# Object is 'twitching' on the ground in my physics simulator

1. Jul 28, 2011

### EricMiddleton

I am working on a simple physics engine right now and when the object eventually comes to a rest, it starts 'twitching' (moving up/down slightly very quickly) because gravity pulls it through the floor, and then the collision detection pops it back up. Here is the order in which my simulation operates:

Calculate Acceleration (Gravity)
Check prior collisions
Update Position
Update Velocity

The only thing I can think of is that the collisions are being tested before the new positions are updated. Thanks for any help you can provide.

2. Jul 29, 2011

### chiro

Can you post a little more information about the actual code? I know its not feasible to dump every routine (even if you did want to), but it's hard to really know specifically is wrong.

I'm wondering if your velocity vector gets the right update. If the collision code is correct, then if gravity is ok and the floor of your map is on the x-y axis (I'm assuming z points up in this case), then the collision should always move the object to the same point (assuming the object is actually or is treated like a box).

But if the floor of your map is at an angle, what may happen is that due to the orientation of your vector and possibly your routine, you might be re-projecting your object at a different place each time and hence you see the weird movement.

Again this is all speculation, and I can't really give a solid comment without getting more information.

3. Jul 29, 2011

### EricMiddleton

I'm currently writing the prototype in a game called Garry's Mod using an an extension called wiremod, for ease of debugging. The language is called Expression2, and its very easy to understand.

Code (Text):

T = 1/33 #This code is updated every tick (33 times per second)
Acceleration = vec(0, 0, -gravity())

Position += Velocity*T + Acceleration/2*T^2

Velocity += Acceleration*T

holoPos(1, Position) #update the position (necessary to get proper bounding box translations)

for(Loop = 1, Count) #iterate through each object
{
E = holoEntity(Loop)
Box = E:toWorld(E:boxSize()) #Translate the object's bounding box
if(Box:z() < 0) #if it's below the ground
{
Height = Box:z()
Ang = 90 - Velocity:toAngle():pitch() #calculate angle
Dist = -Box:z() / cos(Ang) #calculate how far it's gone
Time = abs(Dist/Velocity:z()) #derive time from the distance
Position -= Velocity * Time #roll back the position
T += Time #speed time back up
Velocity -= Velocity * Time
Velocity *= vec(1, 1, -0.5) #inelastic collision
HitLoop = Loop #for debugging
Velocity -= vec(min(abs(Velocity:x()), Mu*Velocity:z()*Mass)*sign(Velocity:x()), min(abs(Velocity:y()), Mu*Velocity:z()*Mass)*sign(Velocity:y()), 0) * T #friction
}
}
holoPos(1, Position) #update the position again

I also tried moving the collision detection down near the end of the code (as seen above) which seems to help a little, but not eliminate the issue entirely.

4. Jul 29, 2011

### rcgldr

The accelerations calculation will have to determine that objects at rest on a solid surface end up with zero acceleration.