PPU attribute tables: Difference between revisions
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(Worked example, requested by koitsu) |
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Most games for the NES use 16x16 pixel [[metatile]]s (size of ''Super Mario Bros.'' ? block) or 32x32 pixel metatiles (width of '''SMB''' pipe) in order to align the map with the attribute areas. | Most games for the NES use 16x16 pixel [[metatile]]s (size of ''Super Mario Bros.'' ? block) or 32x32 pixel metatiles (width of '''SMB''' pipe) in order to align the map with the attribute areas. | ||
== Worked example == | |||
<div style="clear: both"> | |||
[[File:Thwaite bg with attr grid.png|frame|center|The background in the game ''[[Thwaite]]'', with an overlaid attribute grid.]] | |||
[[File:Thwaite attrs.png|frame|center|Each 16x16 pixel color area has one of four color sets assigned to it, and one byte controls four color areas.]] | |||
[[File:Thwaite palette color sets.png|frame|center|The background palette is divided into four color sets.]] | |||
The byte at $23F2 has color set 3 at top left, 1 at top right, 2 at bottom left, and 2 at bottom right. | |||
Thus its attribute is calculated as follows: | |||
value = (topleft << 0) | (topright << 2) | (bottomleft << 4) | (bottomright << 6) | |||
= (3 << 0) | (1 << 2) | (2 << 4) | (2 << 6) | |||
= $03 | $04 | $20 | $80 | |||
= $A7 | |||
</div> | |||
== Glitches == | |||
Nametable tiles are 8x8 pixels, and the left-side clipping window in [[PPU registers|PPUMASK ($2001)]] is 8 pixels wide, but attribute table tiles are 16x16 pixels. This is why games that use the horizontal or vertical [[mirroring]] mode for diagonal [[PPU scrolling|scrolling]] often have color artifacts on one side of the screen (on the right side in ''Super Mario Bros. 3''; on the trailing side of the scroll in ''Kirby's Adventure''; at the top and bottom in ''Super C''). | Nametable tiles are 8x8 pixels, and the left-side clipping window in [[PPU registers|PPUMASK ($2001)]] is 8 pixels wide, but attribute table tiles are 16x16 pixels. This is why games that use the horizontal or vertical [[mirroring]] mode for diagonal [[PPU scrolling|scrolling]] often have color artifacts on one side of the screen (on the right side in ''Super Mario Bros. 3''; on the trailing side of the scroll in ''Kirby's Adventure''; at the top and bottom in ''Super C''). | ||
Revision as of 00:01, 22 March 2015
The attribute table is a 64-byte array at the end of each nametable that controls which palette is assigned to each part of the background.
Each attribute table, starting at $23C0, $27C0, $2BC0, or $2FC0, is arranged as an 8x8 byte array:
2xx0 2xx1 2xx2 2xx3 2xx4 2xx5 2xx6 2xx7
,-------+-------+-------+-------+-------+-------+-------+-------.
| . | . | . | . | . | . | . | . |
2xC0:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
| . | . | . | . | . | . | . | . |
+-------+-------+-------+-------+-------+-------+-------+-------+
| . | . | . | . | . | . | . | . |
2xC8:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
| . | . | . | . | . | . | . | . |
+-------+-------+-------+-------+-------+-------+-------+-------+
| . | . | . | . | . | . | . | . |
2xD0:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
| . | . | . | . | . | . | . | . |
+-------+-------+-------+-------+-------+-------+-------+-------+
| . | . | . | . | . | . | . | . |
2xD8:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
| . | . | . | . | . | . | . | . |
+-------+-------+-------+-------+-------+-------+-------+-------+
| . | . | . | . | . | . | . | . |
2xE0:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
| . | . | . | . | . | . | . | . |
+-------+-------+-------+-------+-------+-------+-------+-------+
| . | . | . | . | . | . | . | . |
2xE8:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
| . | . | . | . | . | . | . | . |
+-------+-------+-------+-------+-------+-------+-------+-------+
| . | . | . | . | . | . | . | . |
2xF0:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
| . | . | . | . | . | . | . | . |
+-------+-------+-------+-------+-------+-------+-------+-------+
2xF8:| . | . | . | . | . | . | . | . |
`-------+-------+-------+-------+-------+-------+-------+-------'
,---+---+---+---. | | | | | + D1-D0 + D3-D2 + | | | | | +---+---+---+---+ | | | | | + D5-D4 + D7-D6 + | | | | | `---+---+---+---'
Each byte controls the palette of a 32x32 pixel part of the nametable and is divided into four 2-bit areas. Each area covers four tiles or 16x16 pixels, the size of a [?] block in Super Mario Bros. Given palette numbers topleft, topright, bottomleft, bottomright, each in the range 0 to 3, the value of the byte is
value = (topleft << 0) | (topright << 2) | (bottomleft << 4) | (bottomright << 6)
Most games for the NES use 16x16 pixel metatiles (size of Super Mario Bros. ? block) or 32x32 pixel metatiles (width of SMB pipe) in order to align the map with the attribute areas.
Worked example
The byte at $23F2 has color set 3 at top left, 1 at top right, 2 at bottom left, and 2 at bottom right. Thus its attribute is calculated as follows:
value = (topleft << 0) | (topright << 2) | (bottomleft << 4) | (bottomright << 6)
= (3 << 0) | (1 << 2) | (2 << 4) | (2 << 6)
= $03 | $04 | $20 | $80
= $A7
Glitches
Nametable tiles are 8x8 pixels, and the left-side clipping window in PPUMASK ($2001) is 8 pixels wide, but attribute table tiles are 16x16 pixels. This is why games that use the horizontal or vertical mirroring mode for diagonal scrolling often have color artifacts on one side of the screen (on the right side in Super Mario Bros. 3; on the trailing side of the scroll in Kirby's Adventure; at the top and bottom in Super C).
