Semi-transparent darkened rectangle via HDMA windows and color math. More...

#include <snes.h>
#include "res/background.inc"

Macros
#define	RECT_HEIGHT 112
	Height of the darkened rectangle in scanlines.
#define	RECT_WIDTH 176
	Width of the darkened rectangle in pixels.
#define	RECT_X 40
	4bpp background tile data (defined in data.asm, stored in ROM)
#define	RECT_Y 96
	Y coordinate of the darkened rectangle's top edge (scanlines from top).

Functions
int	main (void)
	Entry point: darkened rectangle overlay using HDMA windows and color math.

Variables
u8	hdma_left [128]
	HDMA table for the left window boundary (WH0, $2126).
u8	hdma_right [128]
	HDMA table for the right window boundary (WH1, $2127).

Detailed Description

Semi-transparent darkened rectangle via HDMA windows and color math.

Combines two SNES hardware features to render a darkened rectangle over a background image. HDMA drives the window position registers WH0 ($2126) and WH1 ($2127) every scanline to define the rectangle boundaries. The color math unit is configured to subtract a fixed color intensity inside the window region only (CGWSEL bit 5-4 = 01), producing a tinted overlay.

The HDMA tables use repeat mode (bit 7 set in the line count byte) because WH0/WH1 are write-only registers that must be refreshed every scanline, even when the window position is constant across the rectangle height.

Ported from PVSnesLib "TransparentWindow" example by Digifox. Uses the colormath and window modules with PVSnesLib's register values, so the frames match its original.

SNES Concepts

HDMA repeat mode for per-scanline window boundary updates
Window registers WH0/WH1 ($2126/$2127) defining a rectangular region
Color math subtraction inside the window region (CGWSEL/CGADSUB)
Fixed color source (COLDATA, $2132) for darkening intensity

What to Observe

A background image with a darkened rectangle overlaid in the center
The effect is static; no input is required
The rectangle edges are pixel-sharp (HDMA-defined per scanline)

Modules Used: console, sprite, dma, background, window, colormath, hdma

See also: hdma.h, colormath.h, video.h

Macro Definition Documentation

◆ RECT_HEIGHT

#define RECT_HEIGHT 112

Height of the darkened rectangle in scanlines.

◆ RECT_WIDTH

#define RECT_WIDTH 176

Width of the darkened rectangle in pixels.

◆ RECT_X

#define RECT_X 40

4bpp background tile data (defined in data.asm, stored in ROM)

Background tilemap data for the 32x32 tile grid

16-color palette for the background

X coordinate of the darkened rectangle's left edge (pixels)

◆ RECT_Y

#define RECT_Y 96

Y coordinate of the darkened rectangle's top edge (scanlines from top).

Function Documentation

◆ main()

int main ( void )

Entry point: darkened rectangle overlay using HDMA windows and color math.

Builds HDMA tables to define a rectangle region, loads a background image, then uses the PPU's color math unit to subtract a fixed color intensity inside the window — creating a semi-transparent darkened overlay. This technique is used in RPGs for dialogue boxes, inventory screens, etc.

Returns: Never returns (infinite loop).

Variable Documentation

◆ hdma_left

u8 hdma_left[128]

HDMA table for the left window boundary (WH0, $2126).

Built at startup to define where the darkened rectangle starts horizontally on each scanline. Before and after the rectangle, WH0 is set to 0xFF (disabling the window by placing the left edge beyond the right edge).

◆ hdma_right

u8 hdma_right[128]

HDMA table for the right window boundary (WH1, $2127).

Built at startup to define where the darkened rectangle ends horizontally on each scanline. Before the rectangle, WH1 is set to 0x00 (disabling the window by placing the right edge before the left edge).

Macros

Functions

Variables