Method and apparatus for encoding an image

1. An apparatus for decoding an image, comprising:

an inverse scanning module of generating a quantized block by applying an inverse scan pattern to significant flags, coefficient sign and coefficient levels respectively;

an inverse quantization module of generating a quantization parameter predictor, adding the quantization parameter predictor to a differential quantization parameter to generate a quantization parameter and inversely quantizing the quantized block using the quantization parameter;

an inverse transform module of inversely transforming the inversely quantized block to restore a residual block;

an intra prediction module of generating a prediction block;

an adder of generating a reconstructed block using the residual block and the prediction block,

wherein when a size of a transform unit is equal to or smaller than 8×8, the inverse scan pattern is a horizontal scan for a vertical mode and a predetermined number of intra prediction modes having closest direction to the vertical mode, the inverse scan pattern is a vertical scan for a horizontal mode and the predetermined number of the intra prediction modes having closest direction to the horizontal mode, and the inverse scan pattern is a diagonal scan for all other intra prediction modes, and

when the size of the transform unit is larger than 4×4, the significant flags, the coefficient sign and the coefficient levels are inversely scanned in reverse direction according to the inverse scan pattern to generate plural subsets and the plural subsets are inversely scanned in reverse direction according to the inverse scan pattern to generate the quantized block;

wherein the differential quantization parameter is restored per quantization unit by generating an absolute value and a sign flag of the differential quantization parameter,

if only one of a left quantization parameter of a current coding unit and an above quantization parameter of the current coding unit is available, an average of the available quantization parameter and a previous quantization parameter of the current coding unit is set as the quantization parameter predictor, and

if both of the left quantization parameter and the above quantization parameter are unavailable, the previous quantization parameter is set as the quantization parameter predictor;

wherein if plural coding units are included in the quantization unit, the quantization parameter is generated for the first coding unit containing one or more non-zero coefficients in decoding order, and

wherein an inverse transform type for inversely transforming the inversely quantized block is selected among a DCT-based integer transform and a DST-based integer transform depending on a prediction mode and a size of a transform unit,

wherein the significant flag indicates whether a quantized transform coefficient is zero or not, the coefficient sign indicates a sign of non-zero quantized transform coefficient, and the coefficients level indicates an absolute value of non-zero quantized transform coefficient.

2. The apparatus of claim 1 , wherein an inverse transform type for inversely transforming the inversely quantized block is determined according to a prediction mode and the size of the transform unit.

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

4. The apparatus 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 apparatus of claim 1 , wherein the size of a transform unit is equal to 8×8, and the predetermined number is 8.

6. An apparatus for encoding an image, comprising:

an intra prediction module of generating a prediction block;

a transform module of transforming a residual block to generate a transformed block, wherein the residual block is generated using the prediction block;

a quantization module of generating a quantization parameter and quantizing the transformed block using the quantization parameter; and

a scanning module of scanning the quantized block by applying a scan pattern to significant flags, coefficient sign and coefficient levels respectively,

wherein when a size of a transform unit is equal to or smaller than 8×8, the scan pattern is a horizontal scan for a vertical mode and a predetermined number of intra prediction modes having closest direction to the vertical mode, the scan pattern is a vertical scan for a horizontal mode and the predetermined number of the intra prediction modes having closest direction to the horizontal mode, and the scan pattern is a diagonal scan for all other intra prediction modes, and

when the size of the transform unit is larger than 4×4, the quantized block is divided into plural subsets and the significant flags, the coefficient sign and the coefficient levels are scanned in a direction according to the scan pattern applied to in a unit of a subset and the plural subsets are scanned in a direction according to the scan pattern;

wherein a differential quantization parameter is generated per quantization unit by subtracting a quantization parameter predictor from the quantization parameter,

if only one of a left quantization parameter of a current coding unit and an above quantization parameter of the current coding unit is available, an average of the available quantization parameter and a previous quantization parameter of the current coding unit is set as the quantization parameter predictor, and

if both of the left quantization parameter and the above quantization parameter are unavailable, the previous quantization parameter is set as the quantization parameter predictor;

wherein if plural coding units are included in the quantization unit, the quantization parameter is generated for the first coding unit containing one or more non-zero coefficients in decoding order, and

wherein a transform type for transforming the residual block is selected among a DCT-based integer transform and a DST-based integer transform depending on a prediction mode and a size of a transform unit,

wherein the significant flag indicates whether a quantized transform coefficient is zero or not, the coefficient sign indicates a sign of non-zero quantized transform coefficient, and the coefficients level indicates an absolute value of non-zero quantized transform coefficient.

7. A non-transitory computer-readable storage medium storing encoded video information, comprising: a bit stream stored in the medium, the bit stream comprising entropy-encoded information on a differential quantization parameter and a quantized block, wherein the quantized block is inversely quantized with a quantization parameter to generate a transformed block and the transformed block is inversely transformed to generate a residual block, and a reconstructed block is generated with the residual block and a prediction block, wherein the differential quantization parameter is restored per quantization unit with an absolute value and a sign flag of the differential quantization parameter, and the quantization parameter is generated using the differential quantization parameter and a quantization parameter predictor, if only one of a left quantization parameter of a current coding unit and an above quantization parameter of the current coding unit is available, an average of the available quantization parameter and a previous quantization parameter of the current coding unit is set as the quantization parameter predictor, and if both of the left quantization parameter and the above quantization parameter are unavailable, the previous quantization parameter is set as the quantization parameter predictor; wherein if plural coding units are included in the quantization unit, the quantization parameter is generated for the first coding unit containing one or more non-zero coefficients in decoding order, wherein an inverse transform type for inversely transforming the transformed block is selected among a DCT-based integer transform and a DST-based integer transform depending on a prediction mode and a size of a transform unit, and wherein the information on the quantized block includes a significant flag indicating whether a quantized transform coefficient is zero or not, a coefficient sign indicating a sign of non-zero quantized transform coefficient, and a coefficients level indicating an absolute value of non-zero quantized transform coefficient.