Mosaic

Mosaic is a screen pixellation effect. The mosaic register is located at 2106h, and it can be enabled or disabled on a per-background basis via bits 0-3 (called the mosaic mode select). When enabled for a given background, the output background color (which will then be fed to the compositor and combined with all the other possibly mosaicized backgrounds) of the pixel at screen position (H,V) is:

output_color(H,V) = input_color(floor(H/size)*size, floor(V/size)*size)

where size is the value of bits 4-7 of 2106h plus one. Thus, for each mosaicized background, the shaded output consists of a grid of solid-colored squares where each entire square's color matches that of its top-leftmost input pixel.

The first mosaic square's location is always (0,0) so that the upper-left corner of the square is coincident with the upper-left corner of the screen, disregarding overscan. Sprites are unaffected by the mosaic filter. All background modes support mosaic.^[7]

The mosaic filter has a horizontal and vertical subsystem:

The horizontal mosaic latches remember the color of the top-leftmost pixel of each mosaicized block.
The vertical mosaic latch remembers what the value of the PPU vertical scanline counter was at the top of each mosaicized block.^[3]

On Mode 7's EXTBG, vertical mosaic is enabled when bit 0 of the mosaic register is set, and horizontal mosaic is enabled when bit 1 is set. ^[4]

Examples


Game Name	Mosaic Usage	Animated GIF
Animaniacs	upon opening a door
Axelay	when the Stage 3 boss' final form takes damage
Final Fantasy III (US)	when trying to select a disabled menu option and upon entering a battle
Final Fantasy Mystic Quest	upon entering battles and before/after loading new sections of the map
The Legend of Zelda: A Link to the Past	upon entering/exiting the Lost Woods when trying to slash Agahnim with the Master Sword when being shocked by a bari upon entering/exiting a whirlpool waterway or dungeon warp tile
The Magical Quest Starring Mickey Mouse	when the final boss takes damage, and on the walls of that same battle
Star Ocean	upon entering every battle
Street Fighter II	on the title screen logo
Super Mario World	after selecting and entering a level

Apparent Resolution

This table describes how many complete mosaicized blocks appear on each axis when the mosaic filter is applied (the "apparent" screen resolution, in other words) disregarding overscan. The number of spare pixels off to the right and/or bottom of the screen that make up the last incomplete blocks when the block size does not evenly divide the screen is also given.