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hard collisions vs. soft collisions

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Hi guys,

 

In one of my many ventures into attempting to understand collision detections better, I came upon few posts that mentioned one should not do a hard collision, that is, move the object, detect if it collides, and then move it back so it's out of range. Rather, a programmer should try to predict the collision, and not let it happen in the first place.

 

Of course as a noob, I have been using the former method extensively as it's intuitive (and probably easier to code).

 

My question is, what's the advantage of using the predictive model? What does a game or a physical simulation care whether you adjust a position post collision vs. predictive modelling?

 

Thanks,

 

Mike

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To be honest, predictive doesn't sound that difficult compared to reactice (have to get into it myself though).

Idea:
- possiblePosition = currentPosition + requestedMovement
- check possiblePosition for collisions
- if no collisions newPosition = possiblePosition
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To be honest, predictive doesn't sound that difficult compared to reactice (have to get into it myself though).

Idea:
- possiblePosition = currentPosition + requestedMovement
- check possiblePosition for collisions
- if no collisions newPosition = possiblePosition

 

You need a bounding volume that encloses both the original position and the target position, or some other means of checking intermediate points.  Rendering and updating happens on a per-frame basis, but that doesn't mean the simulation is bound to frames.  Once the collision has "happened" on a frame, you've possibly lost information you need to cause a proper collision response.

 

A naive collision detection algorithm can also run into really bizarre cases where your objects almost super-impose on a frame, and reflection + an object slowing down causes them to still coincide on the next frame.  What do you do then?  Do you create a data structure to keep "already collided flags" on every object pair?  It just gets messy.

Edited by SeraphLance
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To be honest, predictive doesn't sound that difficult compared to reactice (have to get into it myself though).

Idea:
- possiblePosition = currentPosition + requestedMovement
- check possiblePosition for collisions
- if no collisions newPosition = possiblePosition

That's the same thing as reactive, you're just 'reacting' to a position before you set it, but aren't doing anything different.

Unless, ofcourse, your 'check collisions' line implied actual prediciton, in which case your idea is missing the key ingredient :)

As Seraph said, you'd need a bounding volume which is then tested against other objects for collision, if the collision is found then you continue to a more detailed test (since this can be just an AABB test).

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