bar constraint.lua

-- "bar" contains:
  -- pt1 {x, y, vx, vy} or {x, y, oldx, oldy}
  -- pt2 {x, y, vx, vy} or {x, y, oldx, oldy}
  -- dist (intended distance between points)

function updatebar(bar)  
  -- find the current vector between our points
  local dx = bar.pt1.x-bar.pt2.x
  local dy = bar.pt1.y-bar.pt2.y
  
  -- find the distance between the points
  local dist = sqrt(dx*dx + dy*dy)
  
  -- find the current error: how much extra distance do we have?
  -- (we halve it because we're going to apply half-a-push to each point)
  local extra = (dist - bar.dist)*.5
  
  -- maybe you only care about blocking "too far" positions,
  -- while points are allowed to be closer together?
  -- if (extra <= 0) return
  
  -- normalize the direction-vector, then resize it
  -- to match the size of our error
  local pushx = (dx/dist) * extra
  local pushy = (dy/dist) * extra

  -- move both points together or apart (since "extra" might be negative)
  bar.pt1.x-=pushx
  bar.pt1.y-=pushy
  bar.pt2.x+=pushx
  bar.pt2.y+=pushy

  -- if you're storing velocities (euler integration),
  -- you'll need to modify those, too:

  -- (this is a cheapo velocity-fixup method, but it's easy to tune)
  local vstr = .5  -- tuning value

  bar.pt1.vx -= pushx*vstr
  bar.pt1.vy -= pushy*vstr
  bar.pt2.vx += pushx*vstr
  bar.pt2.vy += pushy*vstr
end