Devices and Acceleration in MultiMon Systems

A DirectDraw application should enumerate the devices in a system, choose a device (or perhaps allow the user to choose the device to use), then create a DirectDraw object for that device by using its hardware globally unique identifier (GUID). This technique will ensure the best performance on both MultiMon and single monitor systems and at all cooperative levels.

The currently active display device is referred to as the "default device," or the "null device." The latter name comes from the fact that the currently active display device is enumerated with NULL as its GUID. Many existing applications create a DirectDraw object for the null device, assuming that the device will be hardware accelerated. However, on multiple monitor systems, the null device isn't always hardware accelerated; it depends on what cooperative level is set at the time.

In full-screen exclusive mode, the null device is hardware accelerated, but unaware of any other installed devices. This means that full-screen, exclusive mode applications will run as fast on a MultiMon system as any other system, but will not be able to use built-in support for spanning graphics operations across display devices. Full-screen, exclusive mode applications that need to use multiple devices can create a DirectDraw object for each device they want to use. Note that to create a DirectDraw object for a specific device, you must supply that device's GUID (as it is enumerated when you call DirectDrawEnumerate).

When the normal cooperative level is set, the null device has no hardware acceleration; the null device is, effectively, an emulated logical device that combines the resources of two physical devices. Therefore, the null device has no hardware acceleration at all when the normal cooperative level is set. On the other hand, when the normal cooperative level is set, the null device is capable of automatically spanning graphics operations across monitors. As a result, negative coordinates for blit operations are valid when the logical location of secondary monitor is to the left of the primary monitor.

If your application requires hardware acceleration when the normal cooperative level is set, it must create a single DirectDraw object using a specific device's GUID. Note that when you don't use the null device, you don't get automatic device spanning. That is, blit operations that cross an edge of the primary surface will be clipped (if you are using a clipper) or will fail, returning DDERR_INVALIDRECT.

As a rule on any system, you should set the cooperative level immediately after creating a DirectDraw object, before retrieving the object's capabilities or querying for other interfaces. Additionally, avoid setting the cooperative level multiple times on a MultiMon system. If you need to switch from full-screen to normal mode, it is best to create a new DirectDraw object.