Lesson 15 · Inverse mass - Build a Physics Engine

Inverse mass

Collision math divides by mass constantly, and a wall has effectively infinite mass, so engines store one over the mass instead. Today you compute inverse mass, where a mass of zero means immovable.

The goal

Store a body's inverse mass, defined so that a mass of zero yields an inverse mass of zero.

Start here - the target

TO DO

Scenario: Deriving inverse mass from mass

Givena body

WhenSetMass is called with 2

ThenMass is 2 and InvMass is 0.5

AndSetMass(4) gives InvMass 0.25, and SetMass(0) gives InvMass 0 - a mass of zero means infinite mass, an immovable body

Background

Collision response is full of divisions by mass, and doing them as multiplications by a stored inverse mass is both faster and, more importantly, lets you represent an immovable body cleanly. A wall or the ground should never budge no matter what hits it - that is a body of infinite mass, and 1 / infinity is 0. So the convention is: a mass of 0 is a special marker meaning infinite mass, and its inverse mass is 0. Multiplying any impulse by that zero inverse mass produces no change in velocity, which is exactly what “immovable” means.

Set both together through SetMass so they never drift out of sync. While you are here, add a Restitution field - a number from 0 to 1 for how bouncy the body is, 0 for a dead thud and 1 for a perfect bounce. It just rides along as data today; the resolution chapter is where it earns its keep. The next lesson uses this zero inverse mass to make a static body ignore forces entirely.

Make it work

// add InvMass float64 and Restitution float64 fields to Body
func (b *Body) SetMass(m float64) {
  b.Mass = m
  if m == 0 {
    b.InvMass = 0 // infinite mass: a static, immovable body
  } else {
    b.InvMass = 1 / m
  }
}

CheckpointDONE

A body stores its inverse mass, with zero standing for an immovable body. Commit and stop here.