CSharpImageLibrary.DDS.DDS_BlockHelpers.Decompress8BitBlock C# (CSharp) Method

        internal static byte[] Decompress8BitBlock(byte[] source, int sourceStart, bool isSigned)
        {
            byte[] DecompressedBlock = new byte[16];

            // KFreon: Read min and max colours (not necessarily in that order)
            byte min = source[sourceStart];
            byte max = source[sourceStart + 1];

            byte[] Colours = Build8BitPalette(min, max, isSigned);

            // KFreon: Decompress pixels
            ulong bitmask = (ulong)source[sourceStart + 2] << 0 | (ulong)source[sourceStart + 3] << 8 | (ulong)source[sourceStart + 4] << 16 |   // KFreon: Read all 6 compressed bytes into single.
                (ulong)source[sourceStart + 5] << 24 | (ulong)source[sourceStart + 6] << 32 | (ulong)source[sourceStart + 7] << 40;

            // KFreon: Bitshift and mask compressed data to get 3 bit indicies, and retrieve indexed colour of pixel.
            for (int i = 0; i < 16; i++)
                DecompressedBlock[i] = (byte)Colours[bitmask >> (i * 3) & 0x7];

            return DecompressedBlock;
        }

        // BC5
        internal static void DecompressATI2Block(byte[] source, int sourceStart, byte[] destination, int decompressedStart, int decompressedLineLength, bool unused)
        {
            // Green = +1
            DDS_BlockHelpers.Decompress8BitBlock(source, sourceStart, destination, decompressedStart + 1, decompressedLineLength, false);


            // Red = +2, source + 8 to skip first compressed block.
            DDS_BlockHelpers.Decompress8BitBlock(source, sourceStart + 8, destination, decompressedStart + 2, decompressedLineLength, false);


            // KFreon: Alpha is 255, and blue needs to be calculated
            for (int i = 0; i < 16; i++)
            {
                int offset = GetDecompressedOffset(decompressedStart, decompressedLineLength, i);

                // Get Red and Green on the range -1 - 1
                // *2-1 moves the range from 0 - 1, to -1 - 1
                double green = (destination[offset + 1] / 127.5d) - 1d;
                double red   = (destination[offset + 2] / 127.5d) - 1d;

                // Z solution for: x2 + y2 + z2 = 1, unit normal vectors. Only consider +ve root as ATI2 is a tangent space mapping and Z must be +ve.
                // Also when 1 - x2 - y2 < 0, Z = NaN, but is compensated for in ExpandTo255.
                double Z = Math.Sqrt(1d - (red * red + green * green));

                destination[offset]     = ExpandTo255(Z); // Blue
                destination[offset + 3] = 255;            // Alpha
            }
        }