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The IVideoWindowplug-in distributor (PID) in the filter graph manages full-screen playback. It will swap a video renderer out for a specialist full-screen renderer, stretch a window of a renderer to full screen, or have the renderer implement full-screen playback directly.
To interact in full-screen protocols, a video renderer should send an EC_ACTIVATEnotification whenever its window is either activated or deactivated. In other words, an EC_ACTIVATEnotification should be sent for each WM_ACTIVATEAPP message a renderer receives.
When a renderer is being used in full-screen mode, these notifications manage the switching into and out of that full-screen mode. Window deactivation typically occurs when a user presses ALT+TAB to switch to another window, which the DirectShow full-screen renderer uses as a cue to return to typical rendering mode.
When the EC_ACTIVATEnotification is sent to the filter graph manager upon switching out of full-screen mode, the filter graph manager sends an EC_FULLSCREEN_LOSTnotification to the controlling application. The application might use this notification to restore the state of a full-screen button, for example.
The EC_ACTIVATEnotifications are used internally by DirectShow to manage full-screen switching on cues from the video renderers.