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You can directly access the frame buffer or off-screen surface memory by using the IDirectDrawSurface5::Lockmethod. When you call this method, the lpDestRectparameter is a pointer to a RECTstructure that describes the rectangle on the surface you want to access directly. To request that the entire surface be locked, set lpDestRectto NULL. Also, you can specify a RECTthat covers only a portion of the surface. Providing that no two rectangles overlap, two threads or processes can simultaneously lock multiple rectangles in a surface.
The Lockmethod fills a DDSURFACEDESC2structure with all the information you need to properly access the surface memory. The structure includes information about the pitch (or stride) and the pixel format of the surface, if different from the pixel format of the primary surface. When you finish accessing the surface memory, call the IDirectDrawSurface5::Unlockmethod to unlock it.
While you have a surface locked, you can directly manipulate the contents. The following list describes some tips for avoiding common problems with directly rendering surface memory:
- Never assume a constant display pitch. Always examine the pitch information returned by the IDirectDrawSurface5::Lockmethod. This pitch can vary for a number of reasons, including the location of the surface memory, the type of display card, or even the version of the DirectDraw driver. For more information, see Width vs. Pitch.
- Make certain you blit to unlocked surfaces. DirectDraw blit methods will fail, returning DDERR_SURFACEBUSYor DDERR_LOCKEDSURFACES, if called on a locked surface. Similarly, GDI blit functions fail without returning error values if called on a locked surface that exists in display memory.
- Limit your application's activity while a surface is locked. While a surface is locked, DirectDraw often holds the Win16Mutex (also known as the Win16Lock) so that gaining access to surface memory can occur safely. The Win16Mutex serializes access to GDI and USER dynamic-link libraries, shutting down Windows for the duration between the IDirectDrawSurface5::Lockand IDirectDrawSurface5::Unlockcalls. The IDirectDrawSurface5::GetDCmethod implicitly calls Lock, and the IDirectDrawSurface5::ReleaseDCimplicitly calls Unlock.
- Always copy data aligned to display memory. Copying data
unaligned to display memory can cause the system to suspend
operations if the copy spans memory banks.
Unless you include the DDLOCK_NOSYSLOCK flag when you call the Lockmethod, locking the surface typically causes DirectDraw to take the Win16Mutex. During the Win16Mutex all other applications, including Windows, cease execution. Since the Win16Mutex stops applications from executing, standard debuggers cannot be used while the lock is held. Kernel debuggers can be used during this period. DirectDraw always takes the Win16Mutex when locking the primary surface.
If a blit is in progress when you call IDirectDrawSurface5::Lock, the method will return immediately with an error, as a lock cannot be obtained. To prevent the error, use the DDLOCK_WAIT flag to cause the method to wait until a lock can be successfully obtained.
Locking portions of the primary surface can interfere with the display of a software cursor. If the cursor intersects the locked rectangle, it is hidden for the duration of the lock. If it doesn't intersect the rectangle, it is frozen for the duration of the lock. Neither of these effects occurs if the entire surface is locked.
Last updated on Tuesday, May 18, 2004