I was doing a bit of thinking this morning about one of the conflicting design requirement that @KevinJW mentioned in the TAC meeting:
facilitate the maximally saturated output vs enforce a desaturation towards white
To better understand the behavior of each approach, and therefore the pros and cons, I put together … you guessed it: some more plots!
Here’s the setup. We have a 64 frame animation, featuring two exposure sweeps from 0 to +8 stops, of a 6 patch hue swatch of primary and secondary colors, and a second exposure sweep of the same image with a 15 degree hue rotation. For clarity, each hue swatch is a linear ramp from 0 to full color in x and a linear ramp from full saturation to 80% saturation in y.
There are 4 videos with different display rendering transforms.
First we have a pure inverse eotf + clamp. Note the typical behavior of saturated colors converging to the primary and secondary colors as the values clip in overexposure, and then converging to white.
Next we have the same setup rendered through the aces rec.709 output transform. Note the same behavior as the first video in colors between the primaries and secondaries as they converge to primaries and secondaries in overexposure. (Of course the behavior is a bit more nuanced and nonlinear in the aces transform). (Note: I’ve disable the acescg to rec.709 display colorimetry conversion here so that this aspect doesn’t confuse us.)
Next we have the same image rendered through the NaiveDisplayTransform_v03.nk available in this post. (Note: I’ve disable the acescg to rec.709 display colorimetry conversion here as well)
And then the same with “path to white” disabled.
As usual, here’s the nuke script that generated these plots. overexposure_comparisons.nk (24.4 KB)
A couple of observations.
- The range of brightness expressed by the “hue preserving no path to white” display rendering is very limited. For an SDR display rendering I think this is a huge downside.
- The main argument I’ve seen mentioned for having “no path to white” is HDR. It has been mentioned that very bright and very saturated colors are often desired in HDR. It’s difficult for me to know without being able to test on any HDR display. However my intuition tells me we would still want a “path to white” above a certain threshold, it’s just that this threshold would be different between HDR and SDR. The question is, how different? And could it be handled purely by the difference in rendering transform?
One of the biggest arguments for me in favor of a chromaticity preserving display rendering is more consistent rendering of color in highlights between SDR and HDR. I’m guessing here without an HDR display, but I think the hue twists inherent in a per-channel display rendering approach would be very difficult to undo when using an HDR rendering.
The only person I’ve seen mention this is @daniele, on the colour-science slack, so I thought I’d bring it up here.
4:18 PM Saturday Jan 30th
Daniele Siragusano I am not saying HDR and SDR should be identical. But a yellow candle in SDR should not be orange in HDR I guess
At least I know many colourists which are confused when that happens.