Function encodeBlock

extern/toojpeg/toojpeg.cpp:252–323

run DCT, quantize and write Huffman bit codes

// run DCT, quantize and write Huffman bit codes
int16_t encodeBlock(BitWriter& writer, float block[8][8], const float scaled[8*8], int16_t lastDC,
                    const BitCode huffmanDC[256], const BitCode huffmanAC[256], const BitCode* codewords)
{
  // "linearize" the 8x8 block, treat it as a flat array of 64 floats
  auto block64 = (float*) block;

  // DCT: rows
  for (auto offset = 0; offset < 8; offset++)
    DCT(block64 + offset*8, 1);
  // DCT: columns
  for (auto offset = 0; offset < 8; offset++)
    DCT(block64 + offset*1, 8);

  // scale
  for (auto i = 0; i < 8*8; i++)
    block64[i] *= scaled[i];

  // encode DC (the first coefficient is the "average color" of the 8x8 block)
  auto DC = int(block64[0] + (block64[0] >= 0 ? +0.5f : -0.5f)); // C++11's nearbyint() achieves a similar effect

  // quantize and zigzag the other 63 coefficients
  auto posNonZero = 0; // find last coefficient which is not zero (because trailing zeros are encoded differently)
  int16_t quantized[8*8];
  for (auto i = 1; i < 8*8; i++) // start at 1 because block64[0]=DC was already processed
  {
    auto value = block64[ZigZagInv[i]];
    // round to nearest integer
    quantized[i] = int(value + (value >= 0 ? +0.5f : -0.5f)); // C++11's nearbyint() achieves a similar effect
    // remember offset of last non-zero coefficient
    if (quantized[i] != 0)
      posNonZero = i;
  }

  // same "average color" as previous block ?
  auto diff = DC - lastDC;
  if (diff == 0)
    writer << huffmanDC[0x00];   // yes, write a special short symbol
  else
  {
    auto bits = codewords[diff]; // nope, encode the difference to previous block's average color
    writer << huffmanDC[bits.numBits] << bits;
  }

  // encode ACs (quantized[1..63])
  auto offset = 0; // upper 4 bits count the number of consecutive zeros
  for (auto i = 1; i <= posNonZero; i++) // quantized[0] was already written, skip all trailing zeros, too
  {
    // zeros are encoded in a special way
    while (quantized[i] == 0) // found another zero ?
    {
      offset    += 0x10; // add 1 to the upper 4 bits
      // split into blocks of at most 16 consecutive zeros
      if (offset > 0xF0) // remember, the counter is in the upper 4 bits, 0xF = 15
      {
        writer << huffmanAC[0xF0]; // 0xF0 is a special code for "16 zeros"
        offset = 0;
      }
      i++;