Image decoding method using intra prediction mode

1. A method of image decoding, the method comprising:

generating a quantization block by inversely scanning quantization coefficient information;

generating a transform block by inversely quantizing the quantization block using a quantization step size;

generating a residual block by inversely transforming the transform block;

reconstructing an intra prediction mode group indicator and a prediction mode index of a current block;

constructing a first group including three intra prediction modes using valid intra prediction modes of left and top blocks of the current block;

determining an intra prediction mode corresponding to the prediction mode index in the first group as an intra prediction mode of the current block when the intra prediction mode group indicator indicates the first group;

generating a prediction block on the basis of the determined intra prediction mode of the current block; and

generating a reconstructed block using the residual block and the prediction block,

when only one of the intra prediction modes of the left and top blocks of the current block is available, two intra prediction modes are added to the first group,

wherein when the intra prediction mode of the left block is not equal to the intra prediction mode of the top block, and the intra prediction mode of the left block and the intra prediction mode of the top block are planar mode and DC mode, the first group includes the intra prediction modes of the left and top blocks and a vertical mode, and

wherein a lowest prediction mode index is assigned to the planar mode when the intra prediction mode of the current block does not belong to the first group and the planar mode is not included in the first group.

2. The method of claim 1 , wherein the inversely quantizing the quantization block is performed using a quantization step size derived based on a quantization step size predictor and a residual quantization step size.

3. The method of claim 1 , wherein the intra prediction mode corresponding to the prediction mode index in a second group is determined as the intra prediction mode of the current block when the intra prediction mode group indicator indicates the second group.

4. The method of claim 1 , wherein one prediction block is generated on the basis of the determined intra prediction mode when a size of the prediction unit is equal to a size of the prediction block, and a plurality of prediction blocks are generated on the basis of the determined intra prediction mode when the size of the prediction unit is larger than the size of the prediction block.

5. The method of claim 1 , wherein the second block in the decoding order of the plurality of prediction blocks is generated using a part of pixels of a first prediction block as reference pixels.

6. The method of claim 1 , wherein when the intra prediction modes of the left and top blocks are not equal to each other and the intra prediction modes of the left and top blocks are not the DC mode nor the planar mode, the first group includes the intra prediction modes of the left and top blocks and the planar mode.

7. The method of claim 1 , wherein when the intra prediction modes of the left and top blocks of the current block are not available, the first group includes the planar mode, the DC mode and the vertical mode.

8. The method of claim 1 , wherein the DC mode and the planar mode have lower mode numbers than other intra prediction modes.

9. The method of claim 8 , wherein mode number 0 is assigned to the planar mode.

10. The method of claim 8 , wherein mode number 0 is assigned to the planar mode and mode number 1 is assigned to the DC mode.