Question regarding background priority

[Zstorm999]

Hello,

I’m doing some quick testing with the library, and I have trouble with the background priorities. I have created two backgrounds with different priorities from the same Tiled0 instance. However, no matter what priority I assign to each one of them, the second background that is created is always displayed on top.

What I find strange is that the map inspector from mGBA shows that they have the correct priorities in RAM, they are just not showing in the correct order. Do you know what might cause this ?

Here is my code :

let (tiled, mut vram) = gba.display.video.tiled0();
let mut bg = tiled.background(Priority::P0, RegularBackgroundSize::Background32x32);
let mut bg2 = tiled.background(Priority::P1, RegularBackgroundSize::Background32x32);

let tileset = TileSet::new(tile_sheet::grass.tiles, TileFormat::FourBpp);
vram.set_background_palettes(tile_sheet::grass.palettes);

for y in 0..20u16 {
  for x in 0..30u16 {
    bg.set_tile(
      &mut vram,
      (x, y).into(),
      &tileset,
      TileSetting::new(0, false, false, 0),
    );

    bg2.set_tile(
        &mut vram,
        (x, y).into(),
        &tileset,
        TileSetting::new(1, false, false, 0),
    );
  }
}

bg.commit(&mut vram);
bg2.commit(&mut vram);

bg.show();
bg2.show();

Since 0 is higher priority than 1, I’m expecting to see bg to appear on top, but it’s not the case (I tested the other way around, and bg2 always shows up on top, as if the priorities were just ignored).

[corwinkuiper]

Hi! I checked out your repo.

The issue here comes down to not specifying a transparent colour in your background toml file. You can specify a colour that you don't use, for instance. So your toml file would read:

version = "1.0"

[image.grass]
filename = "tile_sheet.png"
tile_size = "8x8"
transparent_colour = "FF00FF"

The reason for this is that agb by default will choose a transparent colour for you, which in this case was your dominant green colour. For this reason when rendering bg2 every tile was transparent and nothing from that background was rendered. By specifying a transparent colour that you don't use, there will be no transparent colour and every tile will be rendered as you expect.

Good luck with your game :)

[Zstorm999]

Thanks a lot ! I very new to gba development, I didn’t even realize this was a thing.

[gwilymk]

Once you get further with your game development, you might want to look at using partially transparent background tiles so you can have a foreground and a background for your sprites. You can see how we did it in the the-hat-chooses-the-wizard's tile_sheet here: https://github.com/agbrs/agb/tree/master/examples/the-hat-chooses-the-wizard/gfx 😄