Graphics

For every screen-pixel, the fdp outputs two half-pixels (each consisting of a color id and an alpha value).  These are then converted to rgb colors, and blended into a single output by the fda.

(say something like: the fdp is controlled by writing to its external RAM, as well as a few write-only internal registers

Graphics Layers

The fdp has eight different graphics layers which can be used as the source for each half-pixel:

Graphics settings are stored in lineram.  Effectively, each setting is an array of 256 values, one for each scanline.

Most layers have these settings:

Enable

whether the layer is visible

Clip Plane enables (4)

which of the four clip windows to enable

clip window modes (4)

0

outside visible

1

inside visible

clip invert

inverts the final clip mask

priority

which of the 16 priority levels to use (higher number = above other things)

blend mode

which half-pixel(s) to contribute to (neither/A/B/both)

alpha select

which set of alpha values to use (0/1)

Clipping

there are four clip windows, each of which has a left and right value (ranging 0-511).  selecting multiple of these will union their visible areas together (todo explain all this better. but i think it goes here because it's a like top-level feature that applies across layers)

Background Color Layer

a solid color, used if nothing else is present in that half-pixel, or if a priority conflict occurs.  this is a pseudo-layer which lacks most of the common settings.

• alpha select
• color id

Pivot Layer

(we're not sure what the name means). the pivot layer is a 64×64 grid of 8×8 tiles, with textures defined in ram. it can be used for displaying text or pixel graphics.

• per-tile palette and flipping
• global scroll
• per-scanline "pivot port" (unknown feature)

Sprite Layer

the fdp renders up to ~900 16×16 sprites per frame to a single bitmap framebuffer.

Tilemap Layers

each tilemap is a 32×32 (or 64×32) grid of 16×16 tiles.

"extend mode" (global)

select between 32×32 or 64×32 size (no tradeoffs)

scroll (global)

scroll (lineram)

(layers 2 and 3 only)

background color

a solid color, used if nothing else is found for that half-pixel, or if a priority conflict occurs.

pivot layer

a 64×64 grid of 8×8 tiles, with textures defined in ram.  can be used for displaying text or bitmap graphics.

sprite groups (4)

playfields (4)

32×32 (or 64×32) grid of 16×16 tiles. can be scaled vertically from 50% to infinity, and horizontally from 100% to 25600%.