Function ReadHeightmapPNGImageData

src/heightmap.cpp:82–134

* The PNG Heightmap loader. */

 * The PNG Heightmap loader.
 */
static void ReadHeightmapPNGImageData(std::span<uint8_t> map, png_structp png_ptr, png_infop info_ptr)
{
	uint x, y;
	uint8_t gray_palette[256];
	png_bytep *row_pointers = nullptr;
	bool has_palette = png_get_color_type(png_ptr, info_ptr) == PNG_COLOR_TYPE_PALETTE;
	uint channels = png_get_channels(png_ptr, info_ptr);

	/* Get palette and convert it to greyscale */
	if (has_palette) {
		int i;
		int palette_size;
		png_color *palette;
		bool all_gray = true;

		png_get_PLTE(png_ptr, info_ptr, &palette, &palette_size);
		for (i = 0; i < palette_size && (palette_size != 16 || all_gray); i++) {
			all_gray &= palette[i].red == palette[i].green && palette[i].red == palette[i].blue;
			gray_palette[i] = RGBToGreyscale(palette[i].red, palette[i].green, palette[i].blue);
		}

		/**
		 * For a non-gray palette of size 16 we assume that
		 * the order of the palette determines the height;
		 * the first entry is the sea (level 0), the second one
		 * level 1, etc.
		 */
		if (palette_size == 16 && !all_gray) {
			for (i = 0; i < palette_size; i++) {
				gray_palette[i] = 256 * i / palette_size;
			}
		}
	}

	row_pointers = png_get_rows(png_ptr, info_ptr);

	/* Read the raw image data and convert in 8-bit greyscale */
	for (x = 0; x < png_get_image_width(png_ptr, info_ptr); x++) {
		for (y = 0; y < png_get_image_height(png_ptr, info_ptr); y++) {
			uint8_t *pixel = &map[y * png_get_image_width(png_ptr, info_ptr) + x];
			uint x_offset = x * channels;

			if (has_palette) {
				*pixel = gray_palette[row_pointers[y][x_offset]];
			} else if (channels == 3) {
				*pixel = RGBToGreyscale(row_pointers[y][x_offset + 0],
						row_pointers[y][x_offset + 1], row_pointers[y][x_offset + 2]);
			} else {
				*pixel = row_pointers[y][x_offset];
			}
		}
	}
}

/**
 * Reads the heightmap and/or size of the heightmap from a PNG file.