HDRCompare: compare SDR and HDR video files

Brought to you by Etienne Dechamps - GitHub

Description

HDRCompare is a suite of ffmpeg/libavfilter filters for comparing the SDR and HDR versions of otherwise identical video material. It is useful to determine how well HDR is used in various movie releases, for example.

The basic idea is to take the SDR video and then convert it to HDR, without expansion (i.e. preserving the original color and luminance), and then show it side by side with the HDR video, allowing for an easy and fair comparison.

The following filter chains are provided:

• hdr-compare.lavfi presents the left half of the HDR video on the right side, and the right half of the SDR video on the right side. Every second, the two sides switch places.
• hdr-diff.lavfi divides the display in 4 quadrants:
  • Upper left: original HDR video
  • Lower left: original SDR video
  • Upper right: extra luminance in the HDR video
  • Lower right: extra saturation in the HDR video

hdr-compare is useful to get an overall feel of the difference between HDR and SDR across entire scenes. hdr-diff is useful to detect and surface any local differences.

Limitations

• This is a quickly-thrown-together toy, not a tool designed for serious use. Do not expect it to be accurate. No effort was spent fine-tuning various parameters. The SDR to HDR colorspace conversion, in particular, is dubious at best.
  • A more serious attempt would use a proper CMS to do the gamut mapping from SDR to HDR, and would likely leverage a DeltaE formula or similar to highlight the differences in a perceptually accurate manner.
• The SDR and HDR input videos must be as identical as possible (aside from the colorspace difference, obviously).
  • This is especially important for hdr-diff, which will behave very poorly if the input is not frame-by-frame synchronized or has the slightest difference in cropping.
  • They can be different sizes though. Typically, HDR files are 2160p while SDR files are 1080p. We want to focus purely on colour differences, so the filters will always scale both streams to 1080p for a fair comparison.
• The filters assume the HDR input is HDR10 (i.e. BT.2020+PQ). Dolby Vision and HLG are not supported.
• HDR metadata (i.e. peak luminance/maxCLL information) is not preserved, so the HDR tone mapping is unlikely to be optimal.
  • This can be work around by manually specifying the metadata after the filter, but note that mpv won't pass the metadata through to the display, in any case.
• The filters are expensive to compute. On a good PC hdr-compare can typically run in real time. hdr-diff typically won't, though it will come close.

How to use

You can apply the filter in real time using any player that is capable of applying a libavfilter filter chain. mpv is the most obvious choice and it's what the filters were developed with.

Here's an example mpv command line using Powershell:

mpv `
    --lavfi-complex="$((Get-Content -Path .\hdr-compare.lavfi) -notmatch '^\s*#')" `
    --vf=format=colormatrix=bt.2020-ncl:colorlevels=limited:primaries=bt.2020:gamma=pq `
    <PATH TO HDR FILE> --external-file=<PATH TO SDR FILE>

Note: SDR uses relative luminance, but HDR uses absolute luminance. For this reason it is fundamentally difficult to ensure that both are presented at the same average brightness level. Currently the filter maps 100% SDR level to 150 nits in HDR. This is mostly arbitrary and will likely be slightly off for most content. If you observe that the HDR side is brighter overall than the SDR side (or vice-versa), search for the npl= parameter in the relevant .lavfi file to adjust the brightness of the SDR side. Note that you might find that even different scenes of a single movie might call for a different value.