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Tribes 1 Physics, Part Three: Collision

Posted on April 11, 2008 7:47 PM

This article covers the collision physics of Tribes 1, i.e. attempting to actually move the player and what to do when the player runs in to something. This is the most convoluted part of the physics and requires a lot of little touches to get right. Tribes movement and collision handling actually gets a little too low level, so I won't be able to show exactly how it works (it gets down to dealing with the raw triangle lists which I don't think all collision detection systems will let you get at), but it will be detailed enough so that there will be no major differences.


Before I get in to the details, a little explanation is required. When Tribes attempts to move an object, it takes the maximum distance the object will cover in the remaining time (X), then divides the remaining time by ceil(X) and does ceil(X) collision detection loops. This is ostensibly to ensure that an object only moves 1m at a time, but for what reason, I don't know. The engine is obviously capable of fairly arbitrary translations (or else Gambase::GetLosInfo would not work) so I can only imagine many short translations proved to be more efficient than a single large translation.

I am going to be using a single translation, and normally this would not make a difference, but Tribes does something a little weird on collisions which necessitates a rather odd fix-up when you are using a single translation versus slicing them up. Instead of adjusting the position based on velocity when there is no collision and adjusting the velocity & setting the position to the collision point on a collision, Tribes adjusts the position based on the velocity no matter what. This means Tribes will attempt to move the player, hit a surface, adjust the velocity based on the collision, and then move the player anyway from their original position based on the new velocity, usually resulting in the player bouncing ever so slightly away from the contacted surface. There is actually a noticeable difference between the correct way and Tribes way of handling collisions, as the correct way feels slightly velcroish while Tribes feels more fluid.

Things get more complicated when you move the player in a single translation instead of sliced up translations as the weird adjustment on collisions is only done for the last fraction of the translation and not the entire thing. I worked around this by calculating some values which let me figure out how many "non-collision" slices have occurred and only do the weird handling on the last slice.

Collision Code


Ski bugs are caused when the translation loop exceeds the maximum number of collisions. When this happens, Tribes zeros the player's velocity as it is having trouble successfully moving. I think there is something in the velocity bounce that occasionally causes the translation loop to get stuck running in to the surface over and over with no change in velocity. When this happens, there is obviously no right answer as to what to do, but zeroing the velocity is fairly annoying answer. I've found that just ignoring the situation and hoping the next tick results in the player getting dislodged appears to work much better.

Also note that I keep forgetting to add constants (MINDAMAGESPEED and DAMAGESCALE in this post) and need to update the original post to include them. Since nobody is reading this and I am taking a while in getting it together, I do not think anyone will mind.

Tribes 1 Physics Series

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Comments (3)

Hey Andrew,
it totally read this (and all the other articles) a few times already. Good stuff, really interessting things you found out there.

I'll keep coming here, and i know about some guys that do the same.

Thumbs up!

Great stuff Andrew, this should help a few projects out!

wow, this pretty awesome. what happened with the Quakewars mod? This was the FeaRog one right?

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