Screenshot

Port of krom (Peter Lemon)'s Mode7 Perspective demo (PeterLemon/SNES, PPU/Mode7/Perspective). Four HDMA channels each stream one Mode 7 matrix register per scanline — M7A and M7D from a cosine table, M7B from sine, M7C from -sine — so the ground plane is simultaneously perspective-projected (each line scaled by 20480/y, an 80× zoom-out at the horizon shrinking hyperbolically to 0.36× up close) and rotated (48 steps of 7.5°). The sibling example mode7/perspective drives only the diagonal terms (no rotation); this one exercises the full affine matrix.

The tables are krom's exact bytes (res/m7{cos,sin,nsin}.bin, extracted verbatim), their math reverse-engineered and verified: entry(a,y) = trig(2πa/48)·20480/y in 8.8 fixed point — 32256/32256 entries proven by devtools/m7ptables.py verify. The track art is original (9 prefab tiles composed into a 128×128 Mode 7 world).

Controls (krom's map)

D-pad — move over the plane (scroll + pivot together)
L / R — rotate the world in 7.5° steps
Y / A — nudge the pivot on X · X / B — nudge it on Y

SNES Concepts

Per-scanline Mode 7 matrix writes (perspective + rotation combined)
4-channel HDMA coordination (hdmaSetup ×4 + hdmaEnable(0x0F))
HDMA table repointing as a zero-copy animation primitive
Mode 7 interleaved VRAM (dmaCopyVramMode7); out-of-map backdrop (M7SEL=$80)
VBlank-window discipline: repoint right after WaitForVBlank() — re-arming an HDMA channel mid-frame restarts its table walk

Register fidelity vs the original

Register	krom	this port
BGMODE	$07	same (setMode(BG_MODE7, 0))
M7SEL	$80	same (mode7SetSettings)
DMAP0-3 / BBAD0-3	%010 / $1B,$1C,$1D,$1E	same (hdmaSetup ch 0-3)
A1B0-3	banks 1,2,3,1	from the far pointers (SUPERFREE sections)
HDMAEN	%00001111	same (hdmaEnable(0x0F))
BG1HOFS/VOFS, M7X/M7Y	384/768, 512/1152	same pose, same per-frame sync order
Table repoint	A1Tx rewrite after WaitNMI	hdmaSetup ×4 after WaitForVBlank()

Measured parity (luna v1.9.0)

Tables: byte-identical to the reference (extracted verbatim, math verified 32256/32256).
In-bounds projection: point-samples of the rendered frame match the world image through the documented matrix model (same check passes on the reference against its own art).
Rotation parity, the strong number: at +90° (12 L-presses) both ROMs show the same out-of-map black except one green wedge — 590 non-black pixels in both, IoU = 1.0000 (pixel-exact geometry); at 180° both are fully black. The "rotate into the void" behavior is the reference's own (pivot outside the 1024² world, M7SEL=$80 backdrop).

Two integration lessons this port surfaced

hdmaSetup() configures but does not enable — hdmaEnable(mask) is a separate call (the un-enabled state renders a convincing static 1:1 view that LOOKS like a broken perspective; check dma.hdmaen).
Calling hdmaSetup from an nmiSet callback did not take effect (registers unchanged) while the same call works from the main loop — under investigation as a potential NMI-context/ABI interaction; the main-loop placement right after WaitForVBlank() is krom's own pattern and still inside VBlank.

How to Build

cd examples/mode7/perspective_rotate && make

Modules Used

console, dma, background, hdma, input, mode7