/// <summary>
/// Builds a 4-color color table for a DXT1-compressed 4x4 block of texels and then decodes the texels.
/// </summary>
/// <remarks>See https://msdn.microsoft.com/en-us/library/windows/desktop/bb694531(v=vs.85).aspx#BC1 </remarks>
static Color32[] DecodeDXT1TexelBlock(UnityBinaryReader r, bool containsAlpha)
{
// Create the color table.
var colorTable = new Color[4];
colorTable[0] = ColorEx.B565ToColor(r.ReadLEUInt16());
colorTable[1] = ColorEx.B565ToColor(r.ReadLEUInt16());
if (!containsAlpha)
{
colorTable[2] = Color.Lerp(colorTable[0], colorTable[1], 1.0f / 3);
colorTable[3] = Color.Lerp(colorTable[0], colorTable[1], 2.0f / 3);
}
else
{
colorTable[2] = Color.Lerp(colorTable[0], colorTable[1], 1.0f / 2);
colorTable[3] = new Color(0, 0, 0, 0);
}
// Calculate pixel colors.
return(DecodeDXT1TexelBlock(r, colorTable));
}
/// <summary>
/// Decodes a DXT3-compressed 4x4 block of texels.
/// </summary>
/// <remarks>See https://msdn.microsoft.com/en-us/library/windows/desktop/bb694531(v=vs.85).aspx#BC2 </remarks>
static Color32[] DecodeDXT3TexelBlock(UnityBinaryReader r)
{
// Read compressed pixel alphas.
var compressedAlphas = new byte[16];
for (var rowIndex = 0; rowIndex < 4; rowIndex++)
{
var compressedAlphaRow = r.ReadLEUInt16();
for (var columnIndex = 0; columnIndex < 4; columnIndex++)
{
// Each compressed alpha is 4 bits.
compressedAlphas[(4 * rowIndex) + columnIndex] = (byte)((compressedAlphaRow >> (columnIndex * 4)) & 0xF);
}
}
// Calculate pixel alphas.
var alphas = new byte[16];
for (var i = 0; i < 16; i++)
{
var alphaPercent = (float)compressedAlphas[i] / 15;
alphas[i] = (byte)Mathf.RoundToInt(alphaPercent * 255);
}
// Create the color table.
var colorTable = new Color[4];
colorTable[0] = ColorEx.B565ToColor(r.ReadLEUInt16());
colorTable[1] = ColorEx.B565ToColor(r.ReadLEUInt16());
colorTable[2] = Color.Lerp(colorTable[0], colorTable[1], 1.0f / 3);
colorTable[3] = Color.Lerp(colorTable[0], colorTable[1], 2.0f / 3);
// Calculate pixel colors.
var colors = DecodeDXT1TexelBlock(r, colorTable);
for (var i = 0; i < 16; i++)
{
colors[i].a = alphas[i];
}
return(colors);
}
/// <summary>
/// Decodes a DXT5-compressed 4x4 block of texels.
/// </summary>
/// <remarks>See https://msdn.microsoft.com/en-us/library/windows/desktop/bb694531(v=vs.85).aspx#BC3 </remarks>
static Color32[] DecodeDXT5TexelBlock(UnityBinaryReader r)
{
// Create the alpha table.
var alphaTable = new float[8];
alphaTable[0] = r.ReadByte();
alphaTable[1] = r.ReadByte();
if (alphaTable[0] > alphaTable[1])
{
for (var i = 0; i < 6; i++)
{
alphaTable[2 + i] = Mathf.Lerp(alphaTable[0], alphaTable[1], (float)(1 + i) / 7);
}
}
else
{
for (var i = 0; i < 4; i++)
{
alphaTable[2 + i] = Mathf.Lerp(alphaTable[0], alphaTable[1], (float)(1 + i) / 5);
}
alphaTable[6] = 0;
alphaTable[7] = 255;
}
// Read pixel alpha indices.
var alphaIndices = new uint[16];
var alphaIndexBytesRow0 = new byte[3];
r.Read(alphaIndexBytesRow0, 0, alphaIndexBytesRow0.Length);
Array.Reverse(alphaIndexBytesRow0); // Take care of little-endianness.
var alphaIndexBytesRow1 = new byte[3];
r.Read(alphaIndexBytesRow1, 0, alphaIndexBytesRow1.Length);
Array.Reverse(alphaIndexBytesRow1); // Take care of little-endianness.
const uint bitsPerAlphaIndex = 3;
alphaIndices[0] = (uint)Utils.GetBits(21, bitsPerAlphaIndex, alphaIndexBytesRow0);
alphaIndices[1] = (uint)Utils.GetBits(18, bitsPerAlphaIndex, alphaIndexBytesRow0);
alphaIndices[2] = (uint)Utils.GetBits(15, bitsPerAlphaIndex, alphaIndexBytesRow0);
alphaIndices[3] = (uint)Utils.GetBits(12, bitsPerAlphaIndex, alphaIndexBytesRow0);
alphaIndices[4] = (uint)Utils.GetBits(9, bitsPerAlphaIndex, alphaIndexBytesRow0);
alphaIndices[5] = (uint)Utils.GetBits(6, bitsPerAlphaIndex, alphaIndexBytesRow0);
alphaIndices[6] = (uint)Utils.GetBits(3, bitsPerAlphaIndex, alphaIndexBytesRow0);
alphaIndices[7] = (uint)Utils.GetBits(0, bitsPerAlphaIndex, alphaIndexBytesRow0);
alphaIndices[8] = (uint)Utils.GetBits(21, bitsPerAlphaIndex, alphaIndexBytesRow1);
alphaIndices[9] = (uint)Utils.GetBits(18, bitsPerAlphaIndex, alphaIndexBytesRow1);
alphaIndices[10] = (uint)Utils.GetBits(15, bitsPerAlphaIndex, alphaIndexBytesRow1);
alphaIndices[11] = (uint)Utils.GetBits(12, bitsPerAlphaIndex, alphaIndexBytesRow1);
alphaIndices[12] = (uint)Utils.GetBits(9, bitsPerAlphaIndex, alphaIndexBytesRow1);
alphaIndices[13] = (uint)Utils.GetBits(6, bitsPerAlphaIndex, alphaIndexBytesRow1);
alphaIndices[14] = (uint)Utils.GetBits(3, bitsPerAlphaIndex, alphaIndexBytesRow1);
alphaIndices[15] = (uint)Utils.GetBits(0, bitsPerAlphaIndex, alphaIndexBytesRow1);
// Create the color table.
var colorTable = new Color[4];
colorTable[0] = ColorEx.B565ToColor(r.ReadLEUInt16());
colorTable[1] = ColorEx.B565ToColor(r.ReadLEUInt16());
colorTable[2] = Color.Lerp(colorTable[0], colorTable[1], 1.0f / 3);
colorTable[3] = Color.Lerp(colorTable[0], colorTable[1], 2.0f / 3);
// Calculate pixel colors.
var colors = DecodeDXT1TexelBlock(r, colorTable);
for (var i = 0; i < 16; i++)
{
colors[i].a = (byte)Mathf.Round(alphaTable[alphaIndices[i]]);
}
return(colors);
}