Mode 7 -- Rotation and Scaling

Screenshot

What This Example Shows

How to use the SNES Mode 7 – a special background mode that applies an affine transformation (rotation + scaling) to a single 128x128 tile layer. This is the same technique behind F-Zero's ground, Mario Kart's tracks, and Pilotwings' maps.

Prerequisites

Read backgrounds/mode1 first (basic background setup). Mode 7 is fundamentally different from tile-based modes but the initialization flow is similar.

Controls

Button	Action
A	Rotate clockwise
B	Rotate counter-clockwise
D-PAD Up	Zoom out (see more of the plane)
D-PAD Down	Zoom in (magnify)

Build & Run

cd $OPENSNES_HOME
make -C examples/graphics/backgrounds/mode7

Then open mode7.sfc in your emulator (Mesen2 recommended).

How It Works

1. Mode 7 VRAM format

Mode 7 uses a special interleaved VRAM format:

• Even bytes: tilemap entries (128x128 = 16,384 tiles)
• Odd bytes: pixel data (256 tiles, 8x8, 256 colors each)

This is completely different from Modes 0-3 where tilemap and tile data are in separate VRAM regions. The loading is handled by asm_loadMode7Data() in data.asm, which uses dmaCopyVramMode7() to write both the tilemap and tile data into the interleaved format the PPU expects.

2. Initialize the transformation

setMode(BG_MODE7, 0);
mode7Init();
mode7SetScale(0x0200, 0x0200);
mode7SetAngle(0);

mode7Init() sets the rotation center to the screen center (128, 128). Scale 0x0200 = 2.0 in 8.8 fixed-point – this shows the map at a comfortable zoom level. Scale 0x0100 would be 1.0 (one texel per pixel).

3. Update the matrix every frame

if (pad0 & KEY_A) {
    angle++;
    mode7SetAngle(angle);
}

mode7SetAngle() recalculates the 2x2 transformation matrix (registers M7A-M7D at $211B-$211E) using the current angle and scale. The angle is 0-255, mapping to 0-360 degrees.

4. Zoom control

if (pad0 & KEY_UP) {
    if (zscale < 0x0F00) zscale += 16;
    mode7SetScale(zscale, zscale);
    mode7SetAngle(angle);
}

After changing the scale, you must call mode7SetAngle() again because the matrix depends on both angle and scale. The scale range is clamped to 0x0010-0x0F00 – values outside this range produce extreme distortion.

SNES Concepts

Affine Transformation

Mode 7 applies a 2x2 matrix to BG1. The matrix encodes rotation, scaling, and skew. The hardware computes the transform per-scanline using four registers:

Register	Address	Purpose
M7A	$211B	Matrix element A (cosine * scale)
M7B	$211C	Matrix element B (sine * scale)
M7C	$211D	Matrix element C (-sine * scale)
M7D	$211E	Matrix element D (cosine * scale)

One Layer Only

Mode 7 supports exactly one background (BG1). No BG2/BG3/BG4. Sprites still work normally on top, which is how F-Zero renders its cars.

256-Color Palette

Each Mode 7 tile uses 256 colors (8bpp), unlike Modes 0-3 which use 2/4/8 colors per palette. This gives richer visuals but limits you to 256 unique tiles total.

8.8 Fixed-Point Scale

0x0100 = 1.0x (normal), 0x0200 = 2.0x (zoomed out, shows more area), 0x0080 = 0.5x (zoomed in, magnifies). The integer part is the high byte, the fractional part is the low byte.

Project Structure

File	Purpose
main.c	Input handling, rotation/zoom logic
data.asm	Mode 7 tile/tilemap/palette loading helper
res/mode7bg.png	Source image for Mode 7 ground
Makefile	LIB_MODULES := console dma background sprite input mode7

Going Further

• Add scrolling: Use mode7SetPos() to pan across the Mode 7 plane with the D-pad while rotating. This is how F-Zero navigates its tracks.
• Add sprites on top: Load sprite tiles and place an OAM sprite over the rotating background. Mode 7 supports full sprite rendering.
• Explore related examples:
  • backgrounds/mode7_perspective – Per-scanline scaling for pseudo-3D (F-Zero style)
  • effects/hdma_wave – HDMA can modify Mode 7 parameters per scanline