INFO: Advantages of Device-Dependent Bitmaps (94918)

SUMMARY

A Device-Dependent Bitmap (DDB) is much faster than a Device Independent-Bitmap (DIB) to BitBlt(). For this reason, it is often a good strategy under Win32 (as well as under Windows 3.1) to create a DDB from a DIB when caching or calling *Blt() functions.

The slight drawback of memory overhead for the DDB is handled well by Win32. With Windows 3.1, the DDB memory could be marked as discardable. Under Win32, the memory will be paged out if system resources become tight (at least until the next repaint); if the memory is marked as PAGE_READONLY, it can be efficiently reused, [see VirtualProtect() in the Win32 Application Programming Interface (API) Help file].

However, saving the DDB to disk as a mechanism for transfer to other applications or for later display (another invocation) is not recommended. This is because DDBs are driver and driver version dependent. DDBs do not have header information, which is needed for proper translation if passed to another driver or, potentially, to a later version of the driver for the same card.