[Pierre-Marie Baty]

Stefan is right in the principle, because I had the same glitch with the OpenGL port of Wolfenstein 3D (in which I put many MD2 models, which are basically the same thing as MDL models). Umm yep, I assume that that tree is a separate entity with a certain model, not a world brush.

To tell whether an entity is visible or not (and thus to decide if it should be drawn or not) one of the first checks the engine does is checking a direct line of sight to each of the model's "bones". These are particular points of the model which can eventually be articulated, in case the model is a skeleton or some sort of moving stuff. The problem is that some modelers just put 1 bone for their static models, which is the minimum for any model to compile successfully ; however with just 1 bone it's very inaccurate to tell that when this bone is visible the whole entity is, and completely wrong to say that when it is no longer visible, the whole entity is no longer visible either.

This is *one* of the reasons that may explain your problem, however I can't guarantee it's the right one. Try adding more bones at different locations in your model (do this with Milkshape 3D, for example) ; then, assign them the right vertices, recompile the .mdl file and try to see if the glitch vanishes or not.