One-pass variable bit rate moving pictures encoding

• US 6,654,417 B1
• Filed: 10/11/2000
• Issued: 11/25/2003
• Est. Priority Date: 01/26/1998
• Status: Expired due to Term

• Current Assignee: SGS-Thomson Microelectronics (STMicroelectronics NV)
• Original Assignee: STMicroelectronics Asia Pacific Pte Limited (STMicroelectronics NV)
• Inventors: Hui, Yau Wei Lucas
• Primary Examiner(s): Le, Vu
• Application Number:
  US09/600,935
• Time in Patent Office: 1,140 Days
• Field of Search: 375/240.02, 375/240.03, 375/240.05, 375/240.07, 348/419.1, 382/251
• US Class Current: 375/240.03
• CPC Class Codes: H04N 19/105 Selection of the reference unit for prediction within a chosen coding or prediction mode, e.g. adaptive choice of position and number of pixels used for prediction
• 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 19/14 Coding unit complexity, e.g. amount of activity or edge presence estimation H04N19/146 takes precedence
• CPC Class Codes: H04N 19/146 Data rate or code amount at the encoder output
• CPC Class Codes: H04N 19/149 by estimating the code amount by means of a model, e.g. mathematical model or statistical model
• CPC Class Codes: H04N 19/152 by measuring the fullness of the transmission buffer
• CPC Class Codes: H04N 19/154 Measured or subjectively estimated visual quality after decoding, e.g. measurement of distortion use of rate-distortion criteria H04N19/147
• 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 19/60 using transform coding
• CPC Class Codes: H04N 19/61 in combination with predictive coding

Abstract

A method and apparatus for encoding digital video utilizing a single pass variable bit rate encoding procedure. An encoding quality is set and the bit rate for encoding blocks or frames in the sequence of moving pictures is determined to achieve the selected quality according to the complexity of the video sequence. The bit rate is constrained by predetermined upper and lower bit rate limits.

Claims

1. A method for variable bit rate control in a single pass moving pictures encoder, comprising:

selecting a target picture encoding quality;

selecting upper and lower bit rate limits;

encoding at least one picture based on a target bit rate within the upper and lower bit rate limits;

predicting a current bit rate and an encoding quality based on the result of the encoding step;

comparing the encoding quality of the at least one encoded picture with the target picture encoding quality;

adjusting the target bit rate within the upper and lower bit rate limits according to the result of said comparison and the predicted current bit rate, for encoding subsequent pictures; and

repeating, for each picture in a sequence of pictures, said encoding, predicting, comparing and adjusting steps.

2. A method as claimed in claim 1, wherein the step of encoding at least one picture based on the target bit rate includes determining a target bit allocation for the at least one picture using the target bit rate.

3. A method as claimed in claim 2, wherein the pictures to be encoded are arranged in groups of pictures comprising an I-picture and at least one P-picture and/or B-picture, and wherein the target bit allocation is adjusted for each picture or plurality of pictures in each group of pictures.

4. A method as claimed in claim 1, 2 or 3, wherein the picture encoding quality is based on encoding quantization step-size.

5. A method as claimed in claim 1, 2 or 3, wherein the picture encoding quality is based on a mean square error.

6. A method as claimed in claim 1, 2 or 3, wherein the picture encoding quality is based on a signal-to-noise ratio.

7. A method as claimed in claim 5 or 6, wherein the comparing step includes measuring a difference in picture encoding quality between corresponding input and locally decoded pictures.

8. A method as claimed in claim 7, including measuring an average picture activity for the moving pictures, and modifying the measured difference in picture encoding quality on the basis of the average picture activity.

9. A method as claimed in claim 8, including measuring a signal-to-noise ratio for at least one previously encoded picture, and determining said target bit rate based on said predicted current bit rate and on a difference between the measured signal-to-noise ratio and a target signal-to-noise ratio as said target picture encoding quality.

