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. 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). Sprites are unaffected by the mosaic filter.

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
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 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
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.

$2106.4-7	Mosaic Block Size	Image	# of rows of complete blocks	# of columns of complete blocks	# of rows of spare pixels	# of columns of spare pixels
0	1		224	256	0	0
1	2		112	128	0	0
2	3		74	85	2	1
3	4		56	64	0	0
4	5		44	51	4	1
5	6		37	42	2	4
6	7		32	36	0	4
7	8		28	32	0	0
8	9		24	28	8	4
9	10		22	25	4	6
10	11		20	23	4	3
11	12		18	21	8	4
12	13		17	19	3	9
13	14		16	18	0	4
14	15		14	17	14	1
15	16		14	16	0	0

References

• setting example: page 2-4-1 of Book I of the official Super Nintendo development manual
• bit-by-bit breakdown of the mosaic register: page 2-27-3 lbid
• display example of mosaic effect: Appendix A-7 lbid
• https://problemkaputt.de/fullsnes.htm#snesppubgcontrol
• [3] https://board.zsnes.com/phpBB3/viewtopic.php?p=204966#p204966
• [4] anomie's register doc