[FFmpeg-user] ffmpeg MP4/x264 output colours change when input source is different resolution (bug?)

[Michael Koch]

Am 15.09.2022 um 11:01 schrieb Paul B Mahol:
> On 9/15/22, Michael Koch <astroelectronic at t-online.de> wrote:
>> Am 15.09.2022 um 00:30 schrieb Paul B Mahol:
>>> On 9/15/22, Dan <twinbee42 at skytopia.com> wrote:
>>>>> zscale=...,format=yuv420p
>>>> Ohhhh.... okay. I tried that before (except using two -vf commands),
>>>> because
>>>> I
>>>> suspected you might've meant that.
>>>>
>>>> Just tried it again, still no luck. Let me know if I need to tweak
>>>> anything:
>>>>
>>>> ffmpeg.exe -f lavfi -i color=0x19be0f:s=400x578 -crf 0 -vcodec libx264
>>>> -vf
>>>> zscale=w=-1:h=-1,format=yuv420p -t 5 -y "578.mp4"
>>>>
>>>> I'm using Media Player Classic to test the colours, which breaks the pic
>>>> using the 578 pixel
>>>> height. Chrome actually works with both the 576 and 578 pixel height, but
>>>> as
>>>> soon as I
>>>> change the height to 984, then both Media Player Classic AND Chrome show
>>>> it
>>>> broken.
>>>>
>>>> Datascope shows the strange and seemingly unrelated 78,4C,44 values for
>>>> all
>>>> three sizes,
>>>> but it does that even without using zscale at all.
>>> Good, we move forward, that are real values encoded in bitstream.
>>>
>>> Anything RGB values you complain about have nothing directly related
>>> about ffmpeg.
>>>
>>> Also make sure that all software are reinterpreting your files correctly.
>>> The files need to signal correct encoded colorspace/primaries/range/etc
>>> so
>>> it can be correctly displayed on screen.
>> Below is a Windows batch file for reproducing with FFplay. I did use the
>> latest version from Gyan. The color difference is clearly visible.
> Looks like you had hard time understanding text you quoted above.
>
>> ffmpeg -f lavfi -i color=0x19be0f:s=400x576 -vf
>> zscale,setrange=full,format=yuv420p -colorspace bt709 -color_primaries
>> bt709 -crf 0 -vcodec libx264 -t 5 -y out1.mp4
>> ffmpeg -f lavfi -i color=0x19be0f:s=400x578 -vf
>> zscale,setrange=full,format=yuv420p -colorspace bt709 -color_primaries
>> bt709 -crf 0 -vcodec libx264 -t 5 -y out2.mp4
>> start ffplay -left 0 -top 0 out1.mp4
>> start ffplay -left 400 -top 0 out2.mp4
>>
>> When inspecting the files with FFprobe or ExifTool, I see no differences
>> except height and small differences in bitrate and filesize.
> ffplay is broken/buggy and should not be used here, it will display
> differently stuff
> all the time, and uses SDL library by default for video output.
>
> Using datascope filter it clearly shows pixels are exactly same.

You are right that datascope shows no difference. But the issue is also 
reproducible with VLC player.