10. A method as claimed in claim 9, wherein the predicted current bit rate and the target bit rate are determined according to:

SNR_diff=SNR_target−

(SNR_picture+K₅×

ACT_average) ${\mathrm{BR}}_{\mathrm{predicted}}=\frac{\left(S_I+n_P\times <br><br>S_P+n_B\times <br><br>S_B\right)}{\left(1+n_P+n_B\right)}$ 

BR_target=BR_predicted+K₆×

BR_current×

SNR_diff

where;

SNR_picture is the measured signal-to-noise ratio,

SNR_target is the target signal-to-noise ratio,

ACT_average is the measured average pixel activity,

K₅ and K₆ are constants,

SNR_diff is the measured difference in picture encoding quality,

BR_predicted is the predicted current bit rate,

S_I, S_P, and S_B are number of bits generated by said previously encoded I-, P-, B-pictures, respectively

n_P is a total number of P-pictures in the group of pictures,

n_B is a total number of B-pictures in the group of pictures,

BR_current is a previously determined BR_target, and

BR_target is said target bit rate.

11. A method as claimed in claim 4, wherein said target picture encoding quality is based on a target encoding quantization step-size.

12. A method as claimed in claim 11, wherein different target quantization step-sizes are selected for I-, P-, and B-pictures.

13. A method as claimed in claim 11 or 12, including measuring an average quantization step-size for at least one previously encoded picture, and determining said target bit rate based on said predicted current bit rate and a difference between the target encoding quantization step-size and the measured average quantization step-size.

14. A method as claimed in claim 13, wherein maximum and minimum bit rates are selected corresponding to the upper and lower bit rate limits, and wherein the predicted current bit rate and the target bit rate are determined according to:

${\mathrm{BR}}_{\mathrm{predicted}}=\frac{\left(S_I+n_P\times <br><br>S_P+n_B\times <br><br>S_B\right)}{\left(1+n_P+n_B\right)}$${\mathrm{BR}}_{\mathrm{target}}={\mathrm{BR}}_{\mathrm{predicted}}+K_2\times <br><br>\frac{\left({\mathrm{QS}}_{\mathrm{average}}-{\mathrm{QS}}_{\mathrm{target}}\right)}{{\mathrm{QS}}_{\mathrm{average}}}$ 

if (BR_target>

BR_max), then BR_target=BR_max

15. A method as claimed in claim 10 or 14, including determining a number of bits R remaining for encoding the group of pictures comprising the steps of:

before encoding a first picture in a said group of pictures, determining R according to $R+=\frac{N}{\mathrm{picture\_rate}}\times <br><br>{\mathrm{BR}}_{\mathrm{target}},\mathrm{then}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{set}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{BR}}_{\mathrm{old\_target}}={\mathrm{BR}}_{\mathrm{target}}$

where;

N is the number of pictures in the group of pictures,

picture_rate is the number of pictures encoded per unit time, and

BR_target is said target bitrate,

and before encoding each subsequent picture in the group of pictures, determining R according to $R+=\frac{\left(N_P+N_B\right)}{\mathrm{picture\_rate}}\times <br><br>\left({\mathrm{BR}}_{\mathrm{target}}-{\mathrm{BR}}_{\mathrm{old\_target}}\right),-S,\mathrm{then}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{set}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{BR}}_{\mathrm{old\_target}}={\mathrm{BR}}_{\mathrm{target}}$

where;

N_P and N_B are the number of P- and B-pictures remaining in the current group of pictures,

BR_target is a subsequently determined target bit rate, and

S is one of S_I, S_P or S_B according to the picture type of the previously encoded picture.

