LinearCast at high world, low relative speed

[DocAce]

Hi all!

I have a somewhat specific problem. In our space game we have a system where flying close to something much larger lets you move with that larger object as your coordinate system, mainly so that things like combat or docking at moving ships are possible.

Unrelatedly, in order to minimize instances of tunneling into or through objects the player-controlled ship uses the motion quality LinearCast. We don't use this for more objects for performance reasons.

In my specific test example I have a capital ship moving at a constant 1km/s, with the player in a spacesuit (technically just a very small and slow ship) going under 10m/s relative to the capital ship. Depending on the relative position I'm seeing some incorrect collisions resulting in position jumps over 10m, which don't happen if the spacesuit uses Discrete motion quality.

Thinking about, this makes sense. The capital ship has moved by over 10m since the last frame, so the CCD cast might begin from a position that used to be free but isn't any more. (Or end at an one, if LinearCast happens before Discrete.)

Barring any hyper-custom solutions (e.g. modifying the start location and velocity used for LinearCast to be relative to the potential obstacle) I've basically resigned myself to having to use Discrete motion quality for the player-controlled ship as well. Unless someone has a bright idea?

[jrouwe]

What happens during the simulation is:

1. Collision detection is done for all objects to check for current overlaps, collision constraints are resolved, velocity is updated etc.
2. All Discrete objects now move to their new position
3. All LinearCast objects will do a collision cast against the world to check for collisions (all from their initial position)
4. Collisions are sorted and then LinearCast objects are placed on the nearest collision they enountered

This means that if your capital ship is Discrete, it will have moved into its new position before the LinearCast player does the collision check. This can indeed mean that the capital ship has moved inside the player when the collision check is performed (if the player was in front of the ship). I don't really see a solution other than to make both bodies Discrete or to make them both LinearCast.

[DocAce]

Thank you for confirming. It may be feasible to switch objects close enough to the player to LinearCast, I will give that a try.

[SirLynix]

Sorry to ask the dumb question but why doesn't Jolt first test all Discrete and LinearCast collisions before moving them to their new position? Why are they updated at two separate times?

[DocAce]

That wouldn't fix anything in this case. Imagine being in front of a wall, both you and the wall going 1km/s forwards. LinearCast would detect a collision with the wall at its old position and then move that wall forward.