====== Dither ======

The Pixel Engine uses a 4×4 perfect-magic-square matrix when DstMode.DitherEnable is set.

^ Y / X ^ 0 ^ 1 ^ 2 ^ 3 ^
| 0 | 0 | 12 | 3 | 15 |
| 1 | 7 | 11 | 4 | 8 |
| 2 | 13 | 1 | 14 | 2 |
| 3 | 10 | 6 | 9 | 5 |

[[registers:pixel-engine:pixel_ditheroffset|DitherOffset]] adds two-bit offsets to the low two integer bits of X and Y, modulo four. This preserves the dither pattern when a window moves.

Let d be the selected matrix value. The reduced-width component calculations are:

<code>
Color6 = min(Color8 + (d >> 2), 255) >> 2
Color5 = min(Color8 + (d >> 1), 255) >> 3
Color4 = min(Color8 +  d,       255) >> 4
Color3 = min(Color8 + (d << 1), 255) >> 5
Color2 = min(Color8 + (d << 2), 255) >> 6
</code>

Sources: [[https://verite.3d32.org/_media/source/v2200spec.pdf#page=34|Technical Spec, p.34]], [[https://verite.3d32.org/_media/source/v2200spec.pdf#page=35|Technical Spec, p.35]]