increasing b-frame is good or not?

[Audionut]

Originally Posted by huang_ch

and x264 does seems to use more b frames after the max is set to 16, but the overall bitrate & PSNR is also decrease a bit.

Hence why people don't allow the encoder to use a maximum of 16 b-frames and set the maximum to say 3 b-frames.

However!!! Your first post states that you did not see any major loss of video quality to your eyes with a max of 16.
So don't bother with metric calculations. Just use max 16.

[Sagittaire]

--crf 24 --ref 5 --mixed-refs --bframes 16 --b-pyramid --b-rdo --bime --weightb --direct auto --filter 4,4 --subme 6 --trellis 2 --analyse all --8x8dct --qpmin 14 --qpmax 28 --ipratio 1.6 --pbratio 1.6 --me umh --progress --no-dct-decimate

1) Well you must use constant size if you want make real comparison. More and more bframe with ratio for bframe imply less size and less quality too simply because average quantizer is more and more high.

2) If you want see bframe adaptive efficacity use crf with ratio = 1.0 for bframe. Here IMO if the adaptive algo is good then size will be less important with constant quality.

3) ipratio = 1.6 is totaly useless. Quality for IFrame will be too high and will produce certainely quality dropp.

[huang_ch]

I've tried to increase the value of -crf(25,26) to get the bitrate of bframe-3 closer to 288Kbps (b-frame16) and the PSNR is also about 0.3 higher than b-frame16 with crf24. Does this mean I should prefer less b-frame with higher crf than higher b-frame with lower crf to encode this content? Even try b-frame1?

[huang_ch]

Originally Posted by Sagittaire

3) ipratio = 1.6 is totaly useless. Quality for IFrame will be too high and will produce certainely quality dropp.

I'm not quit familiar with --ipratio & --pbratio, but after I increase them all to 1.6 from the default 1.4/1.3, the PSNR really increased a bit. But I didn't try to individually increase them, will try later.

[Sagittaire]

Well if you don't know then simply don't touch default setting ...

Use 16 adaptative bframe is simply useless because in most case x264 never use it (no more than 3 bframe for perhaps 99.9% of the cases). IMO best setting are 2 bframes or 3 pyramidal bframes. Use "over setting" for max bframe is simply useless and potentially worst if you use no adaptative bframe.

[MeteorRain]

Originally Posted by huang_ch

I'm not quit familiar with --ipratio & --pbratio, but after I increase them all to 1.6 from the default 1.4/1.3, the PSNR really increased a bit. But I didn't try to individually increase them, will try later.

eg if you use ip/pb ratio 1.6, and if your p-frame is encoded in q24, then i-frame would be q15 and b-frame would be 38~39

mean, i-frame encoded in really good quality, and b-frame encoded in really bad quality.

when you use more b-frames, since the quantizer rises, the filesize reduces, the quality decreases.

if you want a real comparation about the problem, you could consider setting ip/pb ratio to 1.0, or just use -qp instead of -crf. in that case, b-frame is encoded at the same quantizer of p-frame. using more b-frames won't decrease quality lot.

[akupenguin]

Originally Posted by MeteorRain

eg if you use ip/pb ratio 1.6, and if your p-frame is encoded in q24, then i-frame would be q15 and b-frame would be 38~39

No. ip- & pb-ratio are measured in bitrate (as a bonus on top of the different rates inherently caused by different frame types), not in QP.
QP is an exponential scale, so it does not make sense to ever multiply a QP by anything.

[huang_ch]

Originally Posted by akupenguin

No. ip- & pb-ratio are measured in bitrate (as a bonus on top of the different rates inherently caused by different frame types), not in QP.
QP is an exponential scale, so it does not make sense to ever multiply a QP by anything.

Is the ratio used as a constant ratio or an adaptive(maximum) ratio?

[akupenguin]

pb-ratio is constant.
ip-ratio is a maximum, and in 2pass it will decrease for very short GOPs.

[Audionut]

That explains why, with a low ip-ratio it will often result in a lower average P quant than I quant.

Thanks.

[akupenguin]

What explains what?

The reason you can have higher average I quant than average P-quant is that high motion scenes get higher quant than low motion, and high motion scenes have more scenecuts.
That would be true regardless of how ip-ratio works, though for large ip-ratio you'd need a very uneven distribution for the averages to still end up like that.
(Same for P vs B)

[Audionut]

With all the encodes I have done thus far, with the default ip-ratio of 1.4, the P frame quantizers are always are higher value than I frame quantizers.
Say I-frames = quant 17 and P-frames = quant 18

But if I reduce the ip-ratio to 1.05 or even 1.10, then often (not always), I generally end up with P frame quantizers that have a lower value than I frame quantizers.
Say I-frames = quant 17 and P-frames = quant 16.8

I never expanded on why this would be the case.
When you said that ip-ratio is a maximum value and not an absolute value I assumed that was the reason why lower ip-ratio's would result in the findings as stated above.


Thanks for the clarification. Again.

[unmei]

Originally Posted by Sagittaire

Use 16 adaptative bframe is simply useless because in most case x264 never use it (no more than 3 bframe for perhaps 99.9% of the cases).

I definitly wouldn't say that. Or a lot of my anime and esp. cartoon encodings fall into the remaining 0.1%
I never checked the stats file, but from x264 stats at the end of the encode it's not so uncommon for me to have close to 3 B-frames per one P-Frame and in a recent case i had over 3.5 B-frames per P-frame (as per the total number of frames encoded in each mode, thus the overall average).

[huang_ch]

Originally Posted by Audionut

But if I reduce the ip-ratio to 1.05 or even 1.10, then often (not always), I generally end up with P frame quantizers that have a lower value than I frame quantizers.
Say I-frames = quant 17 and P-frames = quant 16.8

Does that mean, if I reduced ip ratio to rather low, and got quant of p higher than I, the overall quality/quant will be improved? Is it the same to b-frames?