Method readPngMetadata

source/core/StarImage.cpp:121–198

}

tuple<Vec2U, PixelFormat> Image::readPngMetadata(IODevicePtr device) {
  png_byte header[8];
  device->readFull((char*)header, sizeof(header));

  if (png_sig_cmp(header, 0, sizeof(header)))
    throw ImageException(strf("File {} is not a png image!", device->deviceName()));

  png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, nullptr, nullptr, nullptr);
  if (!png_ptr)
    throw ImageException("Internal libPNG error");

  // Use custom warning function to suppress cerr warnings
  png_set_error_fn(png_ptr, (png_voidp)device.get(), logPngError, logPngWarning);

  png_infop info_ptr = png_create_info_struct(png_ptr);
  if (!info_ptr) {
    png_destroy_read_struct(&png_ptr, nullptr, nullptr);
    throw ImageException("Internal libPNG error");
  }

  png_infop end_info = png_create_info_struct(png_ptr);
  if (!end_info) {
    png_destroy_read_struct(&png_ptr, &info_ptr, nullptr);
    throw ImageException("Internal libPNG error");
  }

  if (setjmp(png_jmpbuf(png_ptr))) {
    png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
    throw ImageException("Internal error reading png.");
  }

  png_set_read_fn(png_ptr, device.get(), readPngData);

  // Tell libPNG that we read some of the header.
  png_set_sig_bytes(png_ptr, sizeof(header));

  png_read_info(png_ptr, info_ptr);

  png_uint_32 img_width = png_get_image_width(png_ptr, info_ptr);
  png_uint_32 img_height = png_get_image_height(png_ptr, info_ptr);

  png_uint_32 bitdepth = png_get_bit_depth(png_ptr, info_ptr);
  png_uint_32 channels = png_get_channels(png_ptr, info_ptr);

  // Color type. (RGB, RGBA, Luminance, luminance alpha... palette... etc)
  png_uint_32 color_type = png_get_color_type(png_ptr, info_ptr);

  if (color_type == PNG_COLOR_TYPE_PALETTE) {
    png_set_palette_to_rgb(png_ptr);
    channels = 3;
    bitdepth = 8;
  }

  if (color_type == PNG_COLOR_TYPE_GRAY || color_type == PNG_COLOR_TYPE_GRAY_ALPHA) {
    if (bitdepth < 8) {
      png_set_expand_gray_1_2_4_to_8(png_ptr);
      bitdepth = 8;
    }