Rate Control in Video Coding

Abstract

A method of rate control in coding of a video sequence to generate a compressed bit stream is provided that includes computing a sequence base quantization step size for a sequence of pictures in the video sequence, computing a picture base quantization step size for a picture in the sequence of pictures based on the sequence base quantization step size, a type of the picture, and a level of the picture in a rate control hierarchy, and coding the picture using the picture base quantization step size to generate a portion of the compressed bit stream.

1 . A method of rate control in coding of a video sequence to generate a compressed bit stream, the method comprising:
 computing a sequence base quantization step size for a sequence of pictures in the video sequence;   computing a picture base quantization step size for a picture in the sequence of pictures based on the sequence base quantization step size, a type of the picture, and a level of the picture in a rate control hierarchy; and   coding the picture using the picture base quantization step size to generate a portion of the compressed bit stream.   
     
     
         2 . The method of  claim 1 , wherein computing a sequence base quantization step size further comprises:
 when an initial picture in the sequence of pictures is a predicted picture, computing the sequence base quantization step size based on an average number of bits consumed by a previous sequence of pictures in the video sequence and an average quantization step size used in coding an initial picture in the previous sequence of pictures.   
     
     
         3 . The method of  claim 1 , wherein computing a sequence base quantization step size further comprises:
 when an initial picture in the sequence of pictures is an intracoded picture, computing the sequence base quantization step size as a weighted average of average quantization step sizes of previously coded pictures in the video sequence.   
     
     
         4 . The method of  claim 3 , wherein coding the picture further comprises:
 computing a minimum number of bits to be used in coding the picture and a maximum number of bits to be used in coding the picture; and   coding the picture using a number of bits greater than the minimum number of bits and less than the maximum number of bits.   
     
     
         5 . The method of  claim 4 , wherein coding the picture using a number of bits further comprises:
 estimating a number of bits needed to code the picture before coding a sequence of coding blocks in the picture;   increasing a quantization step size for the sequence of coding blocks when the estimated number of bits is larger than the maximum number of bits; and   decreasing the quantization step size for the sequence of coding blocks when the estimated number of bits is smaller than the minimum number of bits.   
     
     
         6 . The method of  claim 4 , wherein coding the picture further comprises:
 adapting the minimum number of bits and the maximum number of bits for rate control of a sequence of coding blocks in the picture when a scene change is detected in a picture immediately preceding the picture in the sequence of pictures.   
     
     
         7 . The method of  claim 1 , further comprising:
 responsive to a scene change in the video sequence, adapting the sequence base quantization step size based on an average of quantization step sizes used in coding an initial non-scene-change picture after the scene change.   
     
     
         8 . An encoder for coding a video sequence to generate a compressed bit stream, the encoder comprising:
 a rate control component configured to generate a quantization step size for quantizing coding blocks in the video sequence; and   a quantizer component configured to quantize the coding blocks using the quantization step size,   wherein the rate control component generates the quantization step size by
 computing a sequence base quantization step size for a sequence of pictures in the video sequence; 
 computing a picture base quantization step size for a picture in the sequence of pictures based on the sequence base quantization step size, a type of the picture, and a level of the picture in a rate control hierarchy. 
   
     
     
         9 . The encoder of  claim 8 , wherein computing a sequence base quantization step size further comprises:
 when an initial picture in the sequence of pictures is a predicted picture, computing the sequence base quantization step size based on an average number of bits consumed by a previous sequence of pictures in the video sequence and an average quantization step size used in coding an initial picture in the previous sequence of pictures.   
     
     
         10 . The encoder of  claim 8 , wherein computing a sequence base quantization step size further comprises:
 when an initial picture in the sequence of pictures is an intracoded picture, computing the sequence base quantization step size as a weighted average of average quantization step sizes of previously coded pictures in the video sequence.   
     
     
         11 . The encoder of  claim 10 , wherein the rate control component is further configured to generate the quantization step size by:
 computing a minimum number of bits to be used in coding the picture and a maximum number of bits to be used in coding the picture, wherein the quantization step size is increased or decreased as the picture is coded to guarantee that a number of bits used to code the picture is greater than the minimum number of bits and less than the maximum number of bits.   
     
     
         12 . The encoder of  claim 11 , wherein the rate control component is further configured to generate the quantization step size by:
 estimating a number of bits needed to code the picture before coding a sequence of coding blocks in the picture;   increasing a quantization step size for the sequence of coding blocks when the estimated number of bits is larger than the maximum number of bits; and   decreasing the quantization step size for the sequence of coding blocks when the estimated number of bits is smaller than the minimum number of bits.   
     
     
         13 . The encoder of  claim 10 , wherein the minimum number of bits and the maximum number of bits is adapted for rate control of a sequence of coding blocks in the picture when a scene change is detected in a picture immediately preceding the picture in the sequence of pictures. 
     
     
         14 . The encoder of  claim 8 , wherein the rate control component is further configured to generate the quantization step size by:
 responsive to a scene change in the video sequence, adapting the sequence base quantization step size based on an average of quantization step sizes used in coding an initial non-scene-change picture after the scene change.   
     
     
         15 . A machine readable medium storing instructions for rate control in coding of a video sequence to generate a compressed bit stream, wherein execution of the instructions by a processor in a video encoder causes the video encoder to perform the actions of:
 computing a sequence base quantization step size for a sequence of pictures in the video sequence;   computing a picture base quantization step size for a picture in the sequence of pictures based on the sequence base quantization step size, a type of the picture, and a level of the picture in a rate control hierarchy; and   coding the picture using the picture base quantization step size to generate a portion of the compressed bit stream.   
     
     
         16 . The machine readable medium of  claim 15 , wherein computing a sequence base quantization step size further comprises:
 when an initial picture in the sequence of pictures is a predicted picture, computing the sequence base quantization step size based on an average number of bits consumed by a previous sequence of pictures in the video sequence and an average quantization step size used in coding an initial picture in the previous sequence of pictures; and   when an initial picture in the sequence of pictures is an intracoded picture, computing the sequence base quantization step size as a weighted average of average quantization step sizes of previously coded pictures in the video sequence.   
     
     
         17 . The machine readable medium of  claim 15 , wherein coding the picture further comprises:
 computing a minimum number of bits to be used in coding the picture and a maximum number of bits to be used in coding the picture; and   coding the picture using a number of bits greater than the minimum number of bits and less than the maximum number of bits.   
     
     
         18 . The machine readable medium of  claim 18 , wherein coding the picture using a number of bits further comprises:
 estimating a number of bits needed to code the picture before coding a sequence of coding blocks in the picture;   increasing a quantization step size for the sequence of coding blocks when the estimated number of bits is larger than the maximum number of bits; and   decreasing the quantization step size for the sequence of coding blocks when the estimated number of bits is smaller than the minimum number of bits.   
     
     
         19 . The machine readable medium of  claim 18 , wherein coding the picture further comprises:
 adapting the minimum number of bits and the maximum number of bits for rate control of a sequence of coding blocks in the picture when a scene change is detected in a picture immediately preceding the picture in the sequence of pictures.   
     
     
         20 . The machine readable medium of  claim 15 , further comprising:
 responsive to a scene change in the video sequence, adapting the sequence base quantization step size based on an average of quantization step sizes used in coding an initial non-scene-change picture after the scene change.