snapshot taken from QCView sometimes blank

I have a program playing quartz compositions in QCViews. Because the QCViews are on a secondary screen, I need to display a (smaller) copy in the main screen (kind of control screen).

So I fire a timer every 0.1 seconds, take a snapshot of the QCView an set the resulting image in a NSImageView.

My problems :

• The snapshot is correct about 90 % of the time but sometimes goes blank. Once this happens all snapshots taken from this QCView are blank until there is a stopRendering / startRendering. This happens even as in the same time another QCView with exactly the same composition is running and delivering good snapshots.

• The snapshots seem to be out of sync with the QCView. I tried to fire the timer with a higher frequency but it did not change anything (I mention this in case it could be related to my main problem of blank images).

Any ide开发者_开发问答as or pointers ?

EDIT:

I tried to reproduce : created a minimal project then copy pasted my code :

    - (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    NSRect r = NSMakeRect(0, 0, nb * 101, 101 );
    feedbackwindow = [[[NSWindow alloc] initWithContentRect: r
                                                         styleMask: NSResizableWindowMask
                                                           backing: NSBackingStoreBuffered
                                                             defer: NO ] retain ];
    for ( int n = 0; n < nb; n++ )
    {
        v[n] = [[[QCView alloc] init] autorelease];
        [v[n] setFrame: NSMakeRect( 101 * n,  0, 100, 100 ) ];
        [[window contentView] addSubview: v[n]];

        if(![v[n] loadCompositionFromFile:[[NSBundle mainBundle] pathForResource:@"clock" ofType:@"qtz"]]) {
            NSLog(@"Could not load composition");
        }

        [v[n] startRendering];

        img[n] = [[[NSImageView alloc] init] autorelease];
        [img[n] setFrame: NSMakeRect(101 * n,  0, 100, 100 ) ];
        [[feedbackwindow contentView] addSubview: img[n]];
    }

    [feedbackwindow orderFront:nil];
    [window orderFront:nil];

    [self setFeedbackTimer: [NSTimer scheduledTimerWithTimeInterval: ( 0.01 )
                                                             target: self
                                                           selector: @selector(updateImage)
                                                           userInfo: nil
                                                            repeats: YES ]];
}

- (void) updateImage
{
    for ( int n = 0; n < nb; n++ )
    {
        NSImage* snapshot = [v[n] snapshotImage];
        [img[n] setImage: snapshot];
    }
}

Alas I don't get the problem...

The snapshots are out of date from the QCViews but perfectly good :(

It seems that the snapshot retrieve the previous image instead of the current one : I made a simple Quartz Composer clock with "system time" -> "time formatter (format = ss)" -> "image with string" -> "sprite" and the snapshot are exactly one second late : they change at the same time as the original clock but display the previous image ...

Sounds like a bug too but probably not the same one that causes invalid images ...