Method and apparatus of deriving quantization parameter

1. A method for decoding an image in intra prediction, comprising:

generating two-dimensional quantized block by applying an inverse scan pattern to significant coefficients, sign flags and levels respectively;

generating a quantization parameter predictor;

adding the quantization parameter predictor and a differential quantization parameter to generate quantization parameter;

inversely quantizing the quantized block using the quantization parameter;

inversely transforming the inversely quantized block to restore a residual block;

generating a prediction block; and

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

wherein when a size of transform unit is equal to 8×8, the inverse scan type is selected among a horizontal scan, a vertical scan and a diagonal scan based on an intra prediction mode, and the significant flags, the sign flags and the levels are inversely scanned in reverse direction according to the selected inverse scan pattern to generate plural subsets and the plural subsets are inversely scanned in reverse direction according to the selected inverse scan pattern to generate the quantized block; and

wherein a transform type for inversely transforming the inversely quantized block is selected among a DCT-based integer transform and a DST-based integer transform based on a size of the transform unit.

2. The method of claim 1 , wherein the plural subsets are generated based on non-zero subset flags.

3. The method of claim 1 , wherein if two or more quantization parameters are available among a left quantization parameter, an above quantization parameter and a previous quantization parameter of a current coding unit, the quantization parameter predictor is generated using two available quantization parameters determined according a predetermined order, and if only one quantization parameter is available, the available quantization parameter is set as the quantization parameter predictor.

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

5. The method of claim 1 , wherein the quantization unit has a size of a coding unit equal to or larger than a reference size, and the reference size is a minimum size of the quantization unit.

6. The method of claim 5 , wherein the minimum size of the quantization unit is one of allowable sizes of the coding unit.