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javacompressiondirectxtexturesrgba

scrambled block trying to decompress BC1 texture compression

I've been trying to implement a BC1 (DXT1) decompression algorithm in Java. Everything seems to work pretty precise but I've ran into problem with some blocks around transparent ones. I've been trying to resolve it for a few hours without success.

In short, after decompressing all blocks everything looks good except for the blocks whose are around transparent ones. During the development I've been checking results with results from DirectXTex (texconv) which is written in C++.

This is my result compared to DirectXTex one: enter image description here

Here is the code I'm using:

BufferedImage decompress(byte[] buffer, int width, int height)

and implementation:

  BufferedImage result = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
  int[] scanline = new int[4 * width]; //stores 4 horizontal lines (width/4 blocks)

  RGBA[] blockPalette = new RGBA[4]; //stores RGBA values of current block

  int bufferOffset = 0;

  for (int row = 0; row < height / 4; row++) {
        for (int col = 0; col < width / 4; col++) {

            short rgb0 = Short.reverseBytes(Bytes.getShort(buffer, bufferOffset));
            short rgb1 = Short.reverseBytes(Bytes.getShort(buffer, bufferOffset + 2));
            int bitmap = Integer.reverseBytes(Bytes.getInt(buffer, bufferOffset + 4));
            bufferOffset += 8;

            blockPalette[0] = R5G6B5.decode(rgb0);
            blockPalette[1] = R5G6B5.decode(rgb1);

            if(rgb0 <= rgb1) {
                int c2r = (blockPalette[0].getRed() + blockPalette[1].getRed()) / 2;
                int c2g = (blockPalette[0].getGreen() + blockPalette[1].getGreen()) / 2;
                int c2b = (blockPalette[0].getBlue() + blockPalette[1].getBlue()) / 2;

                 blockPalette[2] = new RGBA(c2r, c2g, c2b, 255);
                 blockPalette[3] = new RGBA(0, 0, 0, 0);

            } else {
                int c2r = (2 * blockPalette[0].getRed() + blockPalette[1].getRed()) / 3;
                int c2g = (2 * blockPalette[0].getGreen() + blockPalette[1].getGreen()) / 3;
                int c2b = (2 * blockPalette[0].getBlue() + blockPalette[1].getBlue()) / 3;

                int c3r = (blockPalette[0].getRed() + 2 * blockPalette[1].getRed()) / 3;
                int c3g = (blockPalette[0].getGreen() + 2 * blockPalette[1].getGreen()) / 3;
                int c3b = (blockPalette[0].getBlue() + 2 * blockPalette[1].getBlue()) / 3;


                blockPalette[2] = new RGBA(c2r, c2g, c2b, 255);
                blockPalette[3] = new RGBA(c3r, c3g, c3b, 255);

            }

            for (int i = 0; i < 16; i++, bitmap >>= 2) {
                int pi = (i / 4) * width + (col * 4 + i % 4);
                int index = bitmap & 3;
                scanline[pi] = A8R8G8B8.encode(blockPalette[index]);
           }
        }
        //copy scanline to buffered image
        result.setRGB(0, row * 4, width, 4, scanline, 0, width);
  }
  return result;

Does anyone have idea where is the problem? I've been doing exactly the same steps as specification says: Block Compression (Direct3D 10)

Solution

  • For those who are interested, I've found that the problem was in comparing short values.

    I've just changed:

    if(rgb0 <= rgb1) {

    to either

    if(Short.compareUnsigned(rgb0, rgb1) <= 0) {

    or

    if((rgb0 & 0xffff) <= (rgb1 & 0xffff)) {

    and this ensures that color values are compared as unsigned shorts (positive integers).