Family 6 (Colour & effects) · rung 6c.2 — shadow & tint.

Why it matters for your game

Mood is colour. Nightfall, a dungeon, a cave — subtract a grey and the whole screen darkens. Underwater, poison gas, a lava cavern, a sepia flashback, the red flash of taking a hit — add a colour and everything takes that cast. The SNES does this to every pixel of a layer in one PPU pass, so a single register pair changes the entire scene's atmosphere for zero per-frame cost. It is the difference between one tileset and a tileset that reads as day, dusk, and midnight.

Screenshot

What you'll learn

The single new idea: colour math against a fixed colour. The PPU takes the finished main-screen image and adds or subtracts a constant colour per layer — colorMathShadow() subtracts a grey (darken), colorMathTint() adds a colour (cast). This rests on the layers, tiles and palette you already know; the effect sits on top of them and touches no VRAM, CGRAM or OAM.

Because both effects share the one fixed-colour register and the single add/subtract bit, only one is active at a time — which is exactly why the demo cycles through them rather than combining them. The scene is a 4×4 chart of sixteen game-world colours, built procedurally (zero assets), so you can watch the transform land on every hue together.

What to observe / if it breaks

Correct run: the plain colour chart. Press A to cycle: none → shadow (darkens) → underwater (blue cast) → sunset (warm cast) → none. Every swatch shifts together each press.
Nothing changes when you press A: colour math applies only to layers you enabled and that are on the main screen — check setMainScreen(LAYER_BG1) and that the effect targets COLORMATH_BG1.
Only part of the screen is affected: the colour-math condition is windowed. This example leaves it at COLORMATH_ALWAYS; a stray colorMathSetCondition() would clip it to a window region.
Shadow goes fully black / tint blows out white: the fixed-colour intensity is too high — colorMathShadow subtracts, colorMathTint adds, both clamp at the channel limits.

Probe oracle: fx_state (0 none, 1 shadow, 2 underwater, 3 sunset) advances on each A press; the effect matches CGADSUB (add/subtract) and COLDATA (the fixed colour).

Build & run

make

../../../tools/luna-test/bin/luna run -n 3000000 shadow_tint.sfc

Modules used

console, dma, background, colormath, input

Where you are

← previous: transparency · colour math between two layers · → next in family: direct_color · the pixel byte is the colour