Got a bit bored, last night, while waiting for a slow Adventuring session to progress, so tried something out myself. Not particularly brilliant, but I did it anyway.
(Purely in GIMP, using an overlay mask of exactly fitting tile-sized blocks set to a suitable pythagorean intensity from the given centre of interest (which didn't
quite marry up with the visual limit, but was close) and a "nightified" version of the screenshot with everything greyscaled except for remains/bodies (reverted to their traditional green hue) and "memorised creature positions" (set back to the reverse-purple appearance)... This creates
some odd hues, however, where it's partial. I think I should have allowed them to "fade to grey" and leave only "beyond sight" stuff as so coloured.)
Oh, and I play in default tileset (not even squarified, but that was taken account of in my radius calculations).
Daytime, unadulterated. Yes, there
are that many animals around, and I'm sneaking with a load of flingable ammunition ready to try to incapacitate any that might be in danger of spotting me and alerting the cheetahs, lions and mongeese to my presence. (Peregrines are a bugger for that, I've found. that one in the Z+1 insert is one I went on to deliberately avoid, post screenshot.
)
That's from 100% daytime shot at the Adventurer's location, decreasing to slightly below 25% at the extreme limit of normal vision (tried to zero, but didn't look right). "Sight shadows" caused by obstructions (i.e. trees) are left as they were in the raw view, however far away, given that "beyond limit" and "obscured" items are identical.
Note that you cannot see any further than you could in daytime, but a sufficiently close camp-fire will allow a day-visible tile to be seen to an appropriate degree. I did
not implement shadows of trees on camp-fire illuimination, but then I'd already put enough down (as markers, in this particular Adventuring scenario) to illuminate most areas from ENE-ish clockwise round to S, to make the difference moot.
I did not even try to recreate the "night-vision limit" colour scheme for all tiles within the shorter night-time visual limit, though technically as my adventurer is only slightly less than 60% of the drop-off away from the nearest camp-fire (with an arbitrary limit of ten tiles, or pythagorean equivalent), she
should find herself a little bit in the dark.
This looks oddest. Perhaps setting fires to be "100% visible regardless" (even/especially beyond day-time viewing limits?) would help, as it's their "faded-to-grey appearances" (shared with the "just fading" example above, obviously, given they already sustain themselves as 100% fire-lit and are subject only to the fading already seen in that version) that give colours other than the two shades of red and yellow that they should be flickering between. (Flickering is obviously not shown in the original static shot and the resulting mock-ups from it. Could have tried it, but GIF's pallette limit made it not worthwhile to even fake and I didn't feel like converting a string of 'frames' to a proper video file with enough losslessness to look good, either.)
This is something I made by accident. Fire-lit night-time shot with distance fade-off to night-time, but no "memory" surrounds beyond the normal visual limit (although both "memory beyond obstacles" and "memory due to lack of firelight" greyness can be seen). Because it fades to blackness, it also has the illusion of being non-flat or even downright non-euclidean, to my eyes.
(Note, in all the above I also applied the same filtering to the Z+1 insert, assuming that centre-centre was 1 tile away upwards and all others Xdif²+Ydif²+1² away , but trying to tune the fade-off so as to be obvious. I
didn't apply firelight effects, though, I now realise.)