SCGT CentralCollision Points

The text below is JPS's guide to understanding the collision points in Vehicle's VEH files, originally posted in the Speedsims Forums. It appears after JPS granted permission.

Some people have been asking what the Coll_All, Coll_Player and Coll_Top entries are for in the VEH file.
As you have probably suspected, they are used for collision detection (unfortunately, ISI gave them pretty stupid names and that has lead to a lot of confusion as to what the "Player" and "All" entries do). Naturally, for performance reasons, SCGT cannot check every polygon of a car to check if it is colliding with a wall or another car or any other type of object; the sheer amount of calculation required to do that simply doesn't exist in today's computers. But what can be done is a rough/vague check against a short list of coordinates surrounding the car, say 12 coordinates placed at the most likely points of impact on the car. The calculation required to continuously check for collision against this short list of points is relatively easy and fast.

The 3 sets of entries are coordinates that form 3 shapes around the car. Each shape has 4 corners, thus there are 4 entries for each of the 3 shapes.

A picture is definitely needed here:

Collision points diagrams

* In these pictures the points have been placed in exagerated positions so you can see them better, you would certainly want your collision points to be placed better than shown here.

When one of these points is calculated to be in collision with a solid surface (wall, car, whatever) it is deemed a collision and appropriate damage is calculated (if any) and the physics engine applies the required force against the car and the object the car is colliding with (cone, another car, whatever).

It should be obvious to you at this point that all these coordinates need to be in the correct places otherwise the car will experience collisions when it is not really touching anything yet (point(s) placed too far from the sheet metal) or will experience collisions too late (point(s) place too far inside the sheet metal). Don't ask me how to calculate exactly where these points should be placed for any given car, I'm not that smart, so I just keep tweaking them until all the corners and sides of the car touch against a wall just right.

Coll_Top really comes into play the most when the car flips over or rolls, if this shape is not positioned appropriately then the car might act very strange in a roll and when it's upside down.