Mode 7 EXTBG

One Mode 7 plane split into two layers by bit 7 of its pixels, with a sprite passing between them.

SETINI bit 6 (EXTBG) gives Mode 7 a BG2 that shows the same plane as BG1 but reads bit 7 of every pixel as a priority bit. Here the floor's pixels have bit 7 clear and the pillars' have it set, so the ball — a priority-1 sprite — rolls over the floor and behind the pillars of one single plane. Press A to switch EXTBG off: BG1 is shown instead, one layer, and the ball covers the pillars too.

Build & Run

cd $OPENSNES_HOME
make -C examples/mode7/extbg

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

SNES Concepts

• SETINI ($2133) bit 6, Mode 7 EXTBG, through mode7SetExtBg() (the lib keeps a shadow of the write-only register, shared with the other SETINI setters)
• EXTBG priority, front to back: sprites 3, sprites 2, BG2 pixels with bit 7 set, sprites 1, BG1, sprites 0, BG2 pixels with bit 7 clear (anomie's register doc, Mode 7) — hence BG2 alone on the main screen
• One pixel, two colours: BG2 reads bits 0-6, BG1 all eight, so the pillar's 0x83 is colour 3 on BG2 and colour 131 on BG1 (both loaded with the same grey)
• Mode 7 at 1:1: mode7SetScale(0x0200) — the lib's matrix is half the scale — and mode7SetScroll(0, 0) to put plane row 0 on the first line
• The plane built at run time: a 16 KB tilemap in a FAR buffer, three 8bpp tiles, dmaCopyVramMode7() for the interleaved upload; the ball drawn as pixels and encoded with tileEncode4bpp()

What You'll See

• A green checkerboard floor with a row of grey pillars; the yellow ball rolls left and right, hidden behind each pillar as it passes.
• A: EXTBG off — the ball is drawn over the pillars. A again: back.

Testing

testing/manifests/mode7_extbg.toml checks SETINI ($40 / $00 / $40) and the main-screen designation across two A presses, the 1:1 matrix and the scroll, and that the ball moves. The visual baselines capture the occlusion.

Modules Used

Module	Purpose
console	System initialization, WaitForVBlank
dma	Mode 7 VRAM upload, palettes, sprite tiles
sprite	The ball (OAM)
input	The A button
mode7	Matrix, scroll, mode7SetExtBg()
tile	The ball's tiles, built from pixels