sheet_fallen

# art.sprite The unlit surface every 2D thing in Nine Bells is drawn with. A sprite is one quad wearing one material, and this shader does the three things the plain unlit shader does not. **Sheet framing.** `frames_x` x `frames_y` cells, `frame` picks one, counting left to right then top to bottom from 0. A whole run cycle lives in one texture and one material, so animating is writing a number rather than swapping an asset. `cell_inset` walks the sampled window in from each cell's edges — asked for a grid, an image generator draws the borders it was told not to, and the inset crops that seam off instead of ringing every frame with it. **Flipping.** `flip_x` mirrors the cell in place, so the doctor faces left without a second sheet. **Keying.** The image generators do not write an alpha channel; asked for a transparent background they paint one. `key_mode` cuts it out here instead: | `key_mode` | What it keys | |---|---| | 0 | off — use the texture's own alpha | | 1 | plain RGB distance from `key_color` | | 2 | the grey/white transparency checkerboard a generator paints | | 3 | **lean** — how far a pixel leans toward `key_color`'s own channels and away from the others | **Mode 3 is the one to use against generated art on a flat backdrop.** It takes the backdrop *and* the half-strength spill a generator paints for smoke, glass and soft edges, while a warm rim light or a bone-white mask that shares part of the hue survives. Mode 1 cannot do that: the texture is sampled in linear space while `key_color` is authored in display terms, which pushes half-strength background pixels far enough from the key colour to survive any tolerance that still keeps a bone-white mask. Leaning on the hue is immune to that because both terms move together. `key_despill` then pulls the background's hue back out of the pixels that survive, which is what removes the coloured fringe on the cut edge. Against magenta, `key_tolerance` 0.70 and `key_despill` 0.85 are the tuned values. Unlit on purpose. The light in Ashcross is painted — lantern pools, censer glow, firelight — as additive sprites, not as scene lights, so a sprite's colour is exactly what was drawn times its tint, plus `emissive`.