class IRB::RubyModel

def render_frame(i)
  angle = i / 10.0
  dir = Vec.new(*Complex.polar(1, angle).rect, Math.sin(angle)).normalize
  dir2 = Vec.new(*Complex.polar(1, angle - Math::PI/2).rect, 0)
  up = dir.cross(dir2)
  nm = dir.cross(up)
  @faces.each do |vertices|
    v0, v1, v2, = vertices
    if v1.sub(v0).cross(v2.sub(v0)).dot(dir) > 0
      points = vertices.map {|p| [nm.dot(p), up.dot(p)] }
      (points + [points[0]]).each_cons(2) do |p1, p2|
        yield p1, p2
      end
    end
  end
end