16. A method as claimed in claim 15, further comprising the steps of:

obtaining average quantization step sizes Q_I, Q_P, Q_B from previously encoded I-, P-, B-pictures, respectively; and

determining the target bit allocation T_I, T_P, T_B for a subsequent respective I-, P-, or B-picture according to;

$T_I=\frac{R}{1+\frac{N_P<br><br>X_P}{X_I<br><br>K_P}+\frac{N_B<br><br>X_B}{X_I<br><br>K_B}}$$T_P=\frac{R}{N_P+\frac{N_B<br><br>K_P<br><br>X_B}{K_B<br><br>X_P}}$$T_B=\frac{R}{N_B+\frac{N_P<br><br>K_B<br><br>X_P}{K_P<br><br>X_B}}$

where;

X_I=S_IQ_I, X_P=S_PQ_P, X_B=S_B,Q_B,

K_P and K_B are constants.

17. A method as claimed in claim 16, wherein the target bit allocations T_I, T_P, T_B have a lower limit T_min determined according to:

$T_{\min }=\frac{{\mathrm{BR}}_{\mathrm{target}}}{K_3\times <br><br>\mathrm{picture\_rate}}$

where K₃ is a constant.

18. A method as claimed in any one of claims 1 to 17, wherein the target bit allocation is used to encode an input picture, comprising the steps of:

deriving macroblocks from the input picture;

calculating a quantization step-size for each said macroblock according to the target bit allocation and its utilization;

performing motion compensation if necessary to each macroblock;

frequency transforming each said macroblock or motion compensated macroblock;

quantizing each transformed macroblock using the calculated quantization step-size; and

coding the quantized macroblock.

19. A method as claimed in claim 18, wherein the target quantization step-size is used as a lower limit for the determined quantization step-size.

20. A method as claimed in claim 18 or 19, wherein the determined quantization step-size for each macroblock is modulated by a corresponding macroblock activity masking factor, and the quantization is performed using the modulated quantization step-size.

21. A control apparatus for a single pass moving pictures encoder, comprising:

an input for receiving a target picture encoding quality;

an input for receiving upper and lower bit rate limits; and

a controller for controlling the encoder so as to encode at least one picture according to a target bit rate within the upper and lower bit rate limits, predict a current bit rate and an encoding quality based on the result of the encoding, compare the encoding quality of the at least one encoded picture with the target picture encoding quality, and adjust the target bit rate according to the result of said comparison and the predicted current bit rate for encoding subsequent pictures.

22. A control apparatus as claimed in claim 21, wherein encoding the at least one picture based on the target bit rate includes determining a target bit allocation for the at least one picture using the target bit rate.

23. A control apparatus as claimed in claim 22, wherein the pictures to be encoded are arranged in groups of pictures comprising an I-picture and at least one P-picture and/or B-picture, and wherein the controller adjusts the target bit allocation for each picture or plurality of pictures in each group of pictures.

24. A control apparatus as claimed in claim 21, 22 or 23, wherein the picture encoding quality is based on encoding quantization step-size.

25. A control apparatus as claimed in claim 21, 22 or 23, wherein the picture encoding quality is based on a mean square error.

26. A control apparatus as claimed in claim 21, 22 or 23, wherein the picture encoding quality is based on a signal-to-noise ratio.

27. A control apparatus as claimed in claim 25 or 26, wherein the controller includes a local picture decoder and a comparator for measuring a difference in picture encoding quality between corresponding input and locally decoded pictures.

28. A control apparatus as claimed in claim 27, wherein the controller includes a picture activity processor for measuring an average picture activity for the moving pictures, and wherein the controller modifies the measured difference in picture encoding quality on the basis of the average picture activity.

29. A control apparatus as claimed in claim 28, wherein the controller includes a signal-to-noise processor for measuring a signal-to-noise ratio for at least one previously encoded picture, and a bit rate predictor for predicting the current bit rate, and wherein the controller adjusts said target bit rate based on the predicted current bit rate and on a difference between the measured signal-to-noise ratio and a target signal-to-noise ratio as said target picture encoding quality.

