15.01.2022, 18:23
Just to complete the comparison, in the chart below you can find the value of Netflix VMAF for the movies encoded with x265 (CRF=22) and NVenc with CQ=27 (aka CRF=27).
![[Image: VMAF-CRF22-vs-CQ27-small.jpg]](https://i.ibb.co/1RfVcjs/VMAF-CRF22-vs-CQ27-small.jpg)
I think that VMAF is the best metric to compare rate factors that try to improve the perceived quality. The VMF obtained by NVEnc of 95.28 is very near to VMF of 95.31 obtained with the x265 software encoder. So at cost of an increase of about 5% in the BitRate, the NVEnc is able to encode at about 5x the speed of x265 with a comparable quality. Moreover the Constant Quality (aka CRF) of NVEnc is very near to the average "quantization" while it seems that the CRF used by x265 is not linked to the final average "quantization" of the encoded movie. As a rule of thumb it is necessary to increase of about 20% the CRF used by x265 to obtain the equivalent CQ value used by NVEnc.
Another interesting chart is the following
![[Image: VMAF-CQP-vs-CQ-small.jpg]](https://i.ibb.co/ry4vYR8/VMAF-CQP-vs-CQ-small.jpg)
In this case I compared the movie encoded with NVenc using CQP:20-22-28, with the movie encoded using Constant Quality (aka CRF) = 27.8. The reason why I used the CQ of 27.8 is because, it provides almost the same BitRate obtained by using CQP:20-22-28. In this case the VMAF obtained using the "constant quantizer" is 94.2, while the VMAF obtained with "constant quality" is 95.0. So despite that the size of encoded movies is almost the same, the quality obtained with the "constant quality" is better. The reason is that the CQP uses the same "quantization" for all the frames, while the "constant quality" uses a better algorithm and is able to increase the quality on frames where the details are more visible (bright frames) and lower the quality when they are less visible (dark frames), in this way this rate control is able to allocate better the "quantization". It is some kind of VBR 2-pass encoding where the target is not the size but the overall "quantization", probably this is the reason why NVIDIA called this rate control: "Target Quality".
I think that VMAF is the best metric to compare rate factors that try to improve the perceived quality. The VMF obtained by NVEnc of 95.28 is very near to VMF of 95.31 obtained with the x265 software encoder. So at cost of an increase of about 5% in the BitRate, the NVEnc is able to encode at about 5x the speed of x265 with a comparable quality. Moreover the Constant Quality (aka CRF) of NVEnc is very near to the average "quantization" while it seems that the CRF used by x265 is not linked to the final average "quantization" of the encoded movie. As a rule of thumb it is necessary to increase of about 20% the CRF used by x265 to obtain the equivalent CQ value used by NVEnc.
Another interesting chart is the following
In this case I compared the movie encoded with NVenc using CQP:20-22-28, with the movie encoded using Constant Quality (aka CRF) = 27.8. The reason why I used the CQ of 27.8 is because, it provides almost the same BitRate obtained by using CQP:20-22-28. In this case the VMAF obtained using the "constant quantizer" is 94.2, while the VMAF obtained with "constant quality" is 95.0. So despite that the size of encoded movies is almost the same, the quality obtained with the "constant quality" is better. The reason is that the CQP uses the same "quantization" for all the frames, while the "constant quality" uses a better algorithm and is able to increase the quality on frames where the details are more visible (bright frames) and lower the quality when they are less visible (dark frames), in this way this rate control is able to allocate better the "quantization". It is some kind of VBR 2-pass encoding where the target is not the size but the overall "quantization", probably this is the reason why NVIDIA called this rate control: "Target Quality".