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Asynchronous Rendering

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A display driver that handles one or more graphics DDI drawing operations asynchronously provides GDI access to its bitmaps through the use of EngModifySurface. The driver must also provide a synchronization routine in order to avoid drawing errors if it batches graphics DDI drawing operations.

Such a driver has the option of implementing one of DrvSynchronizeSurface or DrvSynchronize as the synchronization routine. GDI calls one of these routines only when the driver has hooked them in EngAssociateSurface. GDI will call only DrvSynchronizeSurface in drivers that hook both of these synchronization routines.

DrvSynchronizeSurface provides additional information to the driver regarding synchronization events and why they occur. This enables the driver to reduce performance lag due to synchronization. For example, drivers that track which device bitmaps are in the accelerator's queue might be able to return immediately from DrvSynchronizeSurface if the specified surface is not currently in the queue.

In addition to providing a synchronization routine, a driver can also activate a time-based or programmatic flush mechanism by setting the following flags in the flGraphicsCaps2 field of the DEVINFO structure:

  • GCAPS2_SYNCTIMER: Setting this flag causes the driver's synchronization routine to be called periodically. Drivers that batch graphics DDI calls must specify this flag. By doing so, these drivers avoid problems such as lag in a software cursor's movement or in drawing that is performed in bursts.

    GDI passes the DSS_TIMER_EVENT flag to DrvSynchronizeSurface when this synchronization routine is called due to a periodic event.

  • GCAPS2_SYNCFLUSH: Setting this flag causes the driver's synchronization routine to be called whenever the Microsoft Win32 GdiFlush API is called. Drivers that perform asynchronous rendering must specify this flag and provide a synchronization routine.

    GDI passes the DSS_FLUSH_EVENT flag to DrvSynchronizeSurface when this synchronization routine is called due to a flush-based event. See the Microsoft Windows SDK documentation for more information about GdiFlush.

Note

The driver must never batch DirectDraw drawing calls when the destination surface is the visible screen. Such a situation occurs in a windowed DirectX application where the completed frame is updated to the screen via DdBlt and should thus be displayed immediately. This restriction also applies to DirectDraw video port surfaces, which might be flipped asynchronously.