I have a solid colour (ACEScg) going in to Media Layer 1 of Action with ACEScg footage as background.
If I do a soft edge mask the colour seems to saturate and darken on the gradient, rather than the typical fade off I’m used to seeing in Rec709.
Should I be blending things differently somehow? Like a different Blend mode or something?
This is actually a simplified situation but it behaves the same. I was actually using a soft edge mask to write on a graphic which was a simple colour filled matte.
I tested the same setup with source CS, Log4 , which didn’t work (makes sense though) and as Rec2020 which did work.
Things blend a little different in scene linear/ACEScg.
The place where it’s likely to be noticeable is when your fill is a value significantly above 1. Because of the way an A over B comp works (A * Matte + B * Inverted Matte) high scene linear values will appear to overtake the matte.
The reason you’re seeing the saturation rise as the image darkens is because the linear value is heavily saturated, but both because of the way humans perceive color and how displays display it (esp now that we have aces 2), hypersaturated reds, such as one that’s 10, 0, 0 appear salmon/pink. As colors get nearer to 1 they attain maximum visual saturation.
In the attached still I made a red that’s 10, 0, 0 in aces CG and ran it through a basic soft matte. I converted the color to rec709 (where it becomes 1, 0.54, 0,47) and you can see it comps in a more traditional way.
The reason that the linear comp bleeds so much further into the matte is that for a large part of the matte’s gradient, the value of “Fill times Matte” is well above 1.
In any “it’s photographically correct” case these sorts of blending issues are fine as they are how the colors would mix in real life (see, for example, how defocused bright regions seem to eat up the dark regions).
But, here in the world of VFX, where I have never won an argument on technical merits, you may not want this behavior. The easiest way to avoid it is to convert your front and back to ACEScct, comp the two, then convert the comp back to ACEScg. Don’t touch the mattes.
You mention graphics there. I find it’s generally better to do any graphics work in a display colour space so after comp/grade. Unless it’s supposed to be an element in a scene, like a logo on the side of a building or phone screen.
