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 using a corresponding luma intra prediction mode;

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

generating a chroma prediction block of the current chroma block;

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 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 luma differential 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, and

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.

2. 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.

3. The method of claim 1 , 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.

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

5. The method of claim 4 , 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.

6. The method of claim 1 , wherein a luma prediction block is generated using the luma intra prediction mode and luma reference pixels, and the chroma prediction block is generated using the chroma intra prediction mode and chroma reference pixels, and wherein reference pixels are not filtered for the current chroma block which has half the height and half the width of a current luma block.

7. A computer-readable non-transitory storage medium storing image information causing a decoding apparatus to carry out the method for decoding chroma image of claim 1 .

8. A method for encoding chroma image, comprising:

determining a chroma intra prediction mode of a prediction unit in consideration of a corresponding luma intra prediction mode;

determining a size of a current chroma block in consideration of a luma transform size information;

generating a chroma prediction block of the current chroma block;

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

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

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

wherein the size of the quantization unit is one of allowable sizes of a coding unit, and the coding unit becomes the quantization unit if a size of the coding unit is equal to or larger than the minimal size of the quantization 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 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 a luma differential quantization parameter is generated using the luma quantization parameter and a luma quantization parameter predictor, and is signaled,

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