Inverse transformation using pruning for video coding

1. A method for decoding an encoded video bit stream in a video decoder, the method comprising:

determining a scan pattern type for a transform block;

decoding a position of a last non-zero coefficient in the transform block;

selecting a transform order based on the scan pattern type;

and

generating an interim results block based on performing one dimensional (1D) inverse discrete cosine transformation (IDCT) computations on a subset of the transform block according to the selected transform order; and

generating a residual block using the interim results block.

2. The method of claim 1 ,

wherein a first type is a vertical scan pattern;

wherein generating the interim results block comprises performing 1D IDCT computations on only a subset of columns of the transform block, wherein a number of columns in the subset is derived from a column position of the position of the last non-zero coefficient in the transform block; and

wherein generating the residual block comprises performing 1D IDCT computations on all rows of the interim results block.

3. The method of claim 1 ,

wherein a second type is a horizontal scan pattern;

wherein generating the interim results block comprises performing 1D IDCT computations on only a subset of rows of the transform block, wherein a number of rows in the subset is derived from a row position of the position of the last non-zero coefficient in the transform block; and

wherein generating the residual block comprises performing 1D IDCT computations on all columns of the interim results block.

4. The method of claim 1 ,

wherein a first type is an up-right scan pattern;

wherein generating the interim results block comprises performing 1D IDCT computations on only a subset of columns of the transform block, wherein a number of columns in the subset is derived from a column position of the position of the last non-zero coefficient in the transform block; and

wherein generating the residual block comprises performing 1D IDCT computations on all rows of the interim results block.

5. The method of claim 1 ,

wherein a second type is a down-left scan pattern;

wherein generating the interim results block comprises performing 1D IDCT computations on only a subset of rows of the transform block to generate the interim results block, wherein a number of rows in the subset is derived from a row position and a column position of the position of the last non-zero coefficient in the transform block; and

wherein generating the residual block comprises performing 1D IDCT computations on all columns of the interim results block to generate the residual block.

6. The method of claim 1 ,

wherein a first type is a zigzag scan pattern; and

wherein generating the interim results block comprises performing 1D IDCT computations on only a subset of columns of the transform block to generate the interim results block, wherein a number of columns in the subset is derived from a column position and a row position of the position of the last non-zero coefficient in the transform block; and

wherein generating the residual block comprises performing 1D IDCT computations on all rows of the interim results block to generate the residual block.

7. The method of claim 1 ,

wherein a second type is a zigzag scan pattern; and

wherein generating the interim results block comprises performing 1D IDCT computations on only a subset of rows of the transform block to generate the interim results block, wherein a number of rows in the subset is derived from a row position and a column position of the position of the last non-zero coefficient in the transform block; and

wherein generating the residual block comprises performing 1D IDCT computations on all columns of the interim results block to generate the residual block.

8. The method of claim 1 , further comprising:

wherein selecting the transform order comprises selecting a column-row inverse transform order when the scan pattern type is a zigzag scan pattern and a scan direction of a diagonal of the transform block in which the last non-zero coefficient is located is up-right; and

wherein selecting the transform order comprises selecting a row-column inverse transform order when the scan pattern type is the zigzag scan pattern and the scan direction of a diagonal of the transform block in which the last non-zero coefficient is located is down-left.

9. The method of claim 1 , wherein:

determining the scan pattern type is based on prediction modes, wherein the prediction modes include an intra-prediction mode and an inter-prediction mode.

10. The method of claim 9 , wherein:

in response to the inter-prediction mode, the scan pattern type is a fixed up-right scan pattern.