Function BmpReadHeader

src/bmp.cpp:244–311

* Reads bitmap headers, and palette (if any) */

 * Reads bitmap headers, and palette (if any)
 */
bool BmpReadHeader(RandomAccessFile &file, BmpInfo &info, BmpData &data)
{
	info = {};

	/* Reading BMP header */
	if (file.ReadWord() != 0x4D42) return false; // signature should be 'BM'
	file.SkipBytes(8); // skip file size and reserved
	info.offset = file.ReadDword() + file.GetStartPos();

	/* Reading info header */
	uint32_t header_size = file.ReadDword();
	if (header_size < 12) return false; // info header should be at least 12 bytes long

	info.os2_bmp = (header_size == 12); // OS/2 1.x or windows 2.x info header is 12 bytes long

	if (info.os2_bmp) {
		info.width = file.ReadWord();
		info.height = file.ReadWord();
		header_size -= 8;
	} else {
		info.width = file.ReadDword();
		info.height = file.ReadDword();
		header_size -= 12;
	}

	if (file.ReadWord() != 1) return false; // BMP can have only 1 plane

	info.bpp = file.ReadWord();
	if (info.bpp != 1 && info.bpp != 4 && info.bpp != 8 && info.bpp != 24) {
		/* Only 1 bpp, 4 bpp, 8bpp and 24 bpp bitmaps are supported */
		return false;
	}

	/* Reads compression method if available in info header*/
	if ((header_size -= 4) >= 4) {
		info.compression = file.ReadDword();
		header_size -= 4;
	}

	/* Only 4-bit and 8-bit rle compression is supported */
	if (info.compression > 2 || (info.compression > 0 && !(info.bpp == 4 || info.bpp == 8))) return false;

	if (info.bpp <= 8) {
		/* Reads number of colours if available in info header */
		if (header_size >= 16) {
			file.SkipBytes(12);                  // skip image size and resolution
			info.palette_size = file.ReadDword(); // number of colours in palette
			file.SkipBytes(header_size - 16);    // skip the end of info header
		}

		uint maximum_palette_size = 1U << info.bpp;
		if (info.palette_size == 0) info.palette_size = maximum_palette_size;

		/* More palette colours than palette indices is not supported. */
		if (info.palette_size > maximum_palette_size) return false;

		data.palette.resize(info.palette_size);