I’ve been running into some very strange discrepancies between grading in the ACES and YRGB colour spaces in Resolve, and was wondering if anyone might be able to shed some light on them?
In a nutshell, what I’m seeing is the following:
- in YRGB, images respond predictably to LUTs, but yield less subtle variation in skintones
- in ACES, images become violently overexposed when applying LUTs, and contain boosted red tones
- After correction, images in ACES appear to yield more accurate and nuanced colour reproduction
Here’s the original log image (slog3 from a Sony F5) and it’s waveform. The exposure values for her skin and the wall in the background are sitting where I’d expect them to be:
Now in YRGB mode, when I apply the LC709A LUT to the log image, exposure values are lifted to approximately where I’d expect them to go:
LC709A is designed to mimic Alexa’s Rec709 output, and does a nice job of normalising slog3 images, it doesn’t crush the blacks though, so a slight contrast curve is needed to put the darker tones where they belong:
There’s a slight overall green tint to the colour here, but it’s easily corrected out, and we have a normalised image. When apply a straight Filmconvert grade on top of the image, everything ends up right where we’d expect it to go without any further corrections or fine-tuning:
When we flip Resolve over into ACES v1.0 mode though, and apply the same LUTs/Looks, we get very different results. (note I’m not using any IDT or ODT on the footage, merely grading it in the ACES colour space). Applying the LC709A LUT in ACES results in this:
Exposure values blown sky high. Applying the straight Filmconvert look on top does the same thing, only worse:
Now with some contrast curves, we can correct the ACES image back to normalised levels:
However, there is an excessive amount of red in this normalised ACES image. Now for this scene, I had uncorrected 650w tungsten lights coming through daytime windows to simulate late afternoon light, and that would have increased the red in the image, but given the light wasn’t hitting the actress directly, it seems odd that the red levels are so boosted. With the excess red dialled out however, the ACES image yield more nuanced and appealing skintones. Here’s the corrected Filmconvert look, with the excess red removed and the exposure levels brought down to normalised levels:
Compare that to the YRGB Filmconvert image, and you can see how much more nuanced and realistic the skintones look:
And again, ACES with a straight Filmconvert look applied, brightness corrected and excess red removed:
And the YRGB version:
Again the skin looks much less wooden, and more realistic in the ACES version. The richer reds of her lips are more clearly defined from the skin on her face.
Is anyone else experiencing similar strange responses to LUTs in ACES in Resolve? And does anyone know what might be causing the discrepancy?
Any insights would be greatly appreciated.