Notice of Meeting - Output Transforms Architecture VWG - August 23rd, 2023

Output Transforms Architecture VWG Meeting

Wednesday, August 23rd, 2023
1pm PDT Add to calendar (.ics)

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The recording and notes from meeting #115 are now available.

A few comment:

Can we check how people exactly judge SDR Vs HDR. I mean do we have a protocol on how we judge the match. I want to make sure there are no system inherent misalignments which creep into the fine tuning of the model parameters.

It is a logical contradiction to say that a certain display referred value is above the original scene value because you cannot compare a relative scale with an absolute. You could argue that from a practical standpoint a DRT should not move high saturation chromaticities outwards otherwise it is impossible to predict out of gamut values from a scene referred standpoint.

What always strikes me on Alex’s Cartesian plots of hellwig JMH scales is how the image data “explodes outwards” for bright saturated colours. This does not seem to align with my perception of those stimuli. I have the feeling that the model becomes too “sensitive” for bright saturated colours. Or in other words the Euclidian distribution of those point clouds look strange to me.

If you do RGB ratio constant tonemapping (and I would also suggest you do) special attention needs to be placed on the construction of the achromatic signal. That construction of the norm will determine where each chromaticity lands on the tone mapping function and therefore affects the look of the result.

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That is true. What we really mean is that they are higher relative to our output reference white than the input is relative to our (somewhat arbitrarily chosen) input reference white.

Do they actually explode outwards? Or are you talking about the point in the process where J has been tone mapped down, but M still has its original value? Something that starts with high J and M values will appear to “explode out” if J is compressed but M is not yet scaled.

I think Luke said it well in the last meeting when pointing out we would want a solid foundation to build on. It seems that much of the work being done currently is to correct for distortions introduced by the Hellwig model. I did some initial work for fun porting v42-pex to gaffer. Here is a quick demo showing the difference between the tonescale applied in Hellwig, vs using a luminance norm. Might be a similar but more sane starting point than the tremulous foundation all the rest of the work is currently resting upon.

aces2_luminance_vs_hellwig_tonescale_rename_file_extension_to_gfr.txt (503.4 KB)