In order to understand the difference between hiding geometry and disabling drawing, you need to understand that an OBJ triangle can serve many purposes. Broadly, those purposes are:
- Drawing (the most basic use).
- Collision detection, of which we have three flavors: collision of the plane with the object (“hard surfaces”, or “physics”), collision of the mouse with the panel (manipulators, or clickable triangles) and collisions of the camera with the airplane (“solid camera”, which constrains the camera).
Any given triangle can be drawn and/or used to check for any of these collisions*; attributes change what the triangle is used for.
By default, all triangles are drawn; ATTR_draw_disable marks future triangles as not being drawn. This allows you to make a triangle that is used only for collisions. Examples might include a “hot spot” in front of a region on the panel (the hot spot might be easier to click than a small switch) and an invisible simple mesh to constrain the camera.
By comparison, ANIM_hide effectively removes some triangles from your model (temporarily) for all uses – drawing and collision detection of any kind. If a door is hidden, it’s not only not drawn, but it’s not going to stop the camera moving through it either.
Some key points to these distinctions:
- Categorizing what a triangle is used for (drawing, various flavors of collisions) is static – that is, it is always the same for the object and never changes with datarefs or animation. This is intentional for performance reasons!
- Animation to hide triangles affects the triangle in every way consistently – drawing and collisions.
Generally, you will get better performance improvements by removing categories from a triangle than by hiding it. (That is, it is better to not have manipulators on your cockpit, so it isn’t mouse-click collision-checked, than to hide it.) But the purpose of ATTR_draw_disable and ANIM_hide are different enough that which you use will be determined by the effect you are trying to create.
Finally, note that hiding an object completely (that is, the object does no drawing) does not provide the maximum performance benefit of not having an object at all. ANIM_hide was created to allow authors to create clever effects, not as a performance enhancer!
* This is not quite accurate: airplane-object collision checks are only available in scenery objects, and camera/airplane or mouse/panel collision checks are only available in the cockpit object.
