[x264-devel] x264 level for 1080i content

[BugMaster]

On Thu, 23 Jun 2016 12:13:04 +0200, Weasel Friend wrote:
> Hi,

> today I tried to encode interlaced content with 1920x1080 and 4Mbps
> and was wondering why x264 auto detects level 5.2 for it. When doing
> the same settings but with progressive output the level is detected
> as 5.0. As of my understanding DPB should not be affected by interlaced content?

> x264 command I used:
> x264 - --demuxer y4m --profile main --preset slow --bitrate 4000 --bff --output video.h264

> Relevant output with --bff
> y4m [info]: 1920x1080i 1:1 @ 2997/100 fps (cfr)
> x264 [warning]: interlace + weightp is not implemented
> x264 [warning]: interlaced (1) > level limit (0)
> x264 [info]: profile Main, level 5.2

> Relevant output without --bff
> y4m [info]: 1920x1080p 1:1 @ 2997/100 fps (cfr)
> x264 [info]: profile Main, level 5.0

> I know I can restrict the level to 5.0 but I am curious if there
> are any additional level requirements for interlaced content that I missed?

> Thanks

Hi. I am dunno if this can be counted as x264 bug. You should specify
`--level 4.1` or `--ref 4` in this case. Here why it happens:
1) you use `--preset slow` which have default ref=5;
2) you didn't specified `--level` so x264 do NOT try to reduce ref
count (because it doesn't know to which level);
3) x264 tries to find level that allow specifed DPB (with ref=5) but
because levels from 1.0 to 4.2 don't allow such DPB we skip them;
4) but we skip levels from 5.0 to 5.1 too because the last level that
allowed interlaced encoding was level 4.1;
5) so we stop at level 5.2 because it is the last one we check and we
need to specify some level anyway (even thou level 5.2 also doesn't
allow interlaced encoding).