Method and apparatus for processing a bitstream in a digital video transcoder

• US 20050152449A1
• Filed: 01/12/2004
• Published: 07/14/2005
• Est. Priority Date: 01/12/2004
• Status: Active Grant

• Current Assignee: Google Technology Holdings LLC (Alphabet Inc.)
• Original Assignee: General Instrument Corporation (CommScope Holding Company, Inc.)
• Inventors: Wu, Siu-Wai, Nemiroff, Robert S.
• Granted Patent: US 7,675,970 B2
• Time in Patent Office: Days
• US Class Current: 375/240.30
• CPC Class Codes: H04N 19/115 Selection of the code volume for a coding unit prior to coding
• CPC Class Codes: H04N 19/124 Quantisation
• CPC Class Codes: H04N 19/137 Motion inside a coding unit, e.g. average field, frame or block difference
• CPC Class Codes: H04N 19/14 Coding unit complexity, e.g. amount of activity or edge presence estimation H04N19/146 takes precedence
• CPC Class Codes: H04N 19/142 Detection of scene cut or scene change
• CPC Class Codes: H04N 19/15 by monitoring actual compressed data size at the memory before deciding storage at the transmission buffer
• CPC Class Codes: H04N 19/159 Prediction type, e.g. intra-frame, inter-frame or bidirectional frame prediction
• CPC Class Codes: H04N 19/172 the region being a picture, frame or field
• CPC Class Codes: H04N 19/176 the region being a block, e.g. a macroblock
• CPC Class Codes: H04N 19/18 the unit being a set of transform coefficients
• CPC Class Codes: H04N 19/196 being specially adapted for the computation of encoding parameters, e.g. by averaging previously computed encoding parameters processing of motion vectors H04N19/513
• CPC Class Codes: H04N 19/198 including smoothing of a sequence of encoding parameters, e.g. by averaging, by choice of the maximum, minimum or median value
• CPC Class Codes: H04N 19/40 using video transcoding, i.e. partial or full decoding of a coded input stream followed by re-encoding of the decoded output stream
• CPC Class Codes: H04N 19/48 using compressed domain processing techniques other than decoding, e.g. modification of transform coefficients, variable length coding [VLC] data or run-length data motion estimation in a transform domain H04N19/547; processing of decoded motion vectors H04N19/513
• CPC Class Codes: H04N 19/61 in combination with predictive coding

Abstract

A method and apparatus for processing a bitstream in a digital video transcoder is described. In one example, an adjustment factor is determined that relates a number of bits representing a selected frame defined by the bitstream to a target number of bits for the selected frame. An average spatial activity value among sets of video data is associated with the selected frame. A spatial activity value for a selected set of the sets of video data is determined. A normalized spatial, activity value for the selected set of frequency video data is then computed in response to the average spatial activity value, the spatial activity value, and a function of the adjustment factor. Target bit reduction for each of the sets of video data may be computed using a function of the normalized spatial activity value and the adjustment factor.

Claims

1. A method of processing a bitstream in a digital video transcoder, comprising:

(a) determining an adjustment factor relating a number of bits representing a selected frame defined by said bitstream to a target number of bits for said selected frame;

(b) determining an average spatial activity value among sets of video data associated with said selected frame;

(c) determining a spatial activity value for a selected set of said sets of video data; and

(d) computing a normalized spatial activity value for said selected set of video data in response to said average spatial activity value, said spatial activity value, and a function of said adjustment factor.

2. The method of claim 1, further comprising:

(e) adjusting a quantization value associated with said selected set of video data in response to said normalized spatial activity value.

3. The method of claim 2, further comprising:

(f) repeating steps (c) through (e) for one or more additional selected sets of said sets of video data.

4. The method of claim 3, further comprising:

(g) repeating steps (a) through (f) for one or more additional frames defined by said bitstream.

5. The method of claim 4, wherein said average spatial activity value associated with said selected frame is estimated in response to a previous average spatial activity value associated with a previous frame and a ratio of the number of bits representing said selected frame and a number of bits representing said previous frame.

6. The method of claim 1, wherein said spatial activity value is determined in response to a number of frequency domain coefficients in said selected set of video data.

7. The method of claim 1, wherein said spatial activity value is determined in response to values of frequency domain coefficients in said selected set of video data.

8. The method of claim 1, wherein said spatial activity value is determined in response to a quantization value associated with said selected set of video data.

9. The method of claim 1, wherein said normalized spatial activity value is computed using a second function, defined by:

${\mathrm{N\_act}}_j=\frac{f<br><br>\left(\mathrm{rcFactor}\right)\times <br><br>{\mathrm{act}}_j+\mathrm{avg\_act}}{{\mathrm{act}}_j+f<br><br>\left(\mathrm{rcFactor}\right)\times <br><br>\mathrm{avg\_act}},$

where j represents said selected set of video data, N_act_j is said normalized spatial activity, rcFactor is said adjustment factor, ƒ

is said function of said adjustment factor, act_j is said spatial activity value, and avg_act is said average spatial activity value.

10. The method of claim 9, wherein said function of said average spatial activity value is defined by:

ƒ

(rcFactor)=rcFactor.

