jitter model becomes 'transparent'

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Arne

Two objects: one black screen and a photograph behind it. Close to the two objects, you can't see the photograph on the other side of the black screen.

Further away, you start seeing the photograph behind the black screen, as if the black screen becomes partly transparent.

I imported a .obj-file in jitter, and show elements of this .obj-file by means of 'drawgroup' messages. I make two elements of the .obj file appear: one is a photograph, the other a black screen (the two elements are apart from each other, but close to each other). The two images above illustrate the problem. When you are close to the objects, you can't see the photograph because the black screen is in front of it (that is how I intend it to be). But when you move away from the two objects, you start seeing the photograph through the black screen. Why is that and how can I avoid this 'transparency'?