Method and apparatus of deriving intra prediction mode using most probable mode group
Provided is a method that constructs an MPM group including three intra prediction modes, determines the intra prediction mode of the MPM group specified by the prediction mode index as the intra prediction mode of the current prediction unit if the mode group indicator indicates the MPM group, and derives the intra prediction mode of the current prediction unit using the prediction mode index and the three prediction modes of the MPM group if the mode group indicator does not indicate the MPM group. Accordingly, additional bits resulted from increase of a number of intra prediction mode are effectively reduced. Also, an image compression ratio can be improved by generating a prediction block similar to an original block.
1. A method of deriving an intra prediction mode of a current prediction unit, comprising:
entropy-decoding a mode group indicator and a prediction mode index;
constructing a Most Probable Mode (MPM) group including three intra prediction modes using a left intra prediction mode and an above intra prediction mode;
determining whether the mode group indicator indicates the MPM group or not;
if the mode group indicator indicates the MPM group, determining an intra prediction mode of the MPM group specified by the prediction mode index as the intra prediction mode of the current prediction unit; and
if the mode group indicator does not indicate the MPM group, deriving the intra prediction mode of the current prediction unit using the prediction mode index and the three intra prediction modes of the MPM group,
wherein deriving the intra prediction mode of the current prediction unit using the prediction mode index and the three intra prediction modes of the MPM group comprises ordered sub-steps of:
increasing a value of the prediction mode index by one if the prediction mode index is equal to or greater than a first intra prediction mode of the MPM group;
increasing the value of the prediction mode index by one if the prediction mode index is equal to or greater than a second intra prediction mode of the MPM group;
increasing the value of the prediction mode index by one if the prediction mode index is equal to or greater than a third intra prediction mode of the MPM group; and
determining the value of the prediction mode index as the mode number of the intra prediction mode of the current prediction unit.
2. The method of claim 1 , wherein the first intra prediction mode is an intra prediction mode having lowest mode number, the second intra prediction mode is an intra prediction mode having middle mode number and the third intra prediction mode is an intra prediction mode having highest mode number.
3. The method of claim 1 , wherein the three intra prediction modes are reordered in the order of mode number to determine the first intra prediction mode, the second intra prediction mode and the third intra prediction mode.
4. The method of claim 1 , wherein when the left intra prediction mode is not equal to the above intra prediction mode and at least one of the left intra prediction mode and the above intra prediction mode is a non-directional intra prediction mode, the MPM group includes both the non-directional intra prediction modes.
5. The method of claim 4 , wherein when the left intra prediction mode and the above intra prediction mode are non-directional intra prediction modes, the MPM group includes both the non-directional intra prediction modes and a vertical mode.
6. The method of claim 1 , wherein when only one of a left intra prediction mode and an above intra prediction mode is available, the MPM group is comprised of the available intra prediction mode and two additional intra prediction modes, and the two additional intra prediction modes are determined according to the available intra prediction mode.
7. The method of claim 6 , wherein, if the available intra prediction mode is one of two non-directional intra prediction modes, the two additional intra prediction modes are the other of the two non-directional intra prediction modes and a vertical mode.
8. The method of claim 7 , wherein, if the available intra prediction mode is a planar mode, the two additional intra prediction modes are a DC mode and the vertical mode.
9. The method of claim 6 , wherein, if the available intra prediction mode is one of directional intra prediction modes, the two additional intra prediction modes are two non-directional intra prediction modes.