Programming with QuickTime VR

Drawing in the Prescreen Buffer

The QuickTime VR Manager allows you to define a prescreen buffer imaging completion procedure that is called whenever QuickTime VR finishes drawing a panorama image in the prescreen buffer. Typically, your completion procedure will add graphical elements to the image before the buffer is copied to the screen. For instance, a flight simulator could overlay a heads-up display containing information about the aircraft (its altitude, velocity, and so forth).

You install a prescreen buffer imaging completion procedure by passing its address to the QTVRSetPrescreenImagingCompleteProc function:



 
theErr = QTVRSetPrescreenImagingCompleteProc(theInstance, 

        NewQTVRImagingCompleteProc(MyImagingCompleteProc), 

        (SInt32)&theData, 0);

Listing 2-11 defines a simple completion routine that overlays a picture onto the screen image.

Listing 11 Overlaying images in the prescreen buffer



 
pascal OSErr MyImagingCompleteProc (QTVRInstance, MyDataPtr theDataPtr)

{

    if (theDataPtr->hasLogoPict) {

        GWorldPtr   theOffscreenGWorld;

        GDHandle    theGD;

        Rect        gwRect;

        Rect        picRect;



        // The current graphics world is set to the prescreen buffer.

        GetGWorld (&theOffscreenGWorld, &theGD);

        gwRect = (*(theOffscreenGWorld->portPixMap))->bounds;



        picRect = (*(theDataPtr->logoPict))->picFrame;

        OffsetRect (&picRect, -picRect.left, -picRect.top);

        OffsetRect (&picRect, gwRect.right - (picRect.right + 8),

                gwRect.bottom - (picRect.bottom + 8));

        // Draw logo in lower right corner

        DrawPicture (theDataPtr->logoPict, &picRect);

    }

    return noErr;

}

On entry to the prescreen buffer imaging completion routine, the current graphics world is set to QuickTime VR's prescreen buffer. The MyImagingCompleteProc function defined in Listing 2-11 retrieves the dimensions of that buffer and then draws a picture in the lower right corner of that buffer.