It's just an after effects filter, far as I can tell. It looks neat, but it does do some weird stuff, like how it looks like dude missed the bullseye and is sticking it in a buttcheek instead.
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I guess it just looks weird to some people because the smoothing filter blurred her butthole and pussy when that part of her body was already shadowed to begin with. It removed detail when that part of the image wasn't too clear to begin with. I'm really not seeing the "fucking her ass cheek" thing at all though. It never looked weird to me because it pretty clearly looks like the head of his dick is about an inch away from her pussy, even with the blurring from the filter. Thanks to the filterless image that the
dev provided though, it should be pretty evident to everyone that he's not inside her yet.
It is somewhat hilarious seeing people get upset and paranoid about it thinking it's "AI" despite the dev already explaining the tools used and providing development shots proving those theories wrong.
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BTW, to anyone who is commenting on that image before actually
playing the VN, just FYI... that image is on screen in game for about a whole half a second as he's getting ready to fuck her. So frankly, the complaints about that image are much ado about nothing. Here's what it looks like when he's actually inside her...
IMO, I think the postwork is fantastic, but it's not going to be perfect (as if any VN ever is). Because the filter removes detail, then some things are going to look too blurred. It's not really worth nitpicking over when the rest of the VN is so damn good.