[x264-devel] Potential bug in transform bypass 8x8

[Jason Garrett-Glaser]

On Fri, Jun 21, 2013 at 3:22 PM, Alain Champenois <achampen at matrox.com> wrote:
>
>> This is a known bug, but unfortunately there's not much that can be
>> done at this point.
>
> I agree. At this stage changing both encoder/decoder to become compliant
> would be a big hassle.
>
> I would however suggest a way to make x264 compliant with the spec while
> keeping compatibility with current libavcodec/coreavec decoders.
>
> If intra8x8 horizontal and vertical modes are disabled in x264 lossless,
> streams encoded would become compliant with the spec while staying compatible
> with any decoder supporting lossless (including libavcodec/coreavc). libavcodec
> wouldn't have to be changed, and existing streams will continu to be deocded
> properly.
>
> I did a small evaluation using the HEVC test sequences, and the size saving
> went from 65.769% to 65.748% when those 2 modes were disabled in x264 using
> the patch below (full results included attachment).
>
> So with negligible impact on compression and no impact on existing decoders,
> x264 can be made compliant with the spec, which would garanty compatibility
> with any current or future decoders (compliant or not).

This can already be done with --no-8x8dct, so I'm not sure it's worth
bothering hacking into x264.

Jason