You can nail every facial feature and still produce a composite that looks obviously fake. I've watched people burn weeks on identity preservation, get it perfect, then ship an image where two people supposedly standing in the same room are lit by two different suns. The brain catches it instantly, without being able to say why. Lighting coherence is the part nobody writes about, so here's what I've learned making it work.
Why "pasted" is a lighting judgment, not a geometry judgment
Human vision is startlingly bad at absolute measurements and startlingly good at consistency checks. We can't tell you a room's color temperature, but we detect within milliseconds that one face is 5500K daylight and the other 3000K tungsten. Same with shadow direction: nobody can estimate a light's azimuth, but two faces with shadows falling opposite ways read as wrong before you've consciously looked.
Four signals dominate the "pasted" verdict:
Color temperature — the global chromatic cast on skin
