You can also install a transfer procedure and set the GWorld to nil. This results in your transfer procedure being called only as a notification that the track has drawn and that no transfer is taking place.

Inside your transfer procedure you could manipulate the image. Note that calling resource intensive or time consuming routines in your transfer procedure may have an adverse effect on the playback performance of the movie that is playing.

Here's an example of a transfer procedure that will keep a counter of 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 ) ;

[May 01 1995]

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