Family 2 (Backgrounds) · the offset-per-tile modes (2/4/6), after mode2.

Screenshot

Mode 4 gives you an 8bpp (256-colour) BG1, a 2bpp BG2, and — like Mode 2 — BG3 as an offset table that lets every 8-pixel column scroll on its own. The difference is the table: Mode 4 reads one row of it, so each column gets one word, either a vertical offset or a horizontal one.

BG1 here is a diagonal 256-colour gradient. A sine wave is written into the offset row every frame as vertical words, and the columns ripple; press A and the same wave goes in as horizontal words, and the columns slide sideways in 8-pixel steps.

Build & Run

cd $OPENSNES_HOME

make -C examples/backgrounds/mode4

Open mode4.sfc in luna (or any SNES emulator) and press A.

SNES Concepts

Mode 4: setMode(BG_MODE4, 0) — BG1 8bpp, BG2 2bpp, BG3 the offset table (not drawn)
The Mode 4 offset word: bit 15 = 1 vertical, 0 horizontal; bit 13 = apply to BG1 (bit 14 = BG2); the value in bits 0-9, of which a horizontal offset ignores the low 3 (snesdev-wiki, Offset-per-tile)
One row: BG3 row 0 holds the 32 words; word 0 applies to the second visible column — the leftmost column is never offset
BG3VOFS picks the row: bgSetScroll(2, 0, 0) reads row 0. The lib writes BG3's VOFS raw in Modes 2/4/6 (for a displayed layer it writes y - 1; see KNOWN_LIMITATIONS, "Vertical scroll is off by one")
8bpp tiles at run time: 32 tiles from pixels with tileEncode8bpp(), a 256-entry palette computed at boot
Upload timing: the row is built during the picture and DMA'd at the start of VBlank, as in mode2

What You'll See

Diagonal rainbow bands whose columns ripple up and down as a travelling wave; the leftmost column stays put.
A: the columns move sideways instead, 8 pixels at a time. A again: back to vertical.

Testing

testing/manifests/backgrounds_mode4.toml checks the mode and BG3VOFS, the vertical words at frame 100 and the horizontal words after A ($2008, $2010… in VRAM), and that the row is uploaded in blank.

Modules Used

Module	Purpose
console	System initialization, setMode, WaitForVBlank
dma	Palette, tiles, map and the offset row
background	BG1 and BG3 bases, BG3 scroll
input	The A button
math	fixSin for the wave
tile	The 8bpp tiles, built from pixels