Block level rate control
Systems, apparatuses, and methods for performing efficient bitrate control of video compression are disclosed. Logic in a bitrate controller of a video encoder receives a target block bitstream length for a block of pixels of a video frame. When the logic determines a count of previously compressed blocks does not exceed a count threshold, the logic selects a quantization parameter from a full range of available quantization parameters. After encoding the block, the logic determines a parameter based on a first ratio of the achieved block bitstream length to an exponential value of an actual quantization parameter used to generate the achieved block bitstream length. For another block, when the count exceeds the count threshold, the logic generates a quantization parameter based on a ratio of the target block bitstream length to an average of parameters of previously encoded blocks.
1. An apparatus comprising:
logic comprising circuitry; and
wherein the logic is configured to:
receive a target block bitstream length for a single block of pixels of a video frame being compressed, the video frame comprising a plurality of blocks of pixels;
in response to determining a count of previously compressed blocks of pixels of the video frame does not exceed a count threshold, perform an iterative process to select a quantization parameter as an output quantization parameter, wherein to perform the iterative process the logic is configured to:
encode the single block two or more times using at least two different quantization parameters; and
set, as an output quantization parameter, a given quantization parameter of the at least two different quantization parameters;
in response to determining the count exceeds the count threshold, set the output quantization parameter without performing the iterative process;
convey the output quantization parameter to an external quantization stage of an encoder.
2. The apparatus as recited in claim 1 , wherein the logic is further configured to determine a rate control parameter based on a first ratio of an achieved block bitstream length to a natural exponential function of a quantization parameter used to generate the achieved block bitstream length.
3. The apparatus as recited in claim 1 , wherein the given quantization parameter is a quantization parameter determined to result in a desired block bitstream length.
4. The apparatus as recited in claim 2 , wherein the logic is further configured to determine an average rate control parameter using the rate control parameter and previous rate control parameters of previously encoded blocks.
5. The apparatus as recited in claim 1 wherein in further response to determining the count exceeds the count threshold, the logic is further configured to determine a ratio of the target block bitstream length to an average rate control parameter.
6. The apparatus as recited in claim 1 wherein in response to determining the count exceeds the count threshold, the logic is configured to set the output quantization parameter based in part on a natural logarithm of the target block bitstream length.
7. The apparatus as recited in claim 1 , wherein the logic is further configured to update the target block bitstream length based on differences between target block bitstream lengths and achieved block bitstream lengths of previously compressed blocks of pixels of the video frame.
8. The apparatus as recited in claim 1 , wherein the logic is further configured to determine the output quantization parameter independent of motion vectors.
9. A method, comprising:
receiving a target block bitstream length for a single block of pixels of a video frame being compressed, the video frame comprising a plurality of blocks of pixels;
in response to determining a count of previously compressed blocks of pixels of the video frame does not exceed a count threshold, performing an iterative process comprising:
encoding the single block two or more times using at least two different quantization parameters; and
setting, as an output quantization parameter, a given quantization parameter of the at least two different quantization parameters;
in response to determining the count exceeds the count threshold:
setting the output quantization parameter without performing the iterative process; and
conveying the output quantization parameter to an external quantization stage of an encoder.
10. The method as recited in claim 9 , further comprising determining a rate control parameter based on a first ratio of an achieved block bitstream length to a natural exponential function of a quantization parameter used to generate the achieved block bitstream length.
11. The method as recited in claim 10 , wherein the given quantization parameter is a quantization parameter determined to result in a desired block bitstream length.
12. The method as recited in claim 10 , further comprising determining an average rate control parameter using the rate control parameter and previous rate control parameters of previously encoded blocks.
13. The method as recited in claim 9 , wherein in further response to determining the count exceeds the count threshold, the method further comprises determining a ratio of the target block bitstream length to an average rate control parameter.
14. The method as recited in claim 9 , wherein in response to determining the count exceeds the count threshold, the method further comprises setting the output quantization parameter based in part on a natural logarithm of the target block bitstream length.
15. The method as recited in claim 9 , further comprising updating the target block bitstream length based on differences between target block bitstream lengths and achieved block bitstream lengths of previously compressed blocks of pixels of the video frame.
16. A non-transitory computer readable storage medium storing program instructions, wherein the program instructions are executable by a processor to:
receive a target block bitstream length for a single block of pixels of a video frame being compressed, the video frame comprising a plurality of blocks of pixels;
in response to determining a count of previously compressed blocks of pixels of the video frame does not exceed a count threshold, perform an iterative process to select a quantization parameter as an output quantization parameter, wherein to perform the iterative process the logic is configured to:
encode the single block two or more times using at least two different quantization parameters; and
set, as an output quantization parameter, a given quantization parameter of the at least two different quantization parameters;
in response to determining the count exceeds the count threshold, set the
output quantization parameter without performing the iterative process;
convey the output quantization parameter to an external quantization stage of an encoder.
17. The non-transitory computer readable storage medium as recited in claim 16 , wherein the program instructions are further executable by a processor to determine a rate control parameter based on a first ratio of an achieved block bitstream length to a natural exponential function of a quantization parameter used to generate the achieved block bitstream length.
18. The non-transitory computer readable storage medium as recited in claim 16 , wherein the given quantization parameter is a quantization parameter determined to result in a desired block bitstream length.
19. The non-transitory computer readable storage medium as recited in claim 16 , wherein the program instructions are further executable by a processor to determine an average rate control parameter using the rate control parameter and previous rate control parameters of previously encoded blocks.
20. The non-transitory computer readable storage medium as recited in claim 16 , wherein in further response to determining the count exceeds the count threshold, the program instructions are further executable by a processor to determine a ratio of the target block bitstream length to an average rate control parameter.