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Accessing Surface Memory Directly (Direct3D 9)

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You can directly access the surface memory by using the IDirect3DSurface9::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 IDirect3DSurface9::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 IDirect3DSurface9::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 IDirect3DSurface9::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 Direct3D driver. For more information, see Width vs. Pitch (Direct3D 9).
  • 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. Windows 98 uses 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 surface created using IDirect3DDevice9::CreateOffscreenPlainSurface. Back buffer surfaces, which may be accessed using the IDirect3DDevice9::GetBackBuffer and IDirect3DSwapChain9::GetBackBuffer methods, may be locked only if the swap chain was created with the Flags member of D3DPRESENT_PARAMETERS to include D3DPRESENTFLAG_LOCKABLE_BACKBUFFER.

Direct3D Surfaces