ShadowTXD/DxtDecoder.cs at main · ShadowTheHedgehogHacking/ShadowTXD · GitHub

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namespace ShadowTXD;

internal static class DxtDecoder
{
    public static void Decode(byte[] srcData, int width, int height, int dxtType, bool hasAlpha, byte[] dstRgba, int dstWidth, int dstHeight)
    {
        int blockSize = (dxtType == 1) ? 8 : 16;
        int blocksX = width / 4;
        int blocksY = height / 4;

        for (int by = 0; by < blocksY; by++)
        {
            for (int bx = 0; bx < blocksX; bx++)
            {
                int blockIndex = by * blocksX + bx;
                int blockOffset = blockIndex * blockSize;

                DecodeBlock(srcData, blockOffset, dxtType, out byte[,] colors);

                for (int ly = 0; ly < 4; ly++)
                {
                    for (int lx = 0; lx < 4; lx++)
                    {
                        int px = bx * 4 + lx;
                        int py = by * 4 + ly;

                        if (px < dstWidth && py < dstHeight)
                        {
                            int outIdx = (py * dstWidth + px) * 4;
                            dstRgba[outIdx + 0] = colors[ly, lx];
                            dstRgba[outIdx + 1] = colors[ly, lx + 4];
                            dstRgba[outIdx + 2] = colors[ly, lx + 8];
                            dstRgba[outIdx + 3] = colors[ly, lx + 12];
                        }
                    }
                }
            }
        }
    }

    private static void DecodeBlock(byte[] data, int offset, int dxtType, out byte[,] colors)
    {
        colors = new byte[4, 16];

        if (dxtType == 1)
        {
            ushort c0 = (ushort)(data[offset + 0] | (data[offset + 1] << 8));
            ushort c1 = (ushort)(data[offset + 2] | (data[offset + 3] << 8));
            uint indices = (uint)(data[offset + 4] | (data[offset + 5] << 8) | (data[offset + 6] << 16) | (data[offset + 7] << 24));

            byte r0 = Scale5((byte)(c0 >> 11));
            byte g0 = Scale6((byte)((c0 >> 5) & 0x3F));
            byte b0 = Scale5((byte)(c0 & 0x1F));
            byte r1 = Scale5((byte)(c1 >> 11));
            byte g1 = Scale6((byte)((c1 >> 5) & 0x3F));
            byte b1 = Scale5((byte)(c1 & 0x1F));

            uint[] c = new uint[4];
            c[0] = PackColor(r0, g0, b0, 255);
            c[1] = PackColor(r1, g1, b1, 255);

            if (c0 > c1)
            {
                c[2] = PackColor((byte)((2 * r0 + r1) / 3), (byte)((2 * g0 + g1) / 3), (byte)((2 * b0 + b1) / 3), 255);
                c[3] = PackColor((byte)((r0 + 2 * r1) / 3), (byte)((g0 + 2 * g1) / 3), (byte)((b0 + 2 * b1) / 3), 255);
            }
            else
            {
                c[2] = PackColor((byte)((r0 + r1) / 2), (byte)((g0 + g1) / 2), (byte)((b0 + b1) / 2), 255);
                c[3] = PackColor(0, 0, 0, 0);
            }

            for (int i = 0; i < 16; i++)
            {
                byte idx = (byte)((indices >> (i * 2)) & 3);
                uint col = c[idx];
                int row = i / 4;
                int col2 = i % 4;
                colors[row, col2] = (byte)(col & 0xFF);
                colors[row, col2 + 4] = (byte)((col >> 8) & 0xFF);
                colors[row, col2 + 8] = (byte)((col >> 16) & 0xFF);
                colors[row, col2 + 12] = (byte)((col >> 24) & 0xFF);
            }
        }
        else if (dxtType == 2 || dxtType == 3)
        {
            ulong alphaBits = 0;
            for (int i = 0; i < 8; i++)
                alphaBits |= ((ulong)data[offset + i]) << (i * 8);

            ushort c0 = (ushort)(data[offset + 8] | (data[offset + 9] << 8));
            ushort c1 = (ushort)(data[offset + 10] | (data[offset + 11] << 8));
            uint indices = (uint)(data[offset + 12] | (data[offset + 13] << 8) | (data[offset + 14] << 16) | (data[offset + 15] << 24));

