If I recall you need to set your CameraClsMode to 0 so that it does not overwrite the background, and draw the background FIRST.
This means that any areas in renderworld which aren't in view (ie. black) will not be drawn, so your bg will show through. That is why BG is drawn before renderworld and clsmode is set to 0.
This means that any areas in renderworld which aren't in view (ie. black) will not be drawn, so your bg will show through. That is why BG is drawn before renderworld and clsmode is set to 0.