Screenshot

A triangle-shaped window mask driven by HDMA, showing how per-scanline window boundary updates create non-rectangular shapes. Press A, X, or B to apply the window to different background layers.

Ported from PVSnesLib "Window" example.

Controls

Button	Action
A	Window on BG1 only
X	Window on BG2 only
B	Window on both BG1 and BG2

Build & Run

cd $OPENSNES_HOME

make -C examples/graphics/effects/window

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

How It Works

SNES Window System

The SNES PPU has two hardware windows – rectangular regions defined by left and right X coordinates. These windows can mask (hide) portions of any background layer or sprite layer. Window operations cost zero CPU time because they are handled entirely by the PPU.

Making Non-Rectangular Shapes with HDMA

A single window is always a horizontal band. But by changing the left/right boundaries every scanline using HDMA, you can create any shape – circles, triangles, or custom silhouettes.

This example uses two HDMA channels:

Channel 6 writes to WH0 ($2126) – window 1 left boundary
Channel 7 writes to WH1 ($2127) – window 1 right boundary

The Triangle Tables

The HDMA tables define a diamond/triangle shape centered on screen:

Line  0-59:  WH0=255, WH1=0   -> left > right -> window disabled (no mask)
Line 60-91:  WH0 shrinks 127->96, WH1 grows 129->160 -> triangle expands
Line 92-123: WH0 grows 96->127, WH1 shrinks 160->129 -> triangle contracts
Line 124+:   WH0=255, WH1=0   -> window disabled again

The tables use repeat mode (bit 7 set in the line count) because each scanline needs a different value. Non-repeat mode would write once and hold, which does not work when the shape changes per line.

Inversion: Show Inside

By default, TMW (main screen window at $212E) hides pixels inside the window. The code sets the invert flag in W12SEL ($2123) so that pixels outside the window are hidden instead – revealing only the triangle-shaped area.

Switching Layers

When you press A/X/B, the code:

Disables HDMA channels
Resets all window registers via windowInit()
Enables window 1 on the new layer(s) with inversion
Sets TMW for the new layer(s)
Re-enables HDMA

SNES Concepts

W12SEL Register ($2123)

This register controls Window 1 enable and inversion for BG1 and BG2:

Bit	Purpose
0	BG1 Window 1 Invert
1	BG1 Window 1 Enable
4	BG2 Window 1 Invert
5	BG2 Window 1 Enable

Setting both the enable and invert bits for a BG makes pixels outside the window invisible, revealing only the windowed region.

Window Shapes in Commercial Games

This technique was used extensively: spotlight effects in Mega Man X, the title screen reveal in Donkey Kong Country, and dialog box cutouts in RPGs. Any shape that can be described as a left-right boundary pair per scanline can be rendered as a window.

VRAM Layout

Region	Content
$0000	BG1 tilemap (32x32)
$1000	BG2 tilemap (32x32)
$4000	BG1 tiles (4bpp, 16 colors)
$6000	BG2 tiles (4bpp, 16 colors)

Project Structure

File	Purpose
`main.c`	HDMA triangle tables, window setup, input loop
`data.asm`	Two backgrounds (BG1 + BG2) graphics data
`res/bg1.png`	Background 1 (palette slot 0)
`res/bg2.png`	Background 2 (palette slot 1)
`Makefile`	`LIB_MODULES := console sprite dma input background window hdma math`

Going Further

Animated window: Modify the triangle tables each frame to make the shape pulse or rotate. Copy the tables to RAM and adjust the boundary values.
Circle window: Replace the linear expansion/contraction with a circular profile calculated from the equation x = sqrt(r^2 - y^2).
Explore related examples:
- effects/transparent_window – Window + color math for semi-transparent overlays
- effects/hdma_wave – Another HDMA-driven per-scanline effect