From the manual:

Quote:

WARNING W090: Region leaking
The sky box is not completely closed, or an object is outside, or a large object - like a terrain - is penetrating the sky box, or there is no sky box at all. This is normally not a problem, but can increase the compiling time. However the outer surfaces of the sky box should have the NONE rendering mode set in such a case to prevent a frame rate decrease due to scanning invisible surfaces.






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