Method readHeader

external/tga/decoder.cpp:23–83

}

bool Decoder::readHeader(Header& header)
{
  header.idLength = read8();
  header.colormapType = read8();
  header.imageType = read8();
  header.colormapOrigin = read16();
  header.colormapLength  = read16();
  header.colormapDepth = read8();
  header.xOrigin = read16();
  header.yOrigin = read16();
  header.width = read16();
  header.height = read16();
  header.bitsPerPixel = read8();
  header.imageDescriptor = read8();

  // Invalid image size
  if (header.width == 0 ||
      header.height == 0)
    return false;

  // Skip ID string (idLength bytes)
  if (header.idLength > 0) {
    uint8_t i = header.idLength;
    while (i--) {
      uint8_t chr = m_file->read8();
      header.imageId.push_back(chr);
    }
  }

#if 0
  // In the best case the "alphaBits" should be valid, but there are
  // invalid TGA files out there which don't indicate the
  // "alphaBits" correctly, so they could be 0 and use the alpha
  // channel anyway on each pixel.
  int alphaBits = (header.imageDescriptor & 15);
  m_hasAlpha =
    (header.bitsPerPixel == 32 && alphaBits == 8) ||
    (header.bitsPerPixel == 16 && alphaBits == 1);
#else
  // So to detect if a 32bpp or 16bpp TGA image has alpha, we'll use
  // the "alpha histogram" in postProcessImage() to check if there are
  // different alpha values. If there is only one alpha value (all 0
  // or all 255), we create an opaque image anyway. The only exception
  // to this rule is when all pixels are black and transparent
  // (RGBA=0), that is the only case when an image is fully
  // transparent.
  //
  // Note: This same heuristic is used in apps like macOS Preview:
  // https://twitter.com/davidcapello/status/1242803110868893697
  m_hasAlpha =
    (header.bitsPerPixel == 32) ||
    (header.bitsPerPixel == 16);
#endif

  // Read colormap
  if (header.colormapType == 1)
    readColormap(header);