30. A control apparatus as claimed in claim 21, wherein the moving picture encoder includes a frequency transform coefficient quantizer for quantization of the encoded picture data, and wherein the controller comprises a bit rate controller coupled to control the quantization step size of the quantizer, a quantization step size comparator for comparing, as a measure of encoding quality, an actual quantization step size with a target quantization step size based on the target picture encoding quality, a bit allocation processor coupled to control the bit rate controller according to a number of bits remaining for encoding a group of pictures, and a target bit rate estimator coupled to receive the upper and lower bit rate limits and coupled to the bit allocation processor and the bit rate controller for controlling the quantization so that the required bit rate for the quantized picture data is within the upper and lower bit rate limits.

31. A single pass variable bit rate video picture encoder comprising:

a picture input for receiving data for a plurality of moving pictures;

a target quality input for receiving a target quality measure for encoded pictures;

an encoder output for supplying encoded picture data;

a bit rate limit input for receiving upper and lower bit rate limits for the encoded picture data;

a bit rate predictor for predicting a current bit rate;

a frequency transform processor for frequency transform encoding picture data from the picture input;

a coefficient quantizer for quantizing the encoded picture data according to a quantization step size;

an encoding quality estimator for measuring an encoding quality of quantized encoded pictures; and

a bit rate controller for dynamically controlling the quantization step size of the coefficient quantizer based on the predicted current bit rate and a comparison of the target quality and the measured quality so that the encoder output remains within the upper and lower bit rate limits.

32. A video picture encoder as claimed in claim 31, wherein the pictures to be encoded are arranged in groups of pictures comprising an I-picture and at least one P-picture and/or B-picture, the bit rate controller including a frame bit counter for a number of remaining bits available for encoding a group of pictures, and a bit allocation processor for controlling the bit rate controller according to the remaining available bits so as to adjust a target bit allocation for each picture or plurality of pictures in each group of pictures.

33. A video picture encoder as claimed in claim 31 or 32, including a quality comparator for comparing the target quality with the measured encoding quality, and wherein the bit rate controller controls the quantization step size based on a difference between the target and measured qualities.

34. A video picture encoder as claimed in claim 32, including a quality comparator for comparing the target quality with the measured encoding quality wherein the bit rate controller adjusts the target bit allocation to achieve a target bit rate for the picture or plurality of pictures in each group of pictures which is determined on the basis of an output from said quality comparator.

35. A video picture encoder as claimed in claim 31, 32, 33 or 34, wherein the measured encoding quality is based on the quantization step size.

36. A video picture encoder as claimed in claim 33, including a local decoder for decoding the quantized encoded picture data, and a quality measurement processor for determining a quality difference between corresponding input and locally decoded pictures.

37. A video picture encoder as claimed in claim 36, wherein the quality measurement processor determines a difference in signal-to-noise ratio.

38. A video picture encoder as claimed in claim 36, wherein the quality measurement processor determines a mean square error.

39. A video picture encoder as claimed in claim 36, 37 or 38, including a picture activity processor for measuring an average picture activity for the moving pictures, and wherein the quality measurement processor modifies the measured difference in picture encoding quality on the basis of the average picture activity.

40. A video picture encoder as claimed in claim 37, including a signal-to-noise processor for measuring a signal-to-noise ratio for at least one previously encoded picture, and a bit rate predictor for estimating a current bit rate, and wherein the bit rate controller adjusts said target bit allocation based on the estimated current bit rate and on a difference between the measured signal-to-noise ratio and a target signal-to-noise ratio as said target picture encoding quality.

41. A video picture encoder as claimed in claim 31, including a quantization step size comparator for comparing, as a measure of encoding quality, an actual quantization step size with a target quantization step size based on the target picture encoding quality, a bit allocation processor coupled to control the bit rate controller according to a number of bits remaining for encoding a group of pictures, and a target bit rate estimator coupled to receive the upper and lower bit rate limits and coupled to the bit allocation processor and the bit rate controller for controlling the quantization so that the required bit rate for the quantized picture data is within the upper and lower bit rate limits.