verlet integration

Author: monkstone

processing_app/library/vecmath/vec2d/library/verlet_chain/lib/verlet_ball.rb

@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+# Chain link end as a ball
+class VerletBall
+  include Propane::Proxy
+  attr_reader :pos, :pos_old, :push, :radius, :x_bound, :y_bound
+
+  def initialize(pos, push, radius)
+    @pos = pos
+    @push = push
+    @radius = radius
+    @pos_old = Vec2D.new(pos.x, pos.y)
+    # start motion
+    @pos += push
+    @x_bound = Boundary.new(radius, width - radius)
+    @y_bound = Boundary.new(radius, height - radius)
+  end
+
+  def run
+    verlet
+    render
+    bounds_collision
+  end
+
+  def adjust(vec)
+    @pos += vec
+  end
+
+  private
+
+  def verlet
+    pos_temp = Vec2D.new(pos.x, pos.y)
+    @pos += (pos - pos_old)
+    @pos_old = pos_temp
+  end
+
+  def render
+    ellipse(pos.x, pos.y, radius, radius)
+  end
+
+  def bounds_collision
+    if x_bound.exclude? pos.x
+      pos.x = constrain pos.x, x_bound.lower, x_bound.upper
+      pos_old.x = pos.x
+      pos.x = (pos.x <= radius)? pos.x + push.x : pos.x - push.x
+    end
+    return unless y_bound.exclude? pos.y
+    pos.y = constrain pos.y, y_bound.lower, y_bound.upper
+    pos_old.y = pos.y
+    pos.y = (pos.y <= radius)? pos.y + push.y : pos.y - push.y
+  end
+end
+
+# We can easily create and test bounds in ruby
+Boundary = Struct.new(:lower, :upper) do
+  def exclude? val
+    true unless (lower..upper).cover? val
+  end
+end

processing_app/library/vecmath/vec2d/library/verlet_chain/lib/verlet_stick.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+# Chain link end as a ball
+class VerletStick
+  include Propane::Proxy
+  attr_reader :ball_1, :ball_2, :stiffness, :len
+
+  def initialize(ball_1, ball_2, stiffness)
+    @ball_1 = ball_1
+    @ball_2 = ball_2
+    @stiffness = stiffness
+    @len = ball_1.pos.dist(ball_2.pos)
+  end
+
+  def run
+    render
+    constrain_len
+  end
+
+  private
+
+  def render
+    begin_shape
+    vertex(ball_1.pos.x, ball_1.pos.y)
+    vertex(ball_2.pos.x, ball_2.pos.y)
+    end_shape
+  end
+
+  def constrain_len
+    delta = ball_2.pos - ball_1.pos
+    delta_length = delta.mag
+    difference = ((delta_length - len) / delta_length)
+    ball_1.adjust delta * (0.5 * stiffness * difference)
+    ball_2.adjust delta * (-0.5 * stiffness * difference)
+  end
+end

processing_app/library/vecmath/vec2d/library/verlet_chain/verlet_chain.rb

@@ -0,0 +1,3 @@
+# frozen_string_literal: true
+require_relative 'lib/verlet_ball'
+require_relative 'lib/verlet_stick'

processing_app/library/vecmath/vec2d/verlet_integration.rb

@@ -0,0 +1,43 @@
+#!/usr/bin/env jruby -v -W2
+# frozen_string_literal: true
+#
+# Verlet Integration - ragdoll chain
+# after a sketch by Ira Greenberg
+#
+require 'propane'
+
+class VerletIntegration < Propane::App
+  load_library :verlet_chain
+  PARTICLES = 5
+  LINKS = PARTICLES - 1
+
+  attr_reader :sticks, :balls
+
+  def settings
+    size(400, 400)
+  end
+
+  def setup
+    sketch_title 'Verlet Chain'
+    ellipse_mode(RADIUS)
+    rshape = 40
+    tension = 0.05
+    @sticks = []
+    @balls = []
+    (0..PARTICLES).each do |i|
+      push = Vec2D.new(rand(3..6.5), rand(3..6.5))
+      pos = Vec2D.new(width / 2 + (rshape * i), height / 2)
+      balls << VerletBall.new(pos, push, 5.0)
+      next if i.zero?
+      sticks << VerletStick.new(balls[i - 1], balls[i], tension)
+    end
+  end
+
+  def draw
+    background(255)
+    sticks.each(&:run)
+    balls.each(&:run)
+  end
+end
+
+VerletIntegration.new