Method for decoding chroma image using luma quantization

1. A method for decoding chroma image, comprising:

deriving a chroma intra prediction mode of a prediction unit;

determining a size of a current chroma block using luma transform size information;

generating a chroma prediction block of the current chroma block using the chroma intra prediction mode;

generating a chroma residual block of the current chroma block using the chroma intra prediction mode and a chroma quantization parameter; and

generating a chroma reconstructed block adding the chroma prediction block and the chroma residual block,

wherein the chroma quantization parameter is generated using a luma quantization parameter determined per quantization unit and information indicating the relationship between the luma quantization parameter and the chroma quantization parameter,

wherein the luma quantization parameter is generated using a luma quantization parameter predictor and a luma differential quantization parameter, the luma quantization parameter predictor is generated using two quantization parameters which are determined in a predetermined order among a left luma quantization parameter, an above luma quantization parameter and a previous luma quantization parameter,

wherein if both of the left luma quantization parameter and the above luma quantization parameter are available, an average of the left luma quantization parameter and the above luma quantization parameter is set as the luma quantization parameter predictor, and

wherein if only one of the left luma quantization parameter and the above luma quantization parameter is available, an average of the available luma quantization parameter and the luma previous quantization parameter is set as the luma quantization parameter predictor.

2. The method of claim 1 , wherein a minimum size of the quantization unit is adjusted per picture.

3. The method of claim 2 , wherein the minimum size of the quantization unit is derived using a parameter specifying the depth of a minimum size of the quantization unit and the size of the largest coding unit.

4. The method of claim 1 , wherein the differential luma quantization parameter is restored by arithmetic-decoding an encoded differential luma quantization parameter to generate a bin string and by applying inverse binarization to the bin string.

5. The method of claim 4 , wherein the bin string includes a bin string indicating an absolute value of the differential luma quantization parameter and a bin indicating a sign of the differential luma quantization parameter.

6. The method of claim 1 , wherein only one of the left luma quantization parameter, the above luma quantization parameter and the previous luma quantization parameter is available, the luma quantization parameter predictor is set to the available quantization parameter.