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 column position and a row position of a last non-zero coefficient in the transform block;

selecting a column-row inverse transform order when the scan pattern type is a first type;

selecting a row-column inverse transform order when the scan pattern type is a second 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 the first type is a vertical scan pattern;

wherein generating an 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 the column position; and

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

3. The method of claim 1 ,

wherein the second type is a horizontal scan pattern;

wherein generating an 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 the row position; and

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

4. The method of claim 1 ,

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

wherein generating an 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 the column position and the row position; and

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

5. The method of claim 1 ,

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

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

wherein generating a 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 the first type is a zigzag scan pattern; and

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

wherein generating a 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 the second type is a zigzag scan pattern; and

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

wherein generating a 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 a column-row inverse transform order comprises selecting the 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 a row-column inverse transform order comprises selecting the row-column 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 down-left.