So I was house-training a budget work-lamp I'd found on CGT.
https://www.cgtrader.com/3d-models/...85-79976b11720d

It included a nice 4k texture map for body & LEDs, with a separate material zone for the front glass, to be set almost transparent.

So I loaded the map into free Irfan View, painted all but the LED zone black, re-saved as an ambiance mask.

Looked okay until I dialled up the ambiance to ~99, at which point a patch on top of the case lit.

Huh ? WTF ?? That's not right...

An exasperated hour of re-mapping and sundry iterations later, I came back to that texture mask and played a hunch. I 'inverted' its colours. Yeah, sorta-diagonal from the LEDs' now-black rectangle, there was a pale grey smudge on the white field. A very pale grey smudge which, before inversion was, to the eye, indistinguishable from black...

Gotcha.

Re-painted, the mask now played nice.

I don't know how that patch slipped through, but lesson learned...

Not a brilliant render, but a useful test of the super-ambiance in action.

Superfly, 128 pixel-samples, an hour on 'Box'...

Wow, really, a little smudge like that can throw the mask a curve ball like that ? Thank you kindly Nik !