I don't think it's been mentioned as a possible feature in any of the patreon updates yet, but I think it'd be pretty nice to have. Chameleons and other color-changing characters are one obvious application. Another could be adding things like whip marks and red butt cheeks for BDSM scenes. Blushing, going pale, and other complexion changes. Applying or removing makeup. Tattooing is probably someone's fetish? Body painting maybe? Sex outside, getting dirty and grass stained?
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Actually, that might allow a pretty good amount of custom anime-esque expressions, as well. Drawn-on sweat droplets and forehead veins, turning blue, etc. Plus things like body writing, which I definitely see in furry porn all the time (and there's currently one skin request, though it's impossible with the YL1 skin system).
It really seems like a versatile enough feature to be worth including. Especially compared to needing to fade the camera to black every time something minor changes.
I'm mostly thinking things like spanking and mild whipping, having redness/whip marks slowly fade in would be pretty nice. I mean, what if cum inflation (which I'm fairly sure is a LOT more technically complex) had to be done by model swapping, too? That would be pretty unsatisfying, having a skinny character, fading to black, fading back in, and now they're inflated.
If the masks can be added together into a single layer on the fly, that would allow for a LOT of things to be done with only 2 layers and some creativity.
For instance, layer 1 starts out with a cheek blush, then layer 2 is ears. Fade in 1, then 2, then you merge them both to layer 1. Then you change layer 2 to another area of the face/body (starting at 0 opacity again), fade that in over top, and merge again. You get as many different stages of blushing as you want, with only 2 total layers needed. Because they're just based around merging masks instead of merging actual images, you don't have to worry about overlapping areas being more opaque (as long as they're all the same color, which is a reasonable limitation). And because the masks can be very simple coordinate based gradients, it should still be very lightweight for most effects. Basically just reusing some of the character creator code.
It'd even be possible to fade in whip marks one at a time that way, just adding and merging masks (lines or capsule gradients) one at a time.
So you'd only need a maximum of 2 layers for each color you want (maximum number of colors would probably need some performance testing). Especially if the system allows subtractive masks, then it should definitely be possible to do a LOT of different things with only 2 layers, adding and taking away colored areas.