Quote:
Originally Posted by Shevach
Thanks for a prompt reply. Frankly speaking i meant both min/max_display_mastering_luminance (signaled in MasteringDisplayColorVolume SEI) and MaxCLL/MaxFall (signaled in Content Light Level SEI).
What's about MaxCll? After encoding this value might be different from that obtained from source yuv or tiff.
In my opinion, lossy encoding may cause non-negligible changes in min/max_display_mastering_luminance and MaxFall/MaxCll.
Why? If a quantization rounding offset is less 0.5 in an encoder then reconstructed pixels tend to be smaller (leakage of energy). If the quantization rounding offset is above 0.5 then an opposite effect takes place. The quantization rounding offset is like a pump which either inhale or exhale "energy".
Consequently, after encoding MaxFall/MaxCll may differ from those in the original file.
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You are entirely correct that encoding can reduce the MaxFALL and particularly MaxCLL from the source. However, industry standard behavior is to retain the values from the source as indicative of creative intent. Also, with adaptive bitrate switching, different streams could have different encoded metadata values. But Displays don't like mid-stream metadata value changes, so the same values should be used for all bitrates.
THAT said, devices don't make heavy use of MaxFALL and MaxCLL in practice, because knowing what the brightest frame and brightest pixel across an entire movie isn't that useful. Dynamic metadata ala Dolby Vision and SMPTE 2094-40 are much more useful for tone mapping.