Gradient Colors -- HDMA Color Gradients

Screenshot

What This Example Shows

How to create a smooth color gradient across the screen using HDMA to rewrite CGRAM (the palette) every few scanlines. This technique changes the backdrop color (color 0) as the PPU draws each line, creating sky gradients, underwater effects, or sunset transitions.

Prerequisites

Read effects/hdma_wave first (HDMA basics and table format).

Controls

Button	Action
A	Remove the gradient (flat background)
B	Re-enable the gradient

Build & Run

cd $OPENSNES_HOME
make -C examples/graphics/effects/gradient_colors

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

How It Works

1. HDMA targets CGADD ($2121)

hdmaSetup(HDMA_CHANNEL_6, HDMA_MODE_2REG_2X, HDMA_DEST_CGADD, hdmaGradientList);

This configures HDMA mode 3 (2REG_2X), which writes 4 bytes per entry to two consecutive registers:

Byte	Register	Purpose
1	$2121	Set CGRAM address (low)
2	$2121	Set CGRAM address (high) – always 0 for color 0
3	$2122	Write color data (low byte of 15-bit color)
4	$2122	Write color data (high byte of 15-bit color)

2. The gradient table

Each entry in hdmaGradientList is:

[line_count] [cgadd_lo=0x00] [cgadd_hi=0x00] [color_lo] [color_hi]

The table defines color 0 (backdrop) for groups of scanlines. By stepping through blues, purples, and oranges, it creates a smooth vertical gradient.

The table ends with a 0x00 byte (zero line count = end of HDMA table).

3. Toggle at runtime

if (pad0 & KEY_A) hdmaDisableAll();
if (pad0 & KEY_B) enableGradient();

HDMA can be enabled/disabled at any time. When disabled, color 0 reverts to whatever was last written to CGRAM.

SNES Concepts

CGRAM (Color Generator RAM)

The SNES palette holds 256 colors, each stored as a 15-bit value (5 bits per R/G/B channel). Color 0 is the "backdrop" – visible wherever no BG or sprite pixel is drawn. By rewriting color 0 per scanline group via HDMA, each band of the screen displays a different background color.

HDMA Mode 3 (2REG_2X)

This mode writes 4 bytes to 2 consecutive registers, 2 bytes each. It is perfect for CGADD+CGDATA pairs because you need to set the target color address first (CGADD at $2121), then write the color value (CGDATA at $2122).

Non-Repeat vs Repeat Mode

This gradient uses non-repeat entries (bit 7 = 0 in the count byte). The color is written once, then holds for N scanlines. This works because CGRAM latches its value – unlike scroll registers (BG1HOFS etc.) which need repeat mode with fresh writes every scanline.

Project Structure

File	Purpose
main.c	Gradient setup, enable/disable, input handling
data.asm	Background tiles/tilemap/palette, HDMA gradient table
res/opensnes.png	Source background image
Makefile	LIB_MODULES := console dma background sprite hdma input math

Going Further

• Custom gradient: Edit the hdmaGradientList table in data.asm. Each entry is a 15-bit SNES color (BBBBBGGGGGRRRRR in little-endian). Try creating a sunset (orange to purple to dark blue).
• Animated gradient: Copy the table to RAM and rotate the color entries each frame. This creates a flowing color effect like underwater caustics.
• Explore related examples:
  • effects/transparency – Color math blending (another way to modify colors)
  • backgrounds/mode7_perspective – HDMA for per-scanline Mode 7 transforms