            byte r0 = Scale5((byte)(c0 >> 11));
            byte g0 = Scale6((byte)((c0 >> 5) & 0x3F));
            byte b0 = Scale5((byte)(c0 & 0x1F));
            byte r1 = Scale5((byte)(c1 >> 11));
            byte g1 = Scale6((byte)((c1 >> 5) & 0x3F));
            byte b1 = Scale5((byte)(c1 & 0x1F));

            uint[] c = new uint[4];
            c[0] = PackColor(r0, g0, b0, 255);
            c[1] = PackColor(r1, g1, b1, 255);
            c[2] = PackColor((byte)((2 * r0 + r1) / 3), (byte)((2 * g0 + g1) / 3), (byte)((2 * b0 + b1) / 3), 255);
            c[3] = PackColor((byte)((r0 + 2 * r1) / 3), (byte)((g0 + 2 * g1) / 3), (byte)((b0 + 2 * b1) / 3), 255);

            for (int i = 0; i < 16; i++)
            {
                byte aIdx = (byte)((alphaBits >> (i * 4)) & 0xF);
                byte alpha = (byte)(aIdx * 17);

                byte cIdx = (byte)((indices >> (i * 2)) & 3);
                uint col = c[cIdx];

                int row = i / 4;
                int col2 = i % 4;
                colors[row, col2] = (byte)(col & 0xFF);
                colors[row, col2 + 4] = (byte)((col >> 8) & 0xFF);
                colors[row, col2 + 8] = (byte)((col >> 16) & 0xFF);
                colors[row, col2 + 12] = alpha;
            }
        }
        else if (dxtType == 4 || dxtType == 5)
        {
            byte a0 = data[offset + 0];
            byte a1 = data[offset + 1];

            ulong alphaBits = 0;
            for (int i = 0; i < 6; i++)
                alphaBits |= ((ulong)data[offset + 2 + i]) << (i * 8);

            ushort c0 = (ushort)(data[offset + 8] | (data[offset + 9] << 8));
            ushort c1 = (ushort)(data[offset + 10] | (data[offset + 11] << 8));
            uint colorIndices = (uint)(data[offset + 12] | (data[offset + 13] << 8) | (data[offset + 14] << 16) | (data[offset + 15] << 24));

            byte r0 = Scale5((byte)(c0 >> 11));
            byte g0 = Scale6((byte)((c0 >> 5) & 0x3F));
            byte b0 = Scale5((byte)(c0 & 0x1F));
            byte r1 = Scale5((byte)(c1 >> 11));
            byte g1 = Scale6((byte)((c1 >> 5) & 0x3F));
            byte b1 = Scale5((byte)(c1 & 0x1F));

            uint[] c = new uint[4];
            c[0] = PackColor(r0, g0, b0, 255);
            c[1] = PackColor(r1, g1, b1, 255);
            c[2] = PackColor((byte)((2 * r0 + r1) / 3), (byte)((2 * g0 + g1) / 3), (byte)((2 * b0 + b1) / 3), 255);
            c[3] = PackColor((byte)((r0 + 2 * r1) / 3), (byte)((g0 + 2 * g1) / 3), (byte)((b0 + 2 * b1) / 3), 255);

            byte[] alphas = new byte[8];
            alphas[0] = a0;
            alphas[1] = a1;
            for (int i = 2; i < 8; i++)
            {
                if (a0 > a1)
                    alphas[i] = (byte)(((6 - (i - 2)) * a0 + ((i - 2) + 1) * a1) / 7);
                else if (i == 6)
                    alphas[i] = 0;
                else if (i == 7)
                    alphas[i] = 255;
                else
                    alphas[i] = (byte)(((4 - (i - 2)) * a0 + ((i - 2) + 1) * a1) / 5);
            }

            for (int i = 0; i < 16; i++)
            {
                byte aIdx = (byte)((alphaBits >> (i * 3)) & 7);
                byte alpha = alphas[aIdx];
                byte cIdx = (byte)((colorIndices >> (i * 2)) & 3);
                uint col = c[cIdx];
                int row = i / 4;
                int col2 = i % 4;
                colors[row, col2] = (byte)(col & 0xFF);
                colors[row, col2 + 4] = (byte)((col >> 8) & 0xFF);
                colors[row, col2 + 8] = (byte)((col >> 16) & 0xFF);
                colors[row, col2 + 12] = alpha;
            }
        }
    }

    private static byte Scale5(byte v) => (byte)(v * 255 / 31);
    private static byte Scale6(byte v) => (byte)(v * 255 / 63);
    private static uint PackColor(byte r, byte g, byte b, byte a) => (uint)(r | (g << 8) | (b << 16) | ((uint)a << 24));
}