2-pass ABR results in lower quality on grainy sources
Typically occurs on sources with high amounts of grain (analog, shot on film). The used encoder preset makes little to no difference on the behavior.
In case of the used sample, the second encode pass starts to produce better results when the ABR is set to 24Mbit/s. However, in the longer encodes of the same source, the required bitrate for the 2-pass mode to produce better quality can be as high as 50Mbit/s.
Encoder: X265 3.0 Au+24 10b (GCC, MSVC, ICL, makes no difference) CPU: Core i9-7960X (MMX2 SSE2Fast LZCNT SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2) Input to encoder: FFMpeg pipe (w/ -pix_fmt yuv420p10le -strict -1) or direct YUV (makes no difference)
Sample: https://1drv.ms/u/s!Ag6oE4SOsCmDhVFn-idEiRUMMGjd (258MB)
--preset medium --bitrate 15000 --pass 1
x265 [info]: frame I: 3, Avg QP:25.68 kb/s: 33601.28 x265 [info]: frame P: 62, Avg QP:27.25 kb/s: 22102.00 x265 [info]: frame B: 251, Avg QP:29.88 kb/s: 13237.02 x265 [info]: Weighted P-Frames: Y:0.0% UV:0.0% x265 [info]: consecutive B-frames: 3.1% 0.0% 0.0% 1.5% 95.4%
encoded 316 frames in 35.08s (9.01 fps), 15169.68 kb/s, Avg QP:29.32
VMAF 1.3.14 (vmaf_4k_v0.6.1.pkl, harmonic mean)
VMAF: 68.0785 PSNR: 28.9692 SSIM: 0.94678 MS_SSIM: 0.845463
--preset medium --bitrate 15000 --pass 2
x265 [info]: frame I: 3, Avg QP:25.36 kb/s: 34306.88 x265 [info]: frame P: 62, Avg QP:27.51 kb/s: 21258.44 x265 [info]: frame B: 251, Avg QP:30.03 kb/s: 12834.82 x265 [info]: Weighted P-Frames: Y:0.0% UV:0.0% x265 [info]: consecutive B-frames: 3.1% 0.0% 0.0% 1.5% 95.4%
encoded 316 frames in 30.85s (10.24 fps), 14691.41 kb/s, Avg QP:29.50
VMAF 1.3.14 (vmaf_4k_v0.6.1.pkl, harmonic mean)
VMAF: 67.7415 PSNR: 28.9096 SSIM: 0.945906 MS_SSIM: 0.843289
--preset medium --tune grain --bitrate 15000 --pass 1
x265 [info]: frame I: 3, Avg QP:32.00 kb/s: 11861.03 x265 [info]: frame P: 62, Avg QP:30.03 kb/s: 15415.52 x265 [info]: frame B: 251, Avg QP:30.04 kb/s: 14711.44 x265 [info]: Weighted P-Frames: Y:0.0% UV:0.0% x265 [info]: consecutive B-frames: 3.1% 0.0% 0.0% 1.5% 95.4%
encoded 316 frames in 44.01s (7.18 fps), 14822.52 kb/s, Avg QP:30.05
VMAF 1.3.14 (vmaf_4k_v0.6.1.pkl, harmonic mean)
VMAF: 68.4474 PSNR: 28.3956 SSIM: 0.937947 MS_SSIM: 0.830487
--preset medium --tune grain --bitrate 15000 --pass 2
x265 [info]: frame I: 3, Avg QP:29.00 kb/s: 20021.71 x265 [info]: frame P: 62, Avg QP:30.19 kb/s: 15423.09 x265 [info]: frame B: 251, Avg QP:30.13 kb/s: 14732.66 x265 [info]: Weighted P-Frames: Y:0.0% UV:0.0% x265 [info]: consecutive B-frames: 3.1% 0.0% 0.0% 1.5% 95.4%
encoded 316 frames in 38.32s (8.25 fps), 14918.34 kb/s, Avg QP:30.13
VMAF 1.3.14 (vmaf_4k_v0.6.1.pkl, harmonic mean)
VMAF: 68.2208 PSNR: 28.3872 SSIM: 0.93765 MS_SSIM: 0.829982
--preset medium --bitrate 25000 --pass 1
x265 [info]: frame I: 3, Avg QP:21.90 kb/s: 59019.12 x265 [info]: frame P: 62, Avg QP:24.60 kb/s: 34893.77 x265 [info]: frame B: 251, Avg QP:26.94 kb/s: 22434.47 x265 [info]: Weighted P-Frames: Y:0.0% UV:0.0% x265 [info]: consecutive B-frames: 3.1% 0.0% 0.0% 1.5% 95.4%
encoded 316 frames in 41.14s (7.68 fps), 25226.34 kb/s, Avg QP:26.43
VMAF 1.3.14 (vmaf_4k_v0.6.1.pkl, harmonic mean)
VMAF: 72.9672 PSNR: 30.33 SSIM: 0.960212 MS_SSIM: 0.879982
--preset medium --bitrate 25000 --pass 2
x265 [info]: frame I: 3, Avg QP:22.80 kb/s: 48710.52 x265 [info]: frame P: 62, Avg QP:24.76 kb/s: 34046.87 x265 [info]: frame B: 251, Avg QP:27.05 kb/s: 21899.63 x265 [info]: Weighted P-Frames: Y:0.0% UV:0.0% x265 [info]: consecutive B-frames: 3.1% 0.0% 0.0% 1.5% 95.4%
encoded 316 frames in 37.52s (8.42 fps), 24537.48 kb/s, Avg QP:26.56
VMAF 1.3.14 (vmaf_4k_v0.6.1.pkl, harmonic mean)
VMAF: 72.7268 PSNR: 30.2663 SSIM: 0.959774 MS_SSIM: 0.878847
--preset medium --tune grain --bitrate 25000 --pass 1
x265 [info]: frame I: 3, Avg QP:28.00 kb/s: 25337.86 x265 [info]: frame P: 62, Avg QP:27.35 kb/s: 25724.24 x265 [info]: frame B: 251, Avg QP:27.34 kb/s: 24600.01 x265 [info]: Weighted P-Frames: Y:0.0% UV:0.0% x265 [info]: consecutive B-frames: 3.1% 0.0% 0.0% 1.5% 95.4%
encoded 316 frames in 45.51s (6.94 fps), 24827.60 kb/s, Avg QP:27.35
VMAF 1.3.14 (vmaf_4k_v0.6.1.pkl, harmonic mean)
VMAF: 73.1188 PSNR: 29.6759 SSIM: 0.953269 MS_SSIM: 0.865493
--preset medium --tune grain --bitrate 25000 --pass 2
x265 [info]: frame I: 3, Avg QP:26.67 kb/s: 30383.25 x265 [info]: frame P: 62, Avg QP:27.32 kb/s: 25679.16 x265 [info]: frame B: 251, Avg QP:27.28 kb/s: 24592.51 x265 [info]: Weighted P-Frames: Y:0.0% UV:0.0% x265 [info]: consecutive B-frames: 3.1% 0.0% 0.0% 1.5% 95.4%
encoded 316 frames in 41.35s (7.64 fps), 24860.69 kb/s, Avg QP:27.28
VMAF 1.3.14 (vmaf_4k_v0.6.1.pkl, harmonic mean)
VMAF: 73.3299 PSNR: 29.7398 SSIM: 0.953795 MS_SSIM: 0.866939