[FFmpeg-devel] [PATCH] Fix chroma positioning for 4:2:0 pixel format

[Andy Furniss]

Michael Niedermayer wrote:
> On Mon, Feb 06, 2017 at 05:14:14PM +0200, Maksym Veremeyenko wrote:
>> Hi,
>>
>> Attached patch fixes chroma positioning during scaling interlaced 4:2:0.
>>
>> On a first iteration default context value been overwritten by new
>> value and not been update on next iterations for fields. This mean
>> that vertical chroma position remain 128 for field#0 and field#1
>> instead of *64* and *192*.
>>
>> Attached patch use local variable for storing this intermediate
>> value of chroma vertical position not modifying default context
>> value.
>>
>> --
>> Maksym Veremeyenko
>>
>
>>   vf_scale.c |    9 +++++----
>>   1 file changed, 5 insertions(+), 4 deletions(-)
>> 6a3a3966939dd87c45a8e124b0364da30b02728b  0001-Fix-chroma-positioning-for-4-2-0-pixel-format.patch
>>  From 912ecf538b6b2f7a8df4afdfed2d34052162335c Mon Sep 17 00:00:00 2001
>> From: Maksym Veremeyenko <verem at m1.tv>
>> Date: Mon, 6 Feb 2017 17:03:17 +0200
>> Subject: [PATCH] Fix chroma positioning for 4:2:0 pixel format
>
> applied
>
> thx

Nice, testing wise it raises a question to how swscale should work by
default with interl=1 and 420 to rgb (maybe other cases as well).

In summary this patch will appear to fail with a test like.

ffmpeg -i 420-interlaced.ts -ss 11.2 -vf scale=interl=1 -vframes 1 out.png

Will look just as broken as before the patch. The addition of -

-sws_flags +accurate_rnd

fixes it, adding +full_chroma_int looks better still.