[x264-devel] x264 bug: interlaced video missing grain/detail in top 32 lines

[Jason Garrett-Glaser]

On Fri, Mar 12, 2010 at 6:33 AM, Adam Warner <lists at consulting.net.nz> wrote:
> On Fri, 2010-03-12 at 05:42 -0800, Jason Garrett-Glaser wrote:
>> On Fri, Mar 12, 2010 at 5:27 AM, Adam Warner <lists at consulting.net.nz> wrote:
>> > Hi all,
>> >
>> > I have found a bug in x264's interlacing.
>>
>> This is not a bug.  This is intentional, and part of how adaptive
>> quantization works.  If you want to avoid it, don't stick gigantic
>> black bars on the top of your videos.
>
> This explains why I was unable to reproduce the missing grain/detail in
> the top 32 lines of other (borderless) grainy video clips.
>
> It's good to learn about this limitation of the adaptive quantization
> algorithm. Since interlaced fields are expanded into full height frames
> the missing grain/detail is, unfortunately, twice as pronounced.
>
> Thanks for all your great work on x264. I've recently enjoyed reading
> your blogging about the complexities of film grain optimization and
> wavelets. Since you have a big Blu-Ray compatibility release coming up
> soon I wanted to ensure no bugs crept in. Sorry I was mistaken about
> x264's output in this instance being a bug.

Indeed, while "not a bug", it is a weakness.  The problem is that I
don't know of any particularly good method of avoiding the problem.

We could special-case the top and bottom rows, but that would cause
problems if the padding was larger than one row.  The core problem is
a general one of sharp edges between non-complex areas, but whenever I
try to make adaptive quantization ignore such edges, it hurts other
cases in the progress of helping those.

It's hard.

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