Inverse transformation using pruning for video coding

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

decoding a pruned transform flag in the encoded video bit stream, wherein the pruned transform flag indicates that a subset region of a transform block contains all non-zero coefficients of the transform block, wherein the subset region consists of M rows and N columns of the transform block;

inverse scanning only quantized transform coefficients of the subset region decoded from the encoded video bit stream to generate the transform block; and

performing one dimensional (1D) inverse discrete cosine transformation (IDCT) computations on the transform block to generate a residual block, wherein one of a value of M or N of the subset region signaled by the pruned transform flag is used to determine pruning of the 1D IDCT computations, wherein performing the 1D IDCT computations includes:

receiving a transform coefficient and a corresponding IDCT coefficient;

setting a prune flag to indicate pruning when a value of the transform coefficient is zero;

setting the prune flag to indicate no pruning when the value of the transform coefficient is non-zero;

disabling latching of the transform coefficient for multiplication with the IDCT coefficient during a clock cycle and outputting a zero when the prune flag is set to indicate pruning; and

enabling latching of the transform coefficient for multiplication with the IDCT coefficient during a clock cycle and outputting a product of the transform coefficient and the IDCT coefficient when the pruning flag is set to indicate no pruning.

2. The method of claim 1 , wherein M=N=4.

3. The method of claim 1 , wherein performing the 1D IDCT computations comprises:

using a column-row inverse transform order when N>M, wherein 1D IDCT computations are performed on only N columns of the transform block to generate an interim results block; and

using a row-column inverse transform order when M>N, wherein 1D IDCT computations are performed on only M rows of the transform block to generate an interim results block.

4. The method of claim 1 , further comprising:

determining a scan pattern type for the transform block;

decoding a column position X and a row position Y of a last non-zero coefficient in the transform block from the encoded video bit stream;

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

selecting a row-column inverse transform order when the scan pattern type is a second type, wherein performing the 1D IDCT computations further includes:

performing the 1D IDCT computations according to the selected transform order to inversely transform the transform block to generate the residual block.

5. The method of claim 4 ,

wherein the first type is a vertical scan pattern; and

wherein performing the 1D IDCT computations further includes:

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

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

6. The method of claim 4 ,

wherein the second type is a horizontal scan pattern; and

wherein performing the 1D IDCT computations further includes:

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

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

7. The method of claim 4 ,

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

wherein performing the 1D IDCT computations further includes:

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

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

8. The method of claim 4 ,

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

wherein performing the 1D IDCT computations further includes:

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

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

9. The method of claim 4 ,

wherein the first type is a zigzag scan pattern; and

wherein performing the 1D IDCT computations further includes:

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

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

10. The method of claim 4 ,

wherein the second type is a zigzag scan pattern; and

wherein performing the 1D IDCT computations further includes:

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

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

11. The method of claim 4 , 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.