Abstract
This document contains results (bitrate/visual quality) of applying x265 (in ABR mode) on two 4K streams (Sintel and TOS), with different lookahead buffer sizes (in frame slots: 3 frames and 20 frames), with two different bitrates 30Mbps and 20Mbps and with two cutree values on/off.
For video quality both PSNR and VIFP metrics are used.
Note: x265 does not allow lookahead buffer size in frames less or equal to the number of consecutive B-frames.
Original streams
All sequences taken from https://media.xiph.org/
Sintel – 3840×1744 resolution, frame rate 24fps.
TOS – resolution 3840×1712, frame rate 24fps
x265 Configuration
All streams are coded with the following fixed parameters:
max CU size = 64×64, min CU size = 8×8
max TU size = 32×32, TU depth = 1 (for both inter and intra)
ME = hex, range =57, subpel =2 , merge =2
Scenecut=0
Gop size = 30 frames
Bframes = 2 (IPBBPBB..), badapt = yes, no b-pyramid
Refs = 2
Signhide = yes
Tmvp = yes
strong-intra-smoothing = yes (since 32×32 TU used)
deblock, sao = yes
Results
Note: In each table psnr and vifp quality statistics is presented as a pair (mean, std).
Sintel (117 frames, 3840×1744), Cutree = OFF
| Lookahead/Expected Bitrate | 30 Mbps | 20 Mbps |
| 3 frames | 37.8 Mbps
Psnr (m,std): 44.574, 3.232 Vifp (m,std): 0.832, 0.055 |
25.7 Mbps
Psnr (m,std): 43, 3.4 Vifp (m,std): 0.794, 0.06 |
| 20 frames | 37.8
Psnr (m,std): 44.574, 3.232 Vifp (m,std): 0.832, 0.055 |
similar results as
in the above cell |
TOS (128 frames, 3840×1712), Cutree = OFF
| Lookahead/Expected Bitrate | 30 Mbps | 20 Mbps |
| 3 frames | 25.4 Mbps
Psnr (m,std): 42.851, 5.673 Vifp (m,std): 0.711, 0.117 |
17.6 Mbps
Psnr (m,std): 42.333, 5.600 Vifp (m,std): 0.695, 0.117 |
| 20 frames | similar results as
in the above cell |
similar results as
in the above cell |
Sintel (117 frames, 3840×1744), Cutree = ON
| Lookahead/Expected Bitrate | 30 Mbps | 20 Mbps |
| 3 frames | 39.5 Mbps
Psnr: 44.726, 3.175 Vifp: 0.835, 0.057 |
27 Mbps
Psnr: 43.157, 3.384 Vifp 0.797, 0.068 |
| 20 frames | 38.8 Mbps
Psnr: 44.842, 3.346 Vifp: 0.840, 0.060 |
26.8 Mbps
Psnr: 43.385, 3.514 Vifp 0.806, 0.071 |
TOS (128 frames, 3840×1712), Cutree = ON
| Lookahead/Expected Bitrate | 30 Mbps | 20 Mbps |
| 3 frames | 22.8 Mbps
Psnr (m,std): 42.660, 5.530 Vifp (m,std): 0.706, 0.113 |
15.8 Mbps
Psnr (m,std): 42.180,5.452 Vifp (m,std): 0.691, 0.113 |
| 20 frames | 22 Mbps
Psnr (m,std): 42.635, 5.509 Vifp (m,std): 0.706, 0.113 |
15.2 Mbps
Psnr (m,std): 42.183, 5.423 Vifp (m,std): 0.692, 0.111 |
Conclusions
Lookahead buffer length does not impact significantly on coding efficiency regardless target bitrates or cutree values. Therefore minimal lookahead buffer (say 3 frame slots) is sufficient.
23+ years’ programming and theoretical experience in the computer science fields such as video compression, media streaming and artificial intelligence (co-author of several papers and patents).
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