Method for decoding chroma image using luma quantization
Provided is a method that derives a chroma intra prediction mode of a prediction unit, determines a size of a current chroma block using luma transform size information, generates a chroma prediction block of the current chroma block using the chroma intra prediction mode, generates a chroma residual block of the current chroma block using the chroma intra prediction mode and a chroma quantization parameter, generates a chroma reconstructed block adding the chroma prediction block and the chroma residual block, and the chroma quantization parameter is generated using a luma quantization parameter and information indicating the relationship between the luma quantization parameter and the chroma quantization parameter. Therefore, the coding efficiency is improved by adjusting the chroma quantization parameter per picture. Also, the amount of bits for transmitting the luma and chroma quantization parameters is reduced by encoding the luma quantization parameter using neighboring luma quantization parameters.
1. An apparatus for decoding a chroma image, comprising:
an intra prediction unit for deriving a chroma intra prediction mode of a prediction unit, determining a size of a chroma prediction block using luma transform size information and generating the chroma prediction block using the chroma intra prediction mode;
a residual block generating unit for generating a chroma residual block using the chroma intra prediction mode and a chroma quantization parameter; and
an adder for generating a chroma reconstructed block by 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,
the luma quantization parameter is generated using a luma quantization parameter predictor and a luma differential quantization parameter,
if both of a left luma quantization parameter and an 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
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 apparatus of claim 1 , wherein a minimum size of the quantization unit is adjusted per picture.
3. The apparatus of claim 1 , wherein a minimum size of the quantization unit is derived using a parameter specifying a depth of the minimum size of the quantization unit and a size of a largest coding unit.
4. The apparatus of claim 1 , wherein the luma differential quantization parameter is restored by arithmetic-decoding an encoded luma differential quantization parameter to generate a bin string and by applying inverse binarization to the bin string.
5. The apparatus of claim 4 , wherein the bin string includes a bin string indicating an absolute value of the luma differential quantization parameter and a bin indicating a sign of the luma differential quantization parameter.
6. The apparatus of claim 1 , wherein if the chroma intra prediction mode is a derived mode (DM mode), the chroma prediction block is generated using a corresponding luma intra prediction mode.