Mosaic

Mosaic is a screen pixellation effect. It can be enabled or disabled on a per-background basis via bits 0-3 of 2106h. When enabled for a given background, the color 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, 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]

Examples

Game Name	Mosaic Usage	Animated GIF
Final Fantasy III (US)	when trying to select a disabled menu option
Final Fantasy Mystic Quest	upon entering battles and before/after loading new sections of the map
Star Ocean	upon entering every battle
Super Mario World	after selecting and entering a level

Apparent Resolution

This table describes how many mosaicized blocks appear on each axis when the mosaic filter is applied (the "apparent" screen resolution, in other words) disregarding overscan.

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

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