Technical Q&A's

QTMTB 27 - Accessing Decompressed Images (1-May-95)

Q Is there a mechanism that would allow us to access each decompressed image prior to display during playback, so that we can manipulate the image data and then hand it back for display?

A QuickTime 1.6.1 provides a function called SetTrackGWorld. SetTrackGWorld lets you force a track to draw into a particular GWorld, which can be different from that of the entire movie. After the track is drawn, it calls your transfer procedure to copy the track to the actual movie GWorld. When your transfer procedure is set, the current GWorld is set to the correct destination.

You could also install a transfer procedure, and set the GWorld to nil. When you do this, your transfer procedure is called only as a notification that the track has drawn and that no transfer is taking place. You can also manipulate the image inside your transfer procedure. Note that calling resource-intensive or time-consuming routines in your transfer procedure may have an adverse effect on playback performance.

Here's an example of a transfer procedure that keeps a counter of the number of times it has been called and displays this number in the top-left corner of the movie:

pascal OSErr myTrackTransferProc(Track t, long refCon)
{

	TransferDataHandle		myTDH = (TransferDataHandle)refCon ;
	GrafPtr			theNewWorld ;
	GrafPtr			movieGWorld ;
	PixMapHandle			offPixMap ;
	Rect				movieBox ;
	static	long			index = 1 ;
	CGrafPtr			savedWorld ;
	GDHandle			savedDevice ;
	Str255				theString ;
	
	

	movieGWorld = (GrafPtr)((**myTDH).movieGWorld) ;
	theNewWorld = (GrafPtr)((**myTDH).trackGWorld) ;
	movieBox    = (**myTDH).movieRect ;
	
	offPixMap = GetGWorldPixMap( (GWorldPtr)theNewWorld ) ;
	(void) LockPixels( offPixMap ) ;
	
	GetGWorld( &savedWorld, &savedDevice );
	SetGWorld( (CGrafPtr)theNewWorld, nil ) ;
	
	MoveTo ( 15, 15 );
	NumToString ( index++, theString );
	DrawString ( theString );
	
	// copy the image from the offscreen port
	// into the movies port
	
	SetGWorld( savedWorld, savedDevice ) ;
	
	CopyBits( 	&theNewWorld->portBits,
				&movieGWorld->portBits,
				&theNewWorld->portRect,
				&movieBox,
				srcCopy,
				nil ) ;

	(void) UnlockPixels( offPixMap ) ;
}


//---------------------------------------------------------------------
// define a structure to hold all the information we need in the transfer
// proc.

typedef struct {
	GWorldPtr	movieGWorld ;
	GWorldPtr	trackGWorld ;
	Rect		movieRect ;
} TransferData, *TransferDataPtr, **TransferDataHandle ;
	
//This has the original movie gWorld, the one we created for the track and a rect 
// describing the movie.  You can set a movie up to use this in the following way:

	TransferDataHandle	myTDH = (TransferDataHandle)NewHandle( sizeof( 							TransferData )) ;
	Track		aTrack = GetFirstTrackOfType( aMovie, VideoMediaType ) ;
	short		trackDepth = GetFirstVideoTrackPixelDepth( aMovie ) ;
	
	if( myTDH == nil || aTrack == nil || trackDepth < 0)
		return ;
		
	GetTrackDimensions( aTrack, &width, &height ) ;
	
	trackDimensions.right = Fix2Long( width );
	trackDimensions.bottom = Fix2Long( height );
	
	// create the movie gWorld
	theErr = NewGWorld( &theNewWorld, trackDepth, &trackDimensions, nil, 						theNewWorldDevice, 0L ) ;
	CheckError( theErr, "\pCall to NewGWorld failed" );

	GetMovieGWorld( aMovie, &movieGWorld, nil ) ;
		
	(**myTDH).movieGWorld = movieGWorld ;
	(**myTDH).trackGWorld = theNewWorld ;
	
	GetMovieBox( aMovie, &movieBox ) ;
	(**myTDH).movieRect = movieBox ;
	
	SetTrackGWorld( aTrack, (CGrafPtr)theNewWorld, nil, myTrackTransferProc,  				(long)myTDH ) ;

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