asc_tone

ASCENSION's display-referred finish. A post-process pass at priority 150 — after tonemap (60) and vignette (70), before demo_fade (200).

It does four things, all of which are defined on a display value and none of which can be done correctly earlier in the chain:

property	what it does
black	the black point. Display values at or below it become 0, the rest is renormalised over the full range. This is what gives the part a shadow end.
toe, toe_end	shadow contrast below toe_end. toe > 1 deepens.
sh_start, sh_ceil, sh_knee	the highlight shoulder, so the fire keeps tonal range instead of meeting at 255.
dither	triangular-PDF output dither in 1/255ths, applied at the final quantisation.
res_x, res_y	frame size, so the dither seed is one hash per pixel.

Why the black point is here and not in colorGrade

The colorGrade pass pivots contrast around 0.5 in linear, pre-tonemap: (c - 0.5) * k + 0.5. Almost all of ASCENSION lives below 0.069 linear, so any k > 1 drives the frame negative and clamps it flat black. That is why asc_grade pins contrast at 1.000. A black point belongs after the tone curve, on the values the audience sees.

Why every curve acts on luminance

Each curve is evaluated on luma(c) and the result is carried back as a ratio c * (L'/L). A per-channel curve moves the channels at different rates and walks the colour off its hue; scaling the triple leaves hue and (max−min)/max exactly as they were. ASCENSION's green / cyan / magenta budgets are all 0.00 %, and this pass reshapes the entire tonal range without being able to move one pixel into a banned lane.

Why the dither is here

The sky dome carries its own dither and it works — but it is inside the dome's shader, so it lands on the sky and nothing else. The dust medium, the fire, the sand, and every gradient the post chain itself creates are quantised at the end of the chain. This is that point.

Driven per frame from the clock bus by asc_grade.