Drawing a minimap - need an advice

Hello! So im trying to draw a minimap. Everything is good, exept of a size of a rect that showes camera view.
So is there any way to draw a rect/texture,that has size equale to camera view?

Im having something like this for now.
[44022-без-имени-1.jpg|44022]

I answered the same question here, take a look:

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