How to grab an image of an offscreen NSWindow?

I'm trying to get 开发者_如何转开发an image of an offscreen window to use in a CALayer-based animation, but no matter which method I try I cannot get an image out the other side. Here is the code I've been using on a custom NSWindow subclass in my project:

CGImageRef windowImage = CGWindowListCreateImage(CGRectNull, 
                                                 kCGWindowListOptionAll, 
                                                 (CGWindowID)[self windowNumber], 
                                                 kCGWindowImageDefault);

This is what the image should look like:

and here's what I get (ignore the slightly larger size - that's my fault in the rendering of the image):

Can anyone see where I'm going wrong? Strangely, Quartz Debug can see the window image without issue, but the Son of Grab sample code from Apple cannot.

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From the documentation comment for CGWindowListCreateImage in CGWindow.h:

kCGWindowListOptionAll, kCGWindowListOptionOnScreenOnly: Use all on-screen windows in this user session to construct the image. The parameter windowID should be kCGNullWindowID.

That's why you're getting a screenshot of all the windows that are within the rectangle you're interested in.

To take a screenshot of a specific window, use kCGWindowListOptionIncludingWindow.