PPU attribute tables: Difference between revisions
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Each area covers 16×16 pixels or 2×2 tiles, the size of a [?] block in ''Super Mario Bros.'' | Each area covers 16×16 pixels or 2×2 tiles, 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 | Given palette numbers topleft, topright, bottomleft, bottomright, each in the range 0 to 3, the value of the byte is | ||
value = ( | value = (bottomright << 6) | (bottomleft << 4) | (topright << 2) | (topleft << 0) | ||
Or equivalently: | Or equivalently: | ||
| Line 68: | Line 68: | ||
[[File:Thwaite palette color sets.png|frame|center|The background palette is divided into four color sets.]] | [[File:Thwaite palette color sets.png|frame|center|The background palette is divided into four color sets.]] | ||
The byte at $23F2 has color set | The byte at $23F2 has color set 2 at bottom right, 2 at bottom left, 1 at top right, and 3 at top left. | ||
Thus its attribute is calculated as follows: | Thus its attribute is calculated as follows: | ||
value = ( | value = (bottomright << 6) | (bottomleft << 4) | (topright << 2) | (topleft << 0) | ||
= ( | = (2 << 6) | (2 << 4) | (1 << 2) | (3 << 0) | ||
= $ | = $80 | $20 | $04 | $03 | ||
= $A7 | = $A7 | ||
Revision as of 03:09, 4 September 2018
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 32×32 pixel or 4×4 tile part of the nametable and is divided into four 2-bit areas. Each area covers 16×16 pixels or 2×2 tiles, 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 = (bottomright << 6) | (bottomleft << 4) | (topright << 2) | (topleft << 0)
Or equivalently:
7654 3210 |||| ||++- Color bits 3-2 for top left quadrant of this byte |||| ++--- Color bits 3-2 for top right quadrant of this byte ||++------ Color bits 3-2 for bottom left quadrant of this byte ++-------- Color bits 3-2 for bottom right quadrant of this byte
Most games for the NES use 16×16 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 2 at bottom right, 2 at bottom left, 1 at top right, and 3 at top left. Thus its attribute is calculated as follows:
value = (bottomright << 6) | (bottomleft << 4) | (topright << 2) | (topleft << 0)
= (2 << 6) | (2 << 4) | (1 << 2) | (3 << 0)
= $80 | $20 | $04 | $03
= $A7
Glitches
There are some well-known glitches when rendering attributes in NES and Famicom games.
While the attribute table specifies one of four three-color palettes for each 16x16 pixel region, the left-side clipping window in PPUMASK ($2001) is only 8 pixels wide.
This is the reason why games that use either horizontal or vertical mirroring modes for arbitrary-direction 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; and at the top and bottom in Super C).
The game Alfred Chicken hides glitches on the left and right sides by using both left clipping and hiding the right side of the screen under solid-colored sprites. To mask the entire 240-scanline height, this approach would occupy 15 entries of 64 in the sprite table in 8x16 sprite mode, or 30 entries in the 8x8 mode.
