// We want to compress our font textures, because, like, smaller is better,
// right? But a standard DXT compressor doesn't do a great job with fonts that
// are in premultiplied alpha format. Our font data is greyscale, so all of the
// RGBA channels have the same value. If one channel is compressed differently
// to another, this causes an ugly variation in brightness of the rendered text.
// Also, fonts are mostly either black or white, with grey values only used for
// antialiasing along their edges. It is very important that the black and white
// areas be accurately represented, while the precise value of grey is less
// important.
//
// Trouble is, your average DXT compressor knows nothing about these
// requirements. It will optimize to minimize a generic error metric such as
// RMS, but this will often sacrifice crisp black and white in exchange for
// needless accuracy of the antialiasing pixels, or encode RGB differently to
// alpha. UGLY!
//
// Fortunately, encoding monochrome fonts turns out to be trivial. Using DXT3,
// we can fix the end colors as black and white, which gives guaranteed exact
// encoding of the font inside and outside, plus two fractional values for edge
// antialiasing. Also, these RGB values (0, 1/3, 2/3, 1) map exactly to four of
// the possible 16 alpha values available in DXT3, so we can ensure the RGB and
// alpha channels always exactly match.
static void CompressBlock(BitmapUtils.PixelAccessor bitmapData, int blockX, int blockY, SpriteFontAsset options, out BC2Pixel bc2Pixel)
{
long alphaBits = 0;
int rgbBits = 0;
int pixelCount = 0;
for (int y = 0; y < 4; y++)
{
for (int x = 0; x < 4; x++)
{
long alpha;
int rgb;
int value = bitmapData[blockX + x, blockY + y].A;
if (options.IsNotPremultiply)
{
// If we are not pre-multiplied, RGB is always white and we have 4 bit alpha.
alpha = value >> 4;
rgb = 0;
}
else
{
// For pre-multiplied encoding, quantize the source value to 2 bit precision.
if (value < 256 / 6)
{
alpha = 0;
rgb = 1;
}
else if (value < 256 / 2)
{
alpha = 5;
rgb = 3;
}
else if (value < 256 * 5 / 6)
{
alpha = 10;
rgb = 2;
}
else
{
alpha = 15;
rgb = 0;
}
}
// Add this pixel to the alpha and RGB bit masks.
alphaBits |= alpha << (pixelCount * 4);
rgbBits |= rgb << (pixelCount * 2);
pixelCount++;
}
}
// Output the alpha bit mask.
bc2Pixel.AlphaBits = alphaBits;
// Output the two endpoint colors (black and white in 5.6.5 format).
bc2Pixel.EndPoint = 0x0000FFFF;
// Output the RGB bit mask.
bc2Pixel.RgbBits = rgbBits;
}
static Glyph[] ImportFont(SpriteFontAsset options, out float lineSpacing, out float baseLine)
{
// Which importer knows how to read this source font?
IFontImporter importer;
var sourceExtension = (Path.GetExtension(options.Source) ?? "").ToLowerInvariant();
var bitmapFileExtensions = new List <string> {
".bmp", ".png", ".gif"
};
var importFromBitmap = bitmapFileExtensions.Contains(sourceExtension);
importer = importFromBitmap ? (IFontImporter) new BitmapImporter() : new TrueTypeImporter();
// create the list of character to import
var characters = GetCharactersToImport(options);
// Import the source font data.
importer.Import(options, characters);
lineSpacing = importer.LineSpacing;
baseLine = importer.BaseLine;
// Get all glyphs
var glyphs = new List <Glyph>(importer.Glyphs);
// Validate.
if (glyphs.Count == 0)
{
throw new Exception("Font does not contain any glyphs.");
}
if (!importFromBitmap && options.AntiAlias != FontAntiAliasMode.ClearType)
{
foreach (var glyph in importer.Glyphs)
{
BitmapUtils.ConvertGreyToAlpha(glyph.Bitmap, glyph.Subrect);
}
}
// Sort the glyphs
glyphs.Sort((left, right) => left.Character.CompareTo(right.Character));
// Check that the default character is part of the glyphs
if (options.DefaultCharacter != 0)
{
bool defaultCharacterFound = false;
foreach (var glyph in glyphs)
{
if (glyph.Character == options.DefaultCharacter)
{
defaultCharacterFound = true;
break;
}
}
if (!defaultCharacterFound)
{
throw new InvalidOperationException("The specified DefaultCharacter is not part of this font.");
}
}
return(glyphs.ToArray());
}
