Impulse model of collision resolution

Color Problem
• Have a black-box function that returns a bright
color in 24-bit RGB
• Want a paler version of the output
• What to do?
Collision Resolution
Collision resolution
• Pre-collision positions, velocities known
• Collision: black box
• Post-collision positions, velocities known
• Assumption: we know collision location
• Instantaneous change in momentum
• j = ∆P
• Apply within one timestep
• Effectively, infinite force
Aside: Alternatives
• Not the only approach to collision resolution
• "soft body": force proportional to penetration
distance (one-way spring force)
One-body collisions
• Most common case: collision of object with
• Calculations generalize to two-body
– perform calculations in reference frame where
one body is at rest, i.e., add one body's velocity to
the other before starting
• Simpler to set up this way
Collision Normal
• direction in which bodies collide
• often simple:
– line joining centres
– normal of collision point on obstacle (often good
approximation anyway)
Closing Velocity
• velocity with which things collide
• magnitude: dot product of velocity and
collision normal
• If colliding: negative value
• If separating: positive
Post-Collision Velocity
• Perfectly elastic collision: v'.nc =
• Perfectly plastic collision: v'.nc = 0
• "Coefficient of restitution": linear
interpolation between these extremes
–v'.nc = -c
• Contact management: avoid rattling effects of
tiny collisions
• Threshold for contact: if closing velocity
smaller than threshold, set coefficient of
restitution to zero
– and perhaps stop simulating this object for now
• Given output velocity, update velocity of body
using momentum (impulse):
j = -(1+c)(
– Unpacking:
• v is relative velocity
• nc is collision normal
• c is coefficient of restitution
Closing rotational velocity
• Recall that rotation produces instantaneous
linear velocity: v
• so, add this velocity to centre of mass velocity
to get velocity of collision point
– r = distance from body centre to collision point
– if using angular momentum, ω = I-1L
Impulsive torque
• Compute impulse as before: have j = ∆P
• Now, compute impulsive torque ∆L
• Actually simple: ∆L = r x j
– recall τ = r x F, same idea
Wrapping up
• Apply impulse, impulsive torque to both
bodies (one positive, one negative)
• If one body is fixed: effectively infinite mass,
moment of inertia (zero inverse mass) so no
resulting velocity

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