Method for decoding chroma image using luma quantization

1. A method for decoding chroma image, the method 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 a relationship between the luma quantization parameter and the chroma quantization parameter,

wherein a size of the quantization unit is one of allowable sizes of a coding unit,

wherein a minimum size of the quantization unit is determined per picture and a parameter for specifying the minimum size of the quantization unit is included in a picture parameter set,

wherein if a size of the coding unit is equal to or larger than the minimum size of the quantization unit, the coding unit becomes the quantization unit,

wherein the information indicating the relationship between the luma quantization parameter and the chroma quantization parameter is signaled at the picture parameter set or a slice header,

wherein the luma quantization parameter is generated using a luma quantization parameter predictor and a differential luma quantization parameter, the luma quantization parameter predictor is generated using two quantization parameters if at least two of a left luma quantization parameter, an above luma quantization parameter and a previous luma quantization parameter are available,

wherein the two quantization parameters used for generating the luma quantization parameter predictor are determined in an order of the left luma quantization parameter, the above luma quantization parameter and the 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,

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 previous luma quantization parameter is set as the luma quantization parameter predictor,

wherein if both of the left luma quantization parameter and the above luma quantization parameter are not available, the previous luma quantization parameter is set as the luma quantization parameter predictor,

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, and

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.

2. The method of claim 1 , wherein the 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 the minimum size of the quantization unit and the size of the largest coding unit.