private static OldToNewGlyphIndex[] GetIndexMapping(TrueTypeFont font, TrueTypeSubsetEncoding newEncoding)
{
var result = new OldToNewGlyphIndex[newEncoding.Characters.Count + 1];
result[0] = new OldToNewGlyphIndex(0, 0, '\0');
var previousCMap = font.WindowsUnicodeCMap ?? font.WindowsSymbolCMap ?? font.MacRomanCMap;
if (previousCMap == null)
{
throw new InvalidOperationException("Cannot subset font due to missing cmap subtables.");
}
for (var i = 0; i < newEncoding.Characters.Count; i++)
{
var character = newEncoding.Characters[i];
var oldIndex = (ushort)previousCMap.CharacterCodeToGlyphIndex(character);
result[i + 1] = new OldToNewGlyphIndex(oldIndex, (ushort)(i + 1), character);
}
return(result);
}
/// <summary>
/// Generate a subset of the input font containing only the data required for the glyphs specified in the encoding.
/// </summary>
public static byte[] Subset(byte[] fontBytes, TrueTypeSubsetEncoding newEncoding)
{
if (fontBytes == null)
{
throw new ArgumentNullException(nameof(fontBytes));
}
if (newEncoding == null)
{
throw new ArgumentNullException(nameof(newEncoding));
}
var font = TrueTypeFontParser.Parse(new TrueTypeDataBytes(fontBytes));
var indexMapping = GetIndexMapping(font, newEncoding);
using (var stream = new MemoryStream())
{
var copiedTableTags = new SortedSet <string>(StringComparer.Ordinal);
for (var i = 0; i < RequiredTags.Count; i++)
{
var tag = RequiredTags[i];
if (!font.TableHeaders.ContainsKey(tag))
{
throw new InvalidFontFormatException($"Font does not contain table required for subsetting: {tag}.");
}
copiedTableTags.Add(tag);
}
for (var i = 0; i < OptionalTags.Count; i++)
{
var tag = OptionalTags[i];
if (!font.TableHeaders.ContainsKey(tag))
{
continue;
}
copiedTableTags.Add(tag);
}
var offsetSubtable = new TrueTypeOffsetSubtable((byte)copiedTableTags.Count);
offsetSubtable.Write(stream);
// The table directory follows the offset subtable.
var directoryEntries = new DirectoryEntry[copiedTableTags.Count];
// Entries in the table directory must be sorted in ascending order by tag (case sensitive).
// Each table in the font file must have its own table directory entry.
var index = 0;
foreach (var tag in copiedTableTags)
{
var entry = new DirectoryEntry(tag, stream.Position, font.TableHeaders[tag]);
entry.DummyHeader.Write(stream);
directoryEntries[index++] = entry;
}
// Generate the glyph subset.
TrueTypeSubsetGlyphTable trueTypeSubsetGlyphTable = TrueTypeGlyphTableSubsetter.SubsetGlyphTable(font, fontBytes, indexMapping);
// Write the actual tables.
for (var i = 0; i < directoryEntries.Length; i++)
{
var entry = directoryEntries[i];
entry.OutputTableOffset = stream.Position;
if (entry.Tag == TrueTypeHeaderTable.Cmap)
{
var cmapTable = GetCMapTable(font, entry, indexMapping);
cmapTable.Write(stream);
}
else if (entry.Tag == TrueTypeHeaderTable.Glyf)
{
stream.Write(trueTypeSubsetGlyphTable.Bytes, 0, trueTypeSubsetGlyphTable.Bytes.Length);
}
else if (entry.Tag == TrueTypeHeaderTable.Hmtx)
{
var hmtx = GetHorizontalMetricsTable(entry, trueTypeSubsetGlyphTable);
hmtx.Write(stream);
}
else if (entry.Tag == TrueTypeHeaderTable.Loca)
{
var table = new IndexToLocationTable(entry.DummyHeader, IndexToLocationTable.EntryFormat.Long,
trueTypeSubsetGlyphTable.GlyphOffsets);
table.Write(stream);
}
else if (entry.Tag == TrueTypeHeaderTable.Head)
{
// Update indexToLoc format.
var headBytes = GetRawInputTableBytes(fontBytes, entry);
WriteUShort(headBytes, headBytes.Length - 4, IndexToLocLong);
stream.Write(headBytes, 0, headBytes.Length);
// TODO: zero out checksum adjustment bytes.
}
else if (entry.Tag == TrueTypeHeaderTable.Hhea)
{
// Update number of h metrics.
var hheaBytes = GetRawInputTableBytes(fontBytes, entry);
WriteUShort(hheaBytes, hheaBytes.Length - 2, (ushort)trueTypeSubsetGlyphTable.HorizontalMetrics.Length);
stream.Write(hheaBytes, 0, hheaBytes.Length);
}
else if (entry.Tag == TrueTypeHeaderTable.Maxp)
{
// Update number of glyphs.
var maxpBytes = GetRawInputTableBytes(fontBytes, entry);
WriteUShort(maxpBytes, 4, trueTypeSubsetGlyphTable.GlyphCount);
stream.Write(maxpBytes, 0, maxpBytes.Length);
}
else
{
// Copy table as-is.
var buffer = GetRawInputTableBytes(fontBytes, entry);
stream.Write(buffer, 0, buffer.Length);
}
entry.Length = (uint)(stream.Position - entry.OutputTableOffset);
// Tables must start on 4 byte boundaries.
var remainder = stream.Position % 4;
if (remainder > 0)
{
var toAppend = 4 - remainder;
stream.Write(PaddingBytes, 0, (int)toAppend);
}
}
using (var inputBytes = new StreamInputBytes(stream, false))
{
// Update the table directory.
for (var i = 0; i < directoryEntries.Length; i++)
{
var entry = directoryEntries[i];
var actualHeaderExceptChecksum = new TrueTypeHeaderTable(entry.Tag, 0, (uint)entry.OutputTableOffset, entry.Length);
var checksum = TrueTypeChecksumCalculator.Calculate(inputBytes, actualHeaderExceptChecksum);
var actualHeader = new TrueTypeHeaderTable(entry.Tag, checksum, (uint)entry.OutputTableOffset, entry.Length);
stream.Seek(entry.OutputEntryOffset, SeekOrigin.Begin);
actualHeader.Write(stream);
}
}
// TODO: whole font checksum.
var result = stream.ToArray();
return(result);
}
}