You can directly access the surface memory by using the IDirect3DSurface8::LockRect method. When you call this method, the pRect parameter is a pointer to a RECT structure that describes the rectangle on the surface to access directly. To request that the entire surface be locked, set pRect to NULL. Also, you can specify a RECT that 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. Note that a multisample back buffer cannot be locked.

The LockRect method fills a D3DLOCKED_RECT structure with all the information to properly access the surface memory. The structure includes information about the pitch and has a pointer to the locked bits. When you finish accessing the surface memory, call the IDirect3DSurface8::UnlockRect method 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 LockRect method. This pitch can vary for a number of reasons, including the location of the surface memory, the display card type, or even the version of the Microsoft® Direct3D® driver. For more information, see Width vs. Pitch.
• Make certain you copy to unlocked surfaces. Direct3D copy methods will fail if called on a locked surface.
• Limit your application's activity while a surface is locked.
• Always copy data aligned to display memory. Microsoft® Windows® 95 and Windows 98 use a page fault handler, Vflatd.386, to implement a virtual flat-frame buffer for display cards with bank-switched memory. The handler allows these display devices to present a linear frame buffer to Direct3D. Copying data unaligned to display memory can cause the system to suspend operations if the copy spans memory banks.
• A surface may not be locked if it belongs to a resource assigned to the D3DPOOL_DEFAULT memory pool unless it is a dynamic texture or a private (FOURCC) driver format. Back buffer surfaces, which may be accessed using the IDirect3DDevice8::GetBackBuffer and IDirect3DSwapChain8::GetBackBuffer methods, may be locked only if the swap chain was created with the Flags member of the D3DPRESENT_PARAMETERS structure set to include D3DPRESENTFLAG_LOCKABLE_BACKBUFFER.