Render nsimage offscreen and set pointer for decklink videoframe

Question

I'm developing on osx.

I'm trying to set a pointer to pixel data rendered offscreen from an nsimage object. This is my code:

- (id)initWithFrame:(NSImage*)image
{
if (self = [super init])
{
    // set size
    NSRect rect = NSMakeRect(0, 0, [image size].width, [image size].height);

    // set image
    frameImageRef = [image CGImageForProposedRect:NULL context:NULL hints:NULL];

    frameContextRef = CreateARGBBitmapContext(frameImageRef);

    CGContextDrawImage(frameContextRef, rect, frameImageRef);
}
return self;
}

- (int) getBytesPerRow
{
    return (int)CGImageGetBytesPerRow(frameImageRef);
}

With following code i pass set a pointer to my decklink frame

- (void*) renderToDeckLink;
{
    return CGBitmapContextGetData (frameContextRef);
}

Al objects are initializing fine. No errors, everything looks like the image is rendered into memory.

This is my code where i create a decklink frame for playback onto a decklink device:

- (void) createVideoFrame:(NSImage*)image
{
    // clear previous videoframe
    if (videoFrame != NULL)
    {
        videoFrame->Release();
    }

The code on top of this post is located at the renderFrame class

    // create renderFrame object
    renderFrame *frame = [[renderFrame alloc] initWithFrame:image];

    // get displaymode
    IDeckLinkDisplayMode* decklinkdisplaymode = (IDeckLinkDisplayMode*)CurrentRes;

    // create new videoframe on output
    if (deckLinkOutput->CreateVideoFrame((int)decklinkdisplaymode->GetWidth(), (int)decklinkdisplaymode->GetHeight(), [frame getBytesPerRow], bmdFormat8BitARGB, bmdFrameFlagDefault, &videoFrame) != S_OK)
    {
        NSLog(@"error");
        // failed to create new video frame on output
    }

Following line of code is where i pass the pixeldata pointer to the videoframe object:

    // fill frame with buffer
    videoFrame->GetBytes((void**)[frame renderToDeckLink]);

    // schedule frame
    [self scheduleVideoFrame];

    // release pixeldata
   [frame releaseData];
}

Everything is compiling. Pixeldata is being rendered offscreen. The frames are correctly scheduled and playbacked on the decklink device. But all i get is a black screen. So maybe there's something wrong with the pointer to the pixeldata? I can't se where the fault is located.

Any help is more then welcome!

Thx Thomas