What are they ? When do they appear ? How can i make them disappear ?
I know, they cause some 'rendering error', so i can see through what i don't want to.

Holes are bad. Avoid them at all costs.

Detail brushes are worse though... so don't use those to fix holes.

Holes are essentially mathematical errors when generating the level geometry. They're usually caused by unclean, non-grid aligned geometry. They can also appear sometimes with completely clean geometry, although when this happens it's usually an invisible hole which can be fixed by "end of time"ing solid brushes and/or "start of time"ing air brushes. Note that there's really no foolproof way to prevent or repair holes - you basically just try stuff until they go away.

Post a picture of the hole you're getting and I can give you a better idea of how to fix it. (or email me the map file)

Yeah, 'end of time' worked.
I have a ladder with 32 steps, and one of the steps caused a hole, now i 'end of time' -ed the two vertical brushes, and the hole is gone.


Thanks again !

what do the start time and end of time buttons do?

Layering for intersections of geometry. Say you have a room (air brush) with a pillar (solid brush) in it, then you make an air brush to cut out of the pillar. If you "end of time" the air brush, it'll cut out of the pillar. If you "start of time" the air brush, it won't - as it would have been calculated before the pillar - hence the pillar fills in the space.