Apparatus and method for macroblock based rate control in a coding system

• US 6,690,833 B1
• Filed: 06/18/1998
• Issued: 02/10/2004
• Est. Priority Date: 07/14/1997
• Status: Expired due to Term

• Current Assignee: MediaTek, Inc.
• Original Assignee: Sarnoff Corporation (SRI International, Inc.)
• Inventors: Lee, Hung-Ju, Zhang, Ya-Qin, Chiang, Tihao
• Primary Examiner(s): Johns, Andrew W.
• Application Number:
  US09/099,617
• Time in Patent Office: 2,063 Days
• Field of Search: 382/181, 382/190, 382/232, 382/233, 382/235, 382/236, 382/238, 382/239, 382/240, 382/243, 382/244, 382/245, 382/246, 382/248, 382/250, 382/251, 382/252, 382/253, 382/261, 341/67, 395/200.77, 395/200.34, 348/420, 348/397, 348/416, 348/415, 348/701, 348/400, 348/403, 345/202
• US Class Current: 382/236
• CPC Class Codes: H04N 19/115 Selection of the code volume for a coding unit prior to coding
• CPC Class Codes: H04N 19/126 Details of normalisation or weighting functions, e.g. normalisation matrices or variable uniform quantisers
• CPC Class Codes: H04N 19/137 Motion inside a coding unit, e.g. average field, frame or block difference
• CPC Class Codes: H04N 19/146 Data rate or code amount at the encoder output
• CPC Class Codes: H04N 19/147 according to rate distortion criteria rate-distortion as a criterion for motion estimation H04N19/567
• CPC Class Codes: H04N 19/152 by measuring the fullness of the transmission buffer
• 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 19/196 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 19/61 in combination with predictive coding

Abstract

A method and apparatus for selecting a quantizer scale for each macroblock within a frame to optimize the coding rate is presented. A quantizer scale is selected for each macroblock within each frame such that the target bit rate for the frame is achieved while maintaining a uniform visual quality over the entire frame.

Claims

1. A method for allocating bits to encode each frame of an image sequence, each of said frame having at least one block, said method comprising the steps of:

(a) determining a target frame bit rate for the frame; and

(b) allocating said target frame bit rate among the at least one block in accordance with a target block bit rate for the at least one block, wherein said target block bit rate for the at least one block is selected in accordance with a mean absolute difference (Mad) of said block.

2. The method of claim 1, wherein said target block bit rate is determined in accordance with:

3. The method of claim 1, wherein said target block bit rate is adjusted in accordance with a threshold H_q.

4. The method of claim 3, wherein said target block bit rate is adjusted by removing each block within the frame having a sum of absolute difference (SAD) that is less than said threshold H_q from said step (b) of allocating said target frame bit rate among the at least one block.

5. The method of claim 1, wherein said block is a macroblock.

6. The method of claim 1, wherein said target frame bit rate, T_frame, is derived in accordance with:

$T_{\mathrm{frame}}=\frac{R}{\mathrm{Nf}}\times <br><br>\left(1-\mathrm{past\_percent}\right)+T_{\mathrm{previous}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{frame}}\times <br><br>\mathrm{past\_percent}$

where R is a remaining number of bits for the image sequence, Nf is a number of remaining frames in the image sequence, T_{previous frame} is a number of bits used for encoding a previous frame, and past_percent is a constant.

7. The method of claim 1, further comprising the step of:

(c) generating a quantizer scale for said at least one block in accordance with said target block bit rate.

8. The method of claim 7, further comprising the step of:

(c′

) adjusting said quantizer scale in accordance with a previous quantizer scale of a previous block.

9. The method of claim 7, further comprising the step of:

(d) encoding said at least one block with said quantizer scale.

10. Apparatus for encoding each frame of an image sequence, said frame having at least one block, said apparatus comprising:

a motion compensator for generating a predicted image of a current frame;

a transform module for applying a transformation to a difference signal between the current frame and said predicted image, where said transformation produces a plurality of coefficients;

a quantizer for quantizing said plurality of coefficients with at least one quantizer scale; and

a controller for selectively adjusting said at least one quantizer scale for a current frame in response to a target block bit rate for the at least one block, wherein said target block bit rate for the at least one block is selected in accordance with a mean absolute difference (Mad) of said block.

11. The apparatus of claim 10, wherein said target block bit rate for each of a plurality of blocks is selected in accordance with:

12. The apparatus of claim 10, wherein said target macroblock bit rate is adjusted in accordance with a threshold H_q.

13. The apparatus of claim 12, wherein said target block bit rate is adjusted by removing each block within the frame having a sum of absolute difference (SAD) that is less than said threshold H_q.

14. The apparatus of claim 10, wherein said block is a macroblock.

15. The apparatus of claim 10, wherein said target block bit rate is derived from a target frame bit rate.

16. The apparatus of claim 15, wherein said target frame bit rate, T_frame, is derived in accordance with:

$T_{\mathrm{frame}}=\frac{R}{\mathrm{Nf}}\times <br><br>\left(1-\mathrm{past\_percent}\right)+T_{\mathrm{previous}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{frame}}\times <br><br>\mathrm{past\_percent}$

where R is a remaining number of bits for a sequence of frames, Nf is a number of remaining frames in the sequence, T_{previous frame} is a number of bits used for encoding a previous frame, and past_percent is a constant.

17. A computer-readable medium having stored thereon a plurality of instructions, the plurality of instructions including instructions which, when executed by a processor, cause the processor to perform the steps comprising of:

(a) determining a target frame bit rate for the frame; and

(b) allocating said target frame bit rate among the at least one block in accordance with a target block bit rate for the at least one block, wherein said target block bit rate for the at least one block is selected in accordance with a mean absolute difference (Mad) of said block.

18. The computer-readable medium of claim 17, wherein said target block bit rate is determined in accordance with:

19. The computer-readable medium of claim 17, wherein said target block bit rate is adjusted in accordance with a threshold H_q.

20. The computer-readable medium of claim 17, wherein said target frame bit rate, T_frame, is derived in accordance with:

$T_{\mathrm{frame}}=\frac{R}{\mathrm{Nf}}\times <br><br>\left(1-\mathrm{past\_percent}\right)+T_{\mathrm{previous}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{frame}}\times <br><br>\mathrm{past\_percent}$

where R is a remaining number of bits for the image sequence, Nf is a number of remaining frames in the image sequence, T_{previous frame} is a number of bits used for encoding a previous frame, and past percent is a constant.