Subtitle blending appears incorrect in HDR video

When viewing HDR videos with SSA/ASS subtitles, it looks like the SDR subtitles aren’t blended with the HDR video correctly. The following three images show an example of the exact same subtitles on SDR and HDR versions of the same video.

I don’t know what library you use to render subtitles, but libass has some possibly related discussion: add ASS header field "transfer function" (to define HDR vs SDR etc) · Issue #297 · libass/libass · GitHub

AppleTV (HDR)

iPhone (HDR)

iPhone (SDR)

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Do you have a sample video and subtile you would be able to send in so we can look into this?

Thanks! I’ve uploaded two sample videos with the same subtitles that I used to get the above images, one in SDR and the other in HDR. The subtitles are embedded in the videos, but you can extract them with ffmpeg -i hdr.mkv -map 0:s:0 -c copy hdr.ssa.