x264 with --bluray-compat Ref Frames

[ACrowley]

Hi

i made a few Encodes with x264 and --bluray-compat Setting.
I know it overrides the X264 Settings to follow Bluray Specs.

I usally set Ref Frames to --ref 4 for Level 4.1 Compliant 1920x1080 encodes but x264 outputs always 3 with Bluray Compatible ?


Simple x264 cmd :
--level 4.1 --bluray-compat --preset veryslow --open-gop --ref 4 --slices 4 --vbv-bufsize 30000 --vbv-maxrate 40000

I hope somebody can clafiry this x264 behaviour

Greetings

[jpsdr]

If i remember properly, there is something about the ref frame reduced by 1 (or something similar) when pyramid is enabled, which is the default setting. BTW, if bluray-compat doesn't change the pyramid setting (i don't remember if it does), it should pe put on "strict".

[ACrowley]

Thanks !

I found it :


B-Pyramid "Normal" = not reducing Ref Frames
B-Pyramid "Strict" = 1 reduced Ref Frame ,

--bluray-compat overrides to Strict

[blob2500]

Originally Posted by ACrowley

Thanks !

I found it :


B-Pyramid "Normal" = not reducing Ref Frames
B-Pyramid "Strict" = 1 reduced Ref Frame ,

--bluray-compat overrides to Strict

Some time ago I had done some tests to better understand what was meant by "strict".

IbBbPbBbP....

With this option active (B-pyramid "Strict"), it turned out to me that B-frames cannot refer to other B-frames, but only to P-Frames (and I). And of course, much less P-Frames cannot refer to B-Frames, but I and P only. But the "strict" option alone does not limit the number of frames to refer to 1.


The hierarchy is rigorous: I --> P --> B --> b

with "normal": I --> P/B <--> B --> b

[benwaggoner]

However, doesn't the Blu-ray spec require Strict?

[sneaker_ger]

BluRay only allows "strict" or no b-pyramid, but like ACrowley said: --bluray-compat will (silently) enforce that.

[benwaggoner]

The whole definition of "number of reference frames" is a little too ambiguous . I believe there are still actually 4 reference frames max in the decoded picture buffer with BD, but one is a "different" kind from the x264 perspective.

[wyup]

Apart from vbv-maxrate=40000 and vbv-bufsize=30000, what is the max overall bitrate supported for a blu-ray?
It is an additional setting of x264: -B, --bitrate <integer> on Ratecontrol.

[benwaggoner]

Originally Posted by wyup

Apart from vbv-maxrate=40000 and vbv-bufsize=30000, what is the max overall bitrate supported for a blu-ray?
It is an additional setting of x264: -B, --bitrate <integer> on Ratecontrol.

The maxrate and bufsize define the max overall bitrate. The --bitrate parameter defines the mean bitrate, and --crf is often used instead.

[jpsdr]

It means it reduces the ref frame value of 1. If you've set 4 ref frame, it's reduced to 3.