/// If the size of the code is larger than two, we must transmit the length
/// of every symbol in the code. To save space, the lengths are themselves
/// Huffman-coded with a length code, so the lengths of the length code must
/// be stored first.
static void WriteNormalCodeLengths(BitWriter b, List <Huffman.Code> codes)
{
var encodedLengths = EncodeCodeLengths(codes);
var lengthHisto = new Histogram(19);
for (int i = 0; i < encodedLengths.Count; ++i)
{
int sym = encodedLengths[i];
lengthHisto.Hit(sym);
if (sym >= 16)
{
i += 1;
}
}
var lengthCodes = Huffman.BuildCodes(lengthHisto, 7);
int lengthCodeCount = 0;
for (int i = 0; i < 19; ++i)
{
if (lengthHisto[CODE_LENGTH_ORDER[i]] > 0)
{
lengthCodeCount = i + 1;
}
}
if (lengthCodeCount < 4)
{
lengthCodeCount = 4;
}
b.WriteBits(0, 1);
b.WriteBits(lengthCodeCount - 4, 4);
for (int i = 0; i < lengthCodeCount; ++i)
{
b.WriteBits(lengthCodes[CODE_LENGTH_ORDER[i]].Length, 3);
}
b.WriteBits(0, 1);
for (int i = 0; i < encodedLengths.Count; ++i)
{
int sym = encodedLengths[i];
b.WriteCode(lengthCodes[sym]);
if (sym == 16)
{
b.WriteBits(encodedLengths[++i], 2);
}
else if (sym == 17)
{
b.WriteBits(encodedLengths[++i], 3);
}
else if (sym == 18)
{
b.WriteBits(encodedLengths[++i], 7);
}
}
}
/// Encodes the image into entropy-coded bitstream. The isRecursive flag
/// must be true iff the data represents the main ARGB image.
internal static void WriteImageData(BitWriter b, Image image,
bool isRecursive, int colorCacheBits = 0)
{
if (colorCacheBits > 0)
{
if (colorCacheBits > 11)
{
throw new ArgumentException("Too many color cache bits");
}
b.WriteBits(1, 1);
b.WriteBits(colorCacheBits, 4);
}
else
{
b.WriteBits(0, 1);
}
if (isRecursive)
{
b.WriteBits(0, 1); // no meta-Huffman image
}
var encoded = EncodeImageData(image, colorCacheBits);
var histos = new List <Histogram> {
new Histogram(256 + 24 + (colorCacheBits > 0 ? (1 << colorCacheBits) : 0)),
new Histogram(256),
new Histogram(256),
new Histogram(256),
new Histogram(40),
};
for (int i = 0; i < encoded.Count; ++i)
{
histos[0].Hit(encoded[i]);
if (encoded[i] < 256)
{
histos[1].Hit(encoded[i + 1]);
histos[2].Hit(encoded[i + 2]);
histos[3].Hit(encoded[i + 3]);
i += 3;
}
else if (encoded[i] < 256 + 24)
{
histos[4].Hit(encoded[i + 2]);
i += 3;
}
}
var codess = new List <List <Huffman.Code> >();
for (int i = 0; i < 5; ++i)
{
var codes = Huffman.BuildCodes(histos[i], 16);
CodeLengths.WriteCodeLengths(b, codes);
codess.Add(codes);
}
for (int i = 0; i < encoded.Count; ++i)
{
b.WriteCode(codess[0][encoded[i]]);
if (encoded[i] < 256)
{
b.WriteCode(codess[1][encoded[i + 1]]);
b.WriteCode(codess[2][encoded[i + 2]]);
b.WriteCode(codess[3][encoded[i + 3]]);
i += 3;
}
else if (encoded[i] < 256 + 24)
{
b.WriteBits(encoded[i + 1], ExtraBitsCount(encoded[i] - 256));
b.WriteCode(codess[4][encoded[i + 2]]);
b.WriteBits(encoded[i + 3], ExtraBitsCount(encoded[i + 2]));
i += 3;
}
}
}