Wrong tile generation

[pladaria]

Hi,

I'm having issues with some PNGs, it looks like the tile extraction has some problem

:point_right: Download project files: issue2.zip

main.c

#include <genesis.h>

#include "../res/bg.h"

int main(bool hardReset) {
    VDP_setScreenWidth320();
    VDP_setScreenHeight224();
    VDP_setPlaneSize(64, 32, TRUE);

    VDP_drawImageEx(BG_B, &resBgSea, TILE_ATTR_FULL(PAL1, 0, 0, 0, TILE_USERINDEX), 0, 0, FALSE, FALSE);
    VDP_setPalette(PAL1, resBgSea.palette->data);

    while (TRUE) {
        SYS_doVBlankProcess();
    }

    return 0;
}

Source image

Nothing special with this file, just an indexed PNG exported by GIMP

Result

VDP

[heitorgcosta]

I'd assume the issue is the bit depth, which GIMP seems to have exported as 2 bits. Opening the picture through Aseprite and exporting it as an indexed png (8-bit depth) made it so that the image loads correctly with SGDK. I could be wrong though, the issue may be something else.

[pladaria]

Yep, forcing the image to 8bpp solves the issue, so there must be something wrong in the PNG bitstream reading when the samples are stored at 2bpp

[matteusbeus]

yes I've had this problem previously!

[alicesim1]

It turns out that the PNG file; if it is 4 colors, but it is exported by GIMP and it is not fully compatible, only partially with Rescom.

If the PNG is opened for example with Aseprite and the PNG is saved is 4 colors, again if it works perfectly with Rescom.

[heitorgcosta]

Did a bit of digging and it turns out the ImageUtil.java file has a typo specifically on the function that converts 2bpp to 8bpp that rescomp calls.

static byte[] convert2bppTo8bpp(byte[] data)
{
    final byte[] result = new byte[data.length * 4];

    for (int i = 0; i < data.length; i++)
    {
        result[(i * 4) + 0] = (byte) ((data[i] >> 6) & 0x03);
        result[(i * 4) + 1] = (byte) ((data[i] >> 4) & 0x03);
        result[(i * 4) + 2] = (byte) ((data[i] >> 3) & 0x03); <- Should shift by 2 instead of 3
        result[(i * 4) + 3] = (byte) ((data[i] >> 0) & 0x03);
    }

    return result;
}

Adjusting it and re-exporting the rescomp jar fixed the issue.

[pladaria]

Did a bit of digging and it turns out the ImageUtil.java file has a typo specifically on the function that converts 2bpp to 8bpp that rescomp calls. ... Adjusting it and re-exporting the rescomp jar fixed the issue.

good catch!

[Stephane-D]

Indeed good catch ! Just fixed :) Thanks for pointing it 👍