11. The method of claim 9, wherein said second function is approximated using a piecewise continuous function.

12. The method of claim 9, further comprising:

(e) adjusting a quantization value associated with said selected set of video data in accordance with a third function, defined by;

outQL=inQL×

rcFactor×

N_—

act_j,

where outQL is said adjusted quantization value and in QL is said quantization value.

13. A method of processing a bitstream in a digital video transcoder, comprising:

(a) determining an adjustment factor relating a number of bits representing a selected frame defined by said bitstream to a target number of bits for said selected frame;

(b) determining an average spatial activity value among macroblocks associated with said selected frame;

(c) determining a spatial activity value for a selected macroblock; and

(d) computing a normalized spatial activity value for said selected macroblock in response to said average spatial activity value, said spatial activity value, and a function of said adjustment factor.

14. The method of claim 13, further comprising:

(e) adjusting a quantization value associated with said selected macroblock in response to said normalized spatial activity value.

15. The method of claim 14, further comprising:

(f) repeating steps (c) through (e) for one or more additional selected macroblocks of said macroblocks.

16. The method of claim 15, further comprising:

(g) repeating steps (a) through (f) for one or more additional frames defined by said bitstream.

17. The method of claim 16, wherein said average spatial activity value associated with said selected frame is estimated in response to a previous average spatial activity value associated with a previous frame and a ratio of the number of bits representing said selected frame and a number of bits representing said previous frame.

18. The method of claim 17, wherein each frame defined by said bitstream is one of an I-type, a P-type, and a B-type, and wherein said previous frame and said selected frame are of identical types.

19. The method of claim 17, wherein each frame defined by said bitstream is one of a first type and a second type, said first type being defined by an I-frame or a P-frame, and said second type being defined by a B-frame, and wherein said previous frame and said selected frame are of identical types.

20. The method of claim 13, wherein said spatial activity value is determined in response to discrete cosine transform (DCT) coefficients associated with said selected macroblock.

21. The method of claim 20, wherein step (c) further comprises:

(c1) identifying coded luma blocks in said selected macroblock using a coded block pattern recovered from said bitstream;

(c2) determining a number of DCT coefficients among said identified coded luma blocks; and

(c3) defining said spatial activity value with respect to said number of DCT coefficients among said identified coded luma blocks.

22. The method of claim 20, wherein step (c) further comprises:

(c1) identifying coded luma blocks and coded chroma blocks in said selected macroblock using a coded block pattern recovered from said bitstream;

(c2) determining a number of DCT coefficients among said identified coded luma blocks and coded chroma blocks; and

(c3) defining said spatial activity value with respect to said number of DCT coefficients among said identified coded luma blocks and coded chroma blocks.

23. The method of claim 13, wherein said normalized spatial activity value is computed using a second function, defined by:

${\mathrm{N\_act}}_j=\frac{f<br><br>\left(\mathrm{rcFactor}\right)\times <br><br>{\mathrm{act}}_j+\mathrm{avg\_act}}{{\mathrm{act}}_j+f<br><br>\left(\mathrm{rcFactor}\right)\times <br><br>\mathrm{avg\_act}},$

where j represents said selected set of frequency domain coefficients, N_act_j is said normalized spatial activity, rcFactor is said adjustment factor, ƒ

is said function of said adjustment factor, act_j is said spatial activity value, and avg_act is said average spatial activity value.

24. The method of claim 23, wherein said function of said average spatial activity value is defined by:

ƒ

(rcFactor)=rcFactor.

25. The method of claim 23, wherein said second function is approximated using a piecewise continuous function.

26. Apparatus for transcoding an input bitstream having a first bit-rate to an output bitstream having a second bit-rate, comprising:

a decoder for decoding said input bitstream and producing decoded data;

a quantizer for quantizing said decoded data;

an encoder for encoding said quantized decoded data and producing said output bitstream; and

a controller for determining quantization adjustment factors associated with a change from said first bit-rate to said second bit-rate, computing adaptive quantization factors based on said quantization adjustment factors, and controlling a quantization level of said quantizer in accordance with said adaptive quantization factors.

27. Apparatus for processing a bitstream in a digital video transcoder, comprising:

means for relating a number of bits representing a selected frame defined by said bitstream to a target number of bits for said selected frame to define an adjustment factor;

means for determining an average spatial activity value among sets of frequency domain coefficients associated with said selected frame, each of said sets of frequency domain coefficients having at least one frequency domain coefficient;

means for determining a spatial activity value for a selected set of said sets of frequency domain coefficients; and

means for computing a normalized spatial activity value for said selected set of frequency domain coefficients in response to said average spatial activity value, said spatial activity value, and a function of said adjustment factor.

28. A computer readable carrier including program instructions that instruct a computer to perform a method of:

(a) determining an adjustment factor relating a number of bits representing a selected frame defined by said bitstream to a target number of bits for said selected frame;

(b) determining an average spatial activity value among sets of video data associated with said selected frame;

(c) determining a spatial activity value for a selected set of said sets of video data; and

(d) computing a normalized spatial activity value for said selected set of video data in response to said average spatial activity value, said spatial activity value, and a function of said adjustment factor.