Family 2 (Backgrounds) · rung 2.8 — the offset-per-tile modes (2/4/6).

Why it matters for your game

Modes 2, 4 and 6 have a trick no other mode offers: offset-per-tile (OPT) lets every 8-pixel column of a layer scroll independently. That is how you get a flag rippling in the wind, a heat-haze shimmer, water distortion, or cheap per-column parallax — effects that would otherwise cost a scanline of HDMA or a full software redraw. It is niche, but when you want it, nothing else does it as cheaply.

Screenshot

What you'll learn

The single idea: in Mode 2, BG3 stops being a drawable layer and becomes an offset table. For each column of BG1/BG2, the PPU reads a scroll offset straight out of BG3's tilemap:

Per screen column N ≥ 1 (column 0 can't be offset), the PPU reads BG3 tile-row 0 for the horizontal offset word and tile-row 1 for the vertical offset word.
Offset word: bit 0x2000 = apply to BG1 (0x4000 = BG2); the V value is bits 0–9, the H value bits 3–9 (8-pixel-granular).

This example puts a stack of horizontal colour bands on BG1 and writes a sine into BG3's V-offset row, so each column samples the bands at a different height — the bands ripple. The phase advances each frame. Modes 4 and 6 reuse this exact data path (mode 4 packs H/V into one word with a select bit), so this one example teaches the whole OPT family.

What to observe / if it breaks

Correct run: rainbow bands ripple as a travelling wave; each column is at its own vertical offset, all from one 32-entry BG3 table.
Bands stay flat: the offset table never reached VRAM. The classic cause is computing it after WaitForVBlank — the upload then runs past VBlank and the PPU drops it. Build the table during active display, DMA it the instant VBlank starts (what this example does).
Only the leftmost column is wrong: column 0 cannot be offset by OPT — that is hardware, not a bug.
Whole layer shifts uniformly (no ripple): your offset words are missing the 0x2000 enable bit, so the PPU ignores them and uses the base scroll.

Probe oracle: wave_phase advances every frame.

Build & run

make

../../../tools/luna-test/bin/luna run -n 3000000 mode2.sfc

Modules used

console, dma, background, math

Where you are

← the rest of the family covers the everyday modes (mode1, mode0, mode3, mode5); this rung is the specialist OPT modes. For per-scanline (rather than per-column) tricks, see the hdma family.