gfx_overview.txt

In non-GBC mode, I need only 3 background layers. One is a solid color (I could 
use the backdrop but it's used for SGB borders). Then there's the bg and window 
layers with color 0 excluded. Sprites with low priority are placed between the 
"backdrop" and the bg/window layers.

In GBC mode, I'd need 6 layers total. Bg & Window each need:
1: A layer with color 0 of each tile
2: A layer with colors 1-3 of each tile
3: A layer with colors 1-3 of high-priority tiles
Low-priority sprites are placed between 1 and 2. Normal-priority sprites are 
placed between 2 and 3.

Layers are dynamically allocated. If bg & window are not on the same line, 
everything's fine. If they are on the same line, they are each allocated 2 
layers. It detects whether high-priority tiles are necessary. If so it uses:
1. A layer with colors 0-3 of each tile
2. A layer with colors 1-3 of high-priority tiles
Or, if it doesn't use high-priority tiles, satisfy low-priority sprites:
1. A layer with color 0 of each tile
2. A layer with colors 1-3 of each tile
However, if there is a border enabled, there aren't enough layers to do this 
properly.

Both of the ds's "windows" are used.
0: Masks the gameboy window layer so that it doesn't loop around the screen, and 
separates the window from the background.
1: Masks everything to show only the 160x144 region of the gameboy screen.


Bg and Window can use "signed" or "unsigned" tile numbering. In one vram bank, 
there are 0x180 tiles total. Signed and Unsigned tile numbering overlap for 0x80 
tiles. In gbc mode there is a second bank, bringing the total number of 
tiles to 0x300.
I separate signed and unsigned tiles entirely, with a bit of duplication of 
effort. Tiles in bank 1 are placed right after tiles in bank 0. That's 0x200 
tiles each for signed and unsigned. It's easy to just modify the BG registers to 
switch from one set of tiles to another with this setup.

In most cases, gb color 0 corresponds to ds color 1, because of how the DS 
handles the backdrop. By default I refer to gb colors.

Map of vram for tiles:
Base 0: First tile is color 0 for all those "color 0" layers
Base 4: Unsigned tiles, bank 0 (colors 1-3)
Base 5: Unsigned tiles, bank 1 (colors 1-3)
Base 6: Signed tiles, bank 0 (colors 1-3)
Base 7: Signed tiles, bank 1 (colors 1-3)
Base 8: Unsigned tiles, bank 0 (colors 0-3)
Base 9: Unsigned tiles, bank 1 (colors 0-3)
Base 10: Signed tiles, bank 0 (colors 0-3)
Base 11: Signed tiles, bank 1 (colors 0-3)

Base 12+: Super Gameboy border tiles