5.6 KiB
Tile Data
When using the GBA's hardware graphics, if you want to let the hardware do most of the work you have to use Modes 0, 1 or 2. However, to do that we first have to learn about how tile data works inside of the GBA.
Tiles
Fundamentally, a tile is an 8x8 image. If you want anything bigger than 8x8 you need to arrange several tiles so that it looks like whatever you're trying to draw.
As was already mentioned, the GBA supports two different color modes: 4 bits per pixel and 8 bits per pixel. This means that we have two types of tile that we need to model. The pixel bits always represent an index into the PALRAM.
- With 4 bits per pixel, the PALRAM is imagined to be 16 palbank sections of 16 palette entries each. The image data selects the index within the palbank, and an external configuration selects which palbank is used.
- With 8 bits per pixel, the PALRAM is imagined to be a single 256 entry array and the index just directly picks which of the 256 colors is used.
Knowing this, we can write the following definitions:
#[derive(Debug, Clone, Copy, Default)]
#[repr(transparent)]
pub struct Tile4bpp {
data: [u32; 8]
}
#[derive(Debug, Clone, Copy, Default)]
#[repr(transparent)]
pub struct Tile8bpp {
data: [u32; 16]
}
I hope this makes sense so far. At 4bpp, we have 4 bits per pixel, times 8
pixels per line, times 8 lines: 256 bits required. Similarly, at 8 bits per
pixel we'll need 512 bits. Why are we defining them as arrays of u32 values?
Because when it comes time to do bulk copies the fastest way to it will be to go
one whole machine word at a time. If we make the data inside the type be an
array of u32 then it'll already be aligned for fast u32 bulk copies.
Keeping track of the current color depth is naturally the programmer's problem. If you get it wrong you'll see a whole ton of garbage pixels all over the screen, and you'll probably be able to guess why. You know, unless you did one of the other things that can make a bunch of garbage pixels show up all over the screen. Graphics programming is fun like that.
Charblocks
Tiles don't just sit on their own, they get grouped into charblocks. Long ago in the distant past, video games were built with hardware that was also used to make text terminals. So tile image data was called "character data". In fact some guides will even call the regular mode for the background layers "text mode", despite the fact that you obviously don't have to show text at all.
A charblock is 16kb long (0x4000 bytes), which means that the number of tiles
that fit into a charblock depends on your color depth. With 4bpp you get 512
tiles, and with 8bpp there's 256 tiles. So they'd be something like this:
#[derive(Clone, Copy)]
#[repr(transparent)]
pub struct Charblock4bpp {
data: [Tile4bpp; 512],
}
#[derive(Clone, Copy)]
#[repr(transparent)]
pub struct Charblock8bpp {
data: [Tile8bpp; 256],
}
You'll note that we can't even derive Debug or Default any more because the
arrays are so big. Rust supports Clone and Copy for arrays of any size, but the
rest is still size 32 or less. We won't generally be making up an entire
Charblock on the fly though, so it's not a big deal. If we absolutely had to,
we could call core::mem::zeroed(), but we really don't want to be trying to
build a whole charblock at runtime. We'll usually want to define our tile data
as const charblock values (or even parts of charblock values) that we then
load out of the game pak ROM at runtime.
Anyway, with 16k per charblock and only 96k total in VRAM, it's easy math to see that there's 6 different charblocks in VRAM when in a tiled mode. The first four of these are for backgrounds, and the other two are for objects. There's rules for how a tile ID on a background or object selects a tile within a charblock, but since they're different between backgrounds and objects we'll cover that on their own pages.
Image Editing
It's very important to note that if you use a normal image editor you'll get very bad results if you translate that directly into GBA memory.
Imagine you have part of an image that's 16 by 16 pixels, aka 2 tiles by 2 tiles. The data for that bitmap is the 1st row of the 1st tile, then the 1st row of the 2nd tile. However, when we translate that into the GBA, the first 8 pixels will indeed be the first 8 tile pixels, but then the next 8 pixels in memory will be used as the 2nd row of the first tile, not the 1st row of the 2nd tile.
So, how do we fix this?
Well, the simple but annoying way is to edit your tile image as being an 8 pixel wide image and then have the image get super tall as you add more and more tiles. It can work, but it's really impractical if you have any multi-tile things that you're trying to do.
Instead, there are some image conversion tools that devkitpro provides in their gba-dev section. They let you take normal images and then repackage them and export it in various formats that you can then compile into your project.
Ketsuban uses the grit tool, with the following suggestions:
- Include an actual resource file and a file describing it somewhere in your project (see the grit manual for all details involved here).
- In a
build.rsyou rungriton each resource+description pair, such as in this old gist example - Then within your rust code you use the include_bytes! macro to have the formatted resource be available as a const value you can load at runtime.