Fractional quantization step sizes for high bit rates
A video codec uses fractional increments of quantization step size at high bit rates to permit a more continuous variation of quality and/or bit rate as the quantization scale changes. For high bit rate scenarios, the bit stream syntax includes an additional syntax element to specify fractional step increments (e.g., half step) of the normal quantizer scale step sizes.
1. In a computer system that implements a video decoder, a method comprising:
receiving, at the video decoder, encoded data in a bitstream for at least part of a video sequence; and
with the video decoder, decoding the encoded data, including, for a given frame of the video sequence:
decoding a quantizer index for the given frame, wherein the quantizer index indicates a picture quantizer scale for the given frame;
comparing the quantizer index to a comparison value;
responsive to the quantizer index being less than or equal to the comparison value, decoding a half-step quantization parameter field, the half-step quantization parameter field indicating a half-step increment for the picture quantizer scale for the given frame, and selectively adjusting the picture quantizer scale for the given frame according to the half-step increment, wherein syntax of the bitstream allows half-step picture quantization step sizes as specified by the quantizer index and the half-step quantization parameter field when the quantizer index is less than or equal to the comparison value but allows only integer-step picture quantization step sizes as specified by the quantizer index when the quantizer index is greater than the comparison value; and
inverse quantizing at least some quantized transform coefficients for the given frame.
2. The method of claim 1 , wherein the quantizer index is a 5-bit field.
3. The method of claim 1 , wherein the quantizer index is signaled in a frame header for the given frame.
4. The method of claim 1 , wherein the half-step quantization parameter field is a 1-bit field.
5. The method of claim 1 , wherein the half-step quantization parameter field is signaled in a frame header for the given frame.
6. The method of claim 1 , wherein the selectively adjusting the picture quantizer scale for the given frame includes:
if the half-step quantization parameter field is 1, adding ½ to the picture quantizer scale for the given frame; and
otherwise, the half-step quantization parameter field being 0, not adjusting the picture quantizer scale for the given frame.
7. The method of claim 1 , wherein the inverse quantizing includes performing operations according to dequant_coeff=quant_coeff*(2*quant_scale+halfstep), where dequant_coeff is a dequantized transform coefficient, quant_coeff is a quantized transform coefficient, quant_scale is the picture quantizer scale for the given frame, and halfstep is the half-step quantization parameter field.
8. The method of claim 1 , wherein the inverse quantizing includes performing operations according to dequant_coeff=quant_coeff*(2*quant_scale+halfstep)+sign(quant_coeff)*quant_scale, where dequant_coeff is a dequantized transform coefficient, quant_coeff is a quantized transform coefficient, quant_scale is the picture quantizer scale for the given frame, halfstep is the half-step quantization parameter field, and sign(x) is a function defined as +1 for x≥0, or −1 for x<0.
9. One or more computer-readable memory or storage devices having stored thereon encoded data in a bitstream for at least part of a video sequence, the encoded data including a quantizer index and a half-step quantization parameter field for a given frame of the video sequence, wherein the encoded data is organized to facilitate decoding, with a computer system that includes one or more processing units and implements a video decoder, by operations that include, for the given frame:
decoding a quantizer index, wherein the quantizer index indicates a picture quantizer scale for the given frame;
comparing the quantizer index to a comparison value;
responsive to the quantizer index being less than or equal to the comparison value, decoding the half-step quantization parameter field, the half-step quantization parameter field indicating a half-step increment for the picture quantizer scale for the given frame, and selectively adjusting the picture quantizer scale for the given frame according to the half-step increment, wherein syntax of the bitstream allows half-step picture quantization step sizes as specified by the quantizer index and the half-step quantization parameter field when the quantizer index is less than or equal to the comparison value but allows only integer-step picture quantization step sizes as specified by the quantizer index when the quantizer index is greater than the comparison value; and
inverse quantizing at least some quantized transform coefficients for the given frame.
10. The one or more computer-readable memory or storage devices of claim 9 , wherein the quantizer index is a 5-bit field.
11. The one or more computer-readable memory or storage devices of claim 9 , wherein the quantizer index is signaled in a frame header for the given frame.
12. The one or more computer-readable memory or storage devices of claim 9 , wherein the half-step quantization parameter field is a 1-bit field.
13. The one or more computer-readable memory or storage devices of claim 9 , wherein the half-step quantization parameter field is signaled in a frame header for the given frame.
14. The one or more computer-readable memory or storage devices of claim 9 , wherein the selectively adjusting the picture quantizer scale for the given frame includes:
if the half-step quantization parameter field is 1, adding ½ to the picture quantizer scale for the given frame; and
otherwise, the half-step quantization parameter field being 0, not adjusting the picture quantizer scale for the given frame.
15. A computer system comprising:
a buffer, implemented with memory, configured to receive encoded data in a bitstream for at least part of a video sequence; and
a video decoder, implemented with one or more processing units, configured to perform operations to decode the encoded data, the operations including, for a given frame of the video sequence:
decoding a quantizer index for the given frame, wherein the quantizer index indicates a picture quantizer scale for the given frame;
comparing the quantizer index to a comparison value;
responsive to the quantizer index being less than or equal to the comparison value, decoding a half-step quantization parameter field, the half-step quantization parameter field indicating a half-step increment for the picture quantizer scale for the given frame, and selectively adjusting the picture quantizer scale for the given frame according to the half-step increment, wherein syntax of the bitstream allows half-step picture quantization step sizes as specified by the quantizer index and the half-step quantization parameter field when the quantizer index is less than or equal to the comparison value but allows only integer-step picture quantization step sizes as specified by the quantizer index when the quantizer index is greater than the comparison value; and
inverse quantizing at least some quantized transform coefficients for the given frame.
16. The computer system of claim 15 , wherein the quantizer index is a 5-bit field, and wherein the quantizer index is signaled in a frame header for the given frame.
17. The computer system of claim 15 , wherein the half-step quantization parameter field is a 1-bit field, and wherein the half-step quantization parameter field is signaled in a frame header for the given frame.
18. The computer system of claim 15 , wherein the selectively adjusting the picture quantizer scale for the given frame includes:
if the half-step quantization parameter field is 1, adding ½ to the picture quantizer scale for the given frame; and
otherwise, the half-step quantization parameter field being 0, not adjusting the picture quantizer scale for the given frame.
19. The computer system of claim 15 , wherein the inverse quantizing includes performing operations according to dequant_coeff=quant_coeff*(2*quant_scale+halfstep), where dequant_coeff is a dequantized transform coefficient, quant_coeff is a quantized transform coefficient, quant_scale is the picture quantizer scale for the given frame, and halfstep is the half-step quantization parameter field.
20. The computer system of claim 15 , wherein the inverse quantizing includes performing operations according to dequant_coeff=quant_coeff*(2*quant_scale+halfstep)+sign(quant_coeff)*quant_scale, where dequant_coeff is a dequantized transform coefficient, quant_coeff is a quantized transform coefficient, quant_scale is the picture quantizer scale for the given frame, halfstep is the half-step quantization parameter field, and sign(x) is a function defined as +1 for x≥0, or −1 for x<0.