4.6 Bitmap Resources

Win32 can read two types of device-independent bitmaps. The normal type of DIB is the Windows 3.0 DIB format. The other type of DIB is that used for OS/2® versions 1.1 and 1.2. The bitmap resource consists of a single device-independent bitmap and, accordingly, this DIB can be of either format. The two DIBs are distinguished by their header structures. They both have the size of their respective structures as the first DWORD in the header. Both of these structures are documented in the Windows SDK Reference Version 3.0 volume 2, section 7. The header structure for the normal DIB is BITMAPINFOHEADER, while the OS/2 DIB header is called BITMAPCOREHEADER. The correct size (as a DWORD) must be in the first entry of the structure:

[Normal resource header (type = 2)]

[BITMAPINFOHEADER or BITMAPCOREHEADER]
[Color table if not 24 bits per pixel]
[Packed-pixel bitmap]

Note that the color table is optional. All but 24-bit color bitmaps have a color table attached next. This table's length can be computed by 2#BitsPerPixel * 3 bytes for OS/2 bitmaps or 2#BitsPerPixel * 4 bytes for Windows bitmaps. The bitmap image data is placed immediately following the color table.

Note that the bitmap file has an unaligned header structure (BITMAPFILEHEADER structure). This header is not, however, stored in the resource file, as it serves only to identify the type of file (DIB or DDB).