feat: tonemap HDR content to SDR using a libplacebo filter

[JB26]

fixes #236

[dirkvdb]

Thanks! I will try to find some time in the coming week to check and merge

[dirkvdb]

If I download an HDR sample from https://kodi.wiki/view/Samples and compare the output against main I don't see a difference. How should I test this?

[JB26]

Thank you for testing

I wanted to prepare some samples and was using ubuntu 25.10 (I developed the change on arch) and was hit by this back trace :(

[libplacebo @ 0x584e6be18240] Validation failed: images->vsync_duration > 0.0 (src/renderer.c:3312)
[libplacebo @ 0x584e6be18240]   Backtrace:
[libplacebo @ 0x584e6be18240]     #0  /lib/x86_64-linux-gnu/libplacebo.so.349(pl_render_image_mix+0x125d) [0x76e604c8bacd]
[libplacebo @ 0x584e6be18240]     #1  /lib/x86_64-linux-gnu/libavfilter.so.10(+0x2f381b) [0x76e60caf381b]
[libplacebo @ 0x584e6be18240]     #2  /lib/x86_64-linux-gnu/libavfilter.so.10(+0x2f4860) [0x76e60caf4860]
[libplacebo @ 0x584e6be18240]     #3  /lib/x86_64-linux-gnu/libavfilter.so.10(+0x1d765d) [0x76e60c9d765d]
[libplacebo @ 0x584e6be18240]     #4  /lib/x86_64-linux-gnu/libavfilter.so.10(+0x1dc286) [0x76e60c9dc286]
[libplacebo @ 0x584e6be18240]     #5  /home/jbroders/Projekte/ffmpegthumbnailer/libffmpegthumbnailer.so.4(_ZN17ffmpegthumbnailer12MovieDecoder19getScaledVideoFrameERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEbRNS_10VideoFrameE+0x1c0) [0x76e610b4f756]
[libplacebo @ 0x584e6be18240]     #6  /home/jbroders/Projekte/ffmpegthumbnailer/libffmpegthumbnailer.so.4(_ZN17ffmpegthumbnailer16VideoThumbnailer17generateThumbnailERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEERNS_11ImageWriterEP15AVFormatContext+0x1bc) [0x76e610b85b0c]
[libplacebo @ 0x584e6be18240]     #7  /home/jbroders/Projekte/ffmpegthumbnailer/libffmpegthumbnailer.so.4(_ZN17ffmpegthumbnailer16VideoThumbnailer17generateThumbnailERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE24ThumbnailerImageTypeEnumS8_P15AVFormatContext+0x7b) [0x76e610b862a1]
[libplacebo @ 0x584e6be18240]     #8  ./ffmpegthumbnailer(+0x5dfe) [0x584e401a5dfe]
[libplacebo @ 0x584e6be18240]     #9  /lib/x86_64-linux-gnu/libc.so.6(+0x2a575) [0x76e61042a575]
[libplacebo @ 0x584e6be18240]     #10 /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x88) [0x76e61042a628]
[libplacebo @ 0x584e6be18240]     #11 ./ffmpegthumbnailer(+0x54e5) [0x584e401a54e5]

I then tested it in a vm to check if it would work on the daily build of ubuntu 26.04 which shows another error:

ubuntu@ubuntu:~/ffmpegthumbnailer$ ./ffmpegthumbnailer -i ~/Downloads/Mystery\ Box\ Dolby\ Vision\ Profile\ 5.mp4 -o /tmp/foobar.png -t 30 -s 720
[libplacebo @ 0x59a227fe8c80] Found no suitable device, giving up.
[libplacebo @ 0x59a227fe8c80] Failed initializing vulkan device
[libplacebo @ 0x59a227fe8c80] Failed creating Vulkan device!
Error: Failed to create libplacebo filter Generic error in an external library

I guess what I'm trying to say is, that I think I made a mistake by choosing libplacebo. I will try if the zscale filter is more stable and works without vulkan. That means no dolby vision support, but most videos have an hdr fallback layer. So I think it would still be helpful.

Again thanks for checking this out and sorry for the noise :/