Wrong output colours using ffmpeg to convert to h264

[Twinbee]

Hi all, I'm a beginner to ffmpeg (latest 5.1.1 "essentials build" by www.gyan.dev, Windows 10).

Didn't have much luck tracking this down at the Vegas forum or the ffmpeg mailing list, so trying here as a maybe last resort. It's incredibly easy to replicate thankfully. I want to convert numerous frames to make an animation, but I've simplified the problem to using a single image to make a '1 frame video' for the purposes of debugging.

Simply perform this command line with this "original.png" image.:

ffmpeg.exe -i original.png -crf 0 -vcodec libx264 output.mp4

And this command line On this "doubleHeight" image.:

ffmpeg.exe -i doubleHeight.png -crf 0 -vcodec libx264 output.mp4

The double height version is darker than it should be. I've checked the resulting video in both Media Player Classic and Chrome.

If you check the dark green colour on the original PNG images, using an eye dropper tool, they're both R=25,G=74,B=15. However, if you check the same colour on the output MP4s, the colour matches on the original PNG, but not the doubleHeight version, which is R=22,G=66,B=12.

If I use -vcodec libx264rgb, instead of -vcodec libx264, that fixes the issue, but I need libx264 so the output video can work on Chrome and other media players. The problem seems to occur in Media Player Classic with a height greater than or equal to 578, and in Chrome with a height greater than or equal to 720.

What gives?

[Selur]

You did not specify a color matrix for the yuv<>rgb conversion thus ffmpeg is guessing based on the resolution.
-> you might want to read up on vui and how to tell ffmpeg which color matrix to use and how to signal it to the output.

[Twinbee]

Thanks. I asked on the ffmpeg mailing list too, and someone recommended both of these. Maybe you'd prefer to use one over the other?

ffmpeg -i input.png -t 5 -vf "zscale=m=709" -c:v libx264 -crf 0 output.mp4

ffmpeg -i input.png -t 5 -color_range tv -colorspace bt470bg -c:v libx264 -crf 0 output.mp4

[Selur]

a. Whether you use 709 or bt470gb color space should depend on the source.
b. doesn't really matter whether you use zscale or the separate flagging, I often use both.