Intermediate compression of reference frames for transcoding

1. A method for compressing reference frames in a video transcoder, the method comprising:

receiving a compressed input stream in a first compressed format;

decoding the compressed input stream to generate an intermediate stream in an intermediate format, first reference frames in the compressed input stream compressed to intermediate compressed reference frames in the intermediate format, wherein compressing the first reference frames to the intermediate format comprises:

extracting a two-dimensional block of pixels from one of the first reference frames;

transforming the two-dimensional block of pixels from a spatial domain to a frequency domain to generate a coefficient array;

compressing the coefficient array to a fixed bucket size; and

storing the compressed coefficient array to a reference frame storage; and

encoding the intermediate stream to generate a compressed output stream in a second compressed format, the intermediate compressed reference frames compressed to second compressed reference frames in the second compressed format.

2. The method of claim 1 , further comprising processing the intermediate stream in the intermediate format according to a processing function.

3. The method of claim 1 , wherein transforming the two-dimensional block of the pixels comprises:

applying a two-dimensional transform to the block of pixels, wherein the two-dimensional transform preserves a DC component of the block of pixels.

4. The method of claim 1 , wherein compressing the coefficient array to a fixed bucket size comprises:

quantizing AC coefficients of the coefficient array according to an initial quantize value;

encoding the quantized AC coefficients of the coefficient array using an entropy encoding;

determining if the encoded AC coefficients, the quantize value, and a unquantized DC coefficient of the coefficient array fit to the fixed bucket size;

responsive to the encoded AC coefficients, the quantize value, and the unquantized DC coefficient not fitting to the fixed bucket size, adjusting the initial quantize value to an updated quantize value; and

responsive to the encoded AC coefficients, the quantize value, and the unquantized DC coefficient fitting to the fixed bucket size, setting current quantize value as a final quantize value.

5. The method of claim 4 , wherein quantizing AC coefficients of the coefficient array comprises:

shifting each AC coefficient of the coefficient array by the final quantize value; and

further shifting each AC coefficient of the coefficient by an offset value from an offset array, each entry of the offset array corresponding to an entry of the coefficient array.

6. The method of claim 4 , wherein encoding the quantized AC coefficients comprises:

selecting, according to the final quantize value, an encoding parameter array from a table of encoding parameter matrices;

encoding each AC coefficient of the coefficient array according to an encoding parameter from the encoding parameter array corresponding to the quantize value, each entry of the selected encoding parameter array corresponding to an entry of the coefficient array.

7. The method of claim 4 , wherein encoding the quantized AC coefficients comprises applying a Golomb-Rice encoding to the AC coefficients.

8. A non-transitory computer readable storage medium storing instructions thereon for compressing reference frames in a video transcoder, the instructions when executed by a processor cause the processor to execute steps including:

receiving a compressed input stream in a first compressed format;

decoding the compressed input stream to generate an intermediate stream in an intermediate format, first reference frames in the compressed input stream compressed to intermediate compressed reference frames in the intermediate format, wherein compressing the first reference frames to the intermediate format comprises:

extracting a two-dimensional block of pixels from one of the first reference frames;

transforming the two-dimensional block of pixels from a spatial domain to a frequency domain to generate a coefficient array;

compressing the coefficient array to a fixed bucket size; and

storing the compressed coefficient array to a reference frame storage; and

encoding the intermediate stream to generate a compressed output stream in a second compressed format, the intermediate compressed reference frames compressed to second compressed reference frames in the second compressed format.

9. The computer readable storage medium of claim 8 , wherein the steps further include further comprising processing the intermediate stream in the intermediate format according to a processing function.

10. The computer readable storage medium of claim 8 , wherein transforming the two-dimensional block of pixels comprises:

applying a two-dimensional transform to the block of pixels, wherein the two-dimensional transform preserves a DC component of the block of pixels.

11. The computer readable storage medium of claim 8 , wherein compressing the coefficient array to a fixed bucket size comprises:

quantizing AC coefficients of the coefficient array according to an initial quantize value;

encoding the quantized AC coefficients of the coefficient array using an entropy encoding;

determining if the encoded AC coefficients, the quantize value, and an unquantized DC coefficient of the coefficient array fit to the fixed bucket size;

responsive to the encoded AC coefficients, the quantize value, and the unquantized DC coefficient not fitting to the fixed bucket size, adjusting the initial quantize value to an updated quantize value; and

responsive to the encoded AC coefficients, the quantize value, and the unquantized DC coefficient fitting to the fixed bucket size, setting current quantize value as a final quantize value.

12. The computer readable storage medium of claim 11 , wherein quantizing AC coefficients of the coefficient array comprises:

shifting each AC coefficient of the coefficient array by the final quantize value; and

further shifting each AC coefficient of the coefficient by an offset value from an offset array, each entry of the offset array corresponding to an entry of the coefficient array.

13. The computer readable storage medium of claim 11 , wherein encoding the quantized AC coefficients comprises:

selecting, according to the final quantize value, an encoding parameter array from a table of encoding parameter matrices;

encoding each AC coefficient of the coefficient array according to an encoding parameter from the encoding parameter array corresponding to the quantize value, each entry of the selected encoding parameter array corresponding to an entry of the coefficient array.

14. The computer readable storage medium of claim 11 , wherein encoding the quantized AC coefficients comprises applying a Golomb-Rice encoding to the AC coefficients.

15. A system for compressing reference frames in a video transcoder comprising:

an input port configured to receive a compressed input stream in a first compressed format;

a decoder configured to decode the compressed input stream to generate an intermediate stream in an intermediate format, the decoder configured to compress first reference frames in the compressed input stream to intermediate compressed reference frames in the intermediate format, wherein the decoder is further configured to:

extract a two-dimensional block of pixels from one of the first reference frames;

transform the two-dimensional block of pixels from a spatial domain to a frequency domain to generate a coefficient array;

compress the coefficient array to a fixed bucket size; and

store the compressed coefficient array to a reference frame storage;

an encoder configured to encode the intermediate stream to generate a compressed output stream in a second compressed format, the encoder configured to compress the intermediate compressed reference frames to second compressed reference frames in the second compressed format; and

an output port configured to output an output stream in the second compressed format.

16. The system of claim 15 , further comprising a decoder reference frame storage coupled to the decoder, the decoder reference frame storage configured to store the compressed reference frames.

17. The system of claim 15 , further comprising an encoder reference frame storage coupled to the encoder, the encoder reference frame storage configured to store the compressed reference frames.

18. The system of claim 15 , further comprising a processor configured to process the intermediate stream in the intermediate format according to a processing function.