Weird striping in tile graphics on Nintendo DS

Question

I'm working on a pong game for the Nintendo DS. I'm using libnds to set things up and I've come across a very strange behaviour. So far I've only tried it out in emulators, but I use three different ones and they all exhibit this behaviour so I suspect I'm doing something bad.

The actual problem is that when I use background layer 1 or above for my tiled graphics, I get weird stripes all over that layer. If I use background layer 0 the problem goes away, but since that is rendered last, I cannot actually draw over it in another layer, which I want to.

My setup code:

void pong::setup_engine()
{
    // Setup DS graphics engine.
    // -------------------------
    videoSetMode(MODE_5_2D | DISPLAY_BG0_ACTIVE | DISPLAY_BG1_ACTIVE | DISPLAY_BG2_ACTIVE);
    vramSetBankA(VRAM_A_MAIN_BG_0x06000000);
    vramSetBankB(VRAM_B_MAIN_BG_0x06020000);

    u8 *tileMemory = reinterpret_cast<u8 *>(BG_TILE_RAM(1));
    u16 *mapMemory = reinterpret_cast<u16 *>(BG_MAP_RAM(0));

    int bg0 = bgInit(1, BgType_Text8bpp, BgSize_T_256x256, 0, 1);
    int bg1 = bgInit(2, BgType_Bmp16, BgSize_B16_256x256, 0, 0);
    //bgScroll(bg0, 256, 256);

    u16 *ptr = bgGetGfxPtr(bg1);
    for (int y = 10; y < 128*60; y++)
    {
        ptr[y] = 0xFFFF;
    }

    BG_PALETTE[1] = grey0;
    BG_PALETTE[2] = grey1;
    BG_PALETTE[3] = grey2;
    BG_PALETTE[4] = grey3;
    BG_PALETTE[5] = grey4;
    BG_PALETTE[6] = grey5;
    BG_PALETTE[7] = grey6;

    // 32 here is not 32 bytes but 32 half-words, which is 64 bytes.
    swiCopy(CORNER_TILE, tileMemory, 32);
    swiCopy(TOP_TILE, tileMemory + 64, 32);
    swiCopy(SIDE_TILE, tileMemory + (64 * 2), 32);
    swiCopy(MAIN_TILE, tileMemory + (64 * 3), 32);

    swiCopy(MAP, mapMemory, 32*24);
}

In the above code I use layer 1 for bg0, which is my tiled graphics layer. This makes the weird stripes appear; if I were to change it to 0 like this, it would show up as expected:

int bg0 = bgInit(0, BgType_Text8bpp, BgSize_T_256x256, 0, 1);

Any ideas what causes the problem, and what the solution is?

Answer 1

a breif look at background.h makes it look like you are using BG1 and BG2. For mode 5, according to:

http://nocash.emubase.de/gbatek.htm

layer 0 and 1 are normal and 2 and 3 are extended. I dont know what extended means. If you want to just do normal tile stuff you probably want to bgInit 0 and 1 not 1 and 2.

You can change the priorities around at will, layer 0 is not necessarily on top of 1, etc. Look at the BGxCNT registers (BG0CNT, BG1CNT,...) and the priority bits. Now if the priority is a tie then yes the layer number determines who wins.

Answer 2

Two things. For one, the striping is most likely a VRAM conflict, although it's been a while since I did DS dev last. Double check where you're copying your graphics and your tile data. Mode 5 uses two text backgrounds, which should be behaving exactly the same if they're initialized a certain way, so I'm not sure what's going on there.

In any case, since you have two backgrounds working, why not just set their priorities in their registers to swap their z order? Background are only drawn in a particular order by default, you can set their priority to have the system draw them in any order you like.