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In programming, void is used when declaring a function that does not return a value. void* is a pointer to such a function.

The void is the black space outside the world. It is an infinitely large area of nothingness that surrounds every map.

The map can occasionally share its inside space with the void causing an error known as a leak. Loading the map's compiled pointfile plots a course between this void and the closest entity in the map which can help patch leaks.

The void is not physical. It isn't a brush or entity but the lack of entities and brushes.

When viewing the void from inside the map, the backbuffer will not be cleared with black. This causes the HoM effect to appear. The pixels where nothing is drawn the next frame will contain what was drawn to them in the past. Post processing effects like filmgrain, HDR (bloom) and motionblur will indefinitely add their output to the backbuffer; which can result in a solid color or random patterns where the backbuffer is not refreshed properly.

By default a clear only happens when the vieworigin is assumed to be in a 'solid', the void is marked as such.