set of most probable modes (MPM); and

storing the current prediction mode in the MPM.

12. The method of claim 10 , further comprising:

selecting the two previous prediction modes as prediction modes of two neighboring portions of the image.

13. The method of claim 12 , wherein the two neighboring portions of the image include a left portion and a top portion of the image with respect to the portion of the image currently decoded.

14. The method of claim 10 , further comprising:

selecting the two previous prediction modes as most frequently used prediction modes.

15. The method of claim 1 , further comprising:

scaling the previous prediction mode by a weighting factor.

16. A decoder for decoding at least a portion of an image based on a current prediction mode, comprising:

a decoding and parsing module for decoding and parsing bit-stream to produce transform coefficients and control data, wherein the control data includes a prediction mode residue representing a difference between a result of the function of a set of previous prediction modes and an index of the current prediction mode; and

an intra prediction mode generator for determining the current prediction mode based on a combination of the prediction mode residue and the result of the function.

17. The decoder of claim 16 , further comprising:

a memory for storing a set of most probable modes (MPM);

a prediction type module for determining a type of ultra prediction; and

an intra prediction mode selector for selecting an intra prediction mode based on the type of intra prediction and for storing the current prediction mode in the set of MPM.

18. The decoder of claim 16 , wherein the intra prediction mode generator comprises:

a mode estimation module for determining the result of the function;

a combining module for combining the prediction mode residue with a result of the function to produce an intermediate prediction mode;

an adjusting module for selecting the current prediction mode from a set of valid prediction modes using the intermediate prediction mode; and

a mapping module for mapping the current prediction mode to an original mode index.