Loading...
Searching...
No Matches
Mosaic -- PPU Pixel Block Effect
Screenshot

What This Example Shows

How to use the SNES mosaic register to create a pixelation effect, and how to combine it with brightness fading for screen transitions. Press any button to cycle through 4 transition steps: fade out, fade in, mosaic out, mosaic in.

Prerequisites

Read effects/fading first (brightness control basics).

Controls

Button Action
Any button Advance to next transition step

The cycle repeats: fade out -> fade in -> mosaic out -> mosaic in -> (loop)

Build & Run

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

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

How It Works

1. Fade effect (brightness)

void doFadeOut(void) {
for (i = 15; i > 0; i--) {
setBrightness(i);
WaitForVBlank(); WaitForVBlank();
}
setBrightness(0);
}
setBrightness
void setBrightness(u8 brightness)
Set screen brightness.
WaitForVBlank
void WaitForVBlank(void)
Wait for next VBlank period.
i
static u8 i
Definition main.c:156
doFadeOut
void doFadeOut(void)
Fade the screen from full brightness to black over ~30 frames.
Definition main.c:58

setBrightness() writes to register $2100 (INIDISP). Values 0-15 control the master brightness – 15 is full, 0 is black. Two VBlank waits per step gives a smooth ~0.5 second transition.

2. Mosaic effect (pixelation)

void doMosaicOut(void) {
mosaicEnable(MOSAIC_BG1);
mosaicFadeOut(3);
}
doMosaicOut
void doMosaicOut(void)
Activate the hardware mosaic filter and animate pixel blocks growing.
Definition main.c:94
mosaicFadeOut
void mosaicFadeOut(u8 speed)
Animate mosaic fade out (clear -> pixelated)
mosaicEnable
void mosaicEnable(u8 bgMask)
Enable mosaic effect for specified backgrounds.
MOSAIC_BG1
#define MOSAIC_BG1
Definition mosaic.h:30

The library functions mosaicEnable() and mosaicFadeOut() write to register $2106 (MOSAIC). The parameter 3 controls the speed of the transition.

Register $2106 layout:

  • Bits 7-4: Block size (0 = 1x1 pixel, 15 = 16x16 pixel blocks)
  • Bits 3-0: Which backgrounds are affected (bit 0 = BG1, bit 1 = BG2, etc.)

Increasing the block size from 0 to 15 makes pixels progressively larger until the image is a grid of colored squares.

3. Combining both for transitions

Classic SNES transition pattern used in many RPGs:

  1. Mosaic out (image becomes blocky)
  2. Fade to black (blocky image fades)
  3. Load new scene during black
  4. Fade in (new scene appears, still blocky)
  5. Mosaic in (blocks shrink to reveal the scene)

This gives a polished "dissolve" effect.

SNES Concepts

Mosaic Register ($2106)

The SNES PPU provides per-background pixelation with 16 levels. The effect is applied entirely in hardware – zero CPU cost. Each BG layer can be independently mosaic'd by setting its corresponding bit in the low nibble.

Not a Blur

Mosaic replaces pixels with the top-left pixel of each NxN block. It is a decimation effect, not a filter. The result looks like a low-resolution version of the original image.

Sprites Are Unaffected

Mosaic only applies to background layers. Sprites always render at full resolution. Some games use this creatively – mosaic the BG while sprites remain sharp during a transition, making characters stand out against the dissolving background.

Project Structure

File Purpose
main.c Fade/mosaic transition logic, button-driven state machine
data.asm Background tiles, tilemap, and palette data
res/opensnes.png Source background image
Makefile LIB_MODULES := console dma background sprite input mosaic

Going Further

  • Scene transition: Load two different backgrounds. Use the fade/mosaic combination to transition between them – mosaic out scene A, load scene B tiles during force blank, mosaic in scene B.
  • Per-layer mosaic: Enable mosaic on BG1 but not BG2. This lets you pixelate the foreground while the background stays sharp, or vice versa.
  • Explore related examples:
    • effects/fading – Brightness fading in detail
    • effects/hdma_wave – More advanced PPU effects with HDMA