add jbox2d examples

samples/external_library/ruby_gem/jbox2d/README.md

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+### java_args.txt
+
+The `data` folder contains java_args.txt, which currently silently enables the OpenGl-based pipeline using `-Dsun.java2d.opengl=true` add any other java args here, use `True` instead of `true` to get a message.

samples/external_library/ruby_gem/jbox2d/bumpy_surface_noise.rb

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+# The Nature of Code
+# PBox2D example
+# An uneven surface
+
+require 'pbox2d'
+require_relative 'lib/surface'
+
+attr_reader :surface, :box2d, :particles
+
+def setup
+  size(500, 300)
+  smooth 4
+  # Initialize box2d physics and create the world
+  @box2d = Box2D.new(self)
+  box2d.init_options(gravity: [0, -20])
+  box2d.create_world
+  # to later set a custom gravity
+  # box2d.gravity([0, -20])
+  # Create the empty list
+  @particles = []
+  # Create the surface
+  @surface = Surface.new(box2d)
+end
+
+def draw
+  # If the mouse is pressed, we make new particles
+  # We must always step through time!
+  background(138, 66, 54)
+  # Draw the surface
+  surface.display
+  # NB ? reqd to call mouse_pressed value, else method gets called.
+  particles << Particle.new(box2d, mouse_x, mouse_y, rand(2.0..6)) if mouse_pressed?
+  # Draw all particles
+  particles.each(&:display)
+  # Particles that leave the screen, we delete them
+  # (note they have to be deleted from both the box2d world and our list
+  particles.reject!(&:done)
+  # Just drawing the framerate to see how many particles it can handle
+  fill(0)
+  text("framerate: #{frame_rate.to_i}", 12, 16)
+end

samples/external_library/ruby_gem/jbox2d/data/java_args.txt

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+-Dsun.java2d.opengl=true

samples/external_library/ruby_gem/jbox2d/lib/boundary.rb

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+# The boundary class is used to create fixtures
+class Boundary
+  include Processing::Proxy
+  attr_reader :box2d, :x, :y, :w, :h, :b
+  def initialize(b2d, x, y, w, h)
+    @box2d, @x, @y, @w, @h = b2d, x, y, w, h
+    sd = PolygonShape.new
+    box2d_w = box2d.scale_to_world(w / 2)
+    box2d_h = box2d.scale_to_world(h / 2)
+    sd.setAsBox(box2d_w, box2d_h)
+    # Create the body
+    bd = BodyDef.new
+    bd.type = BodyType::STATIC
+    bd.position.set(box2d.processing_to_world(x, y))
+    @b = box2d.create_body(bd)
+    # Attached the shape to the body using a Fixture
+    b.create_fixture(sd, 1)
+  end
+
+  # Draw the boundary, if it were at an angle we'd have to do something fancier
+  def display
+    fill(0)
+    stroke(0)
+    rect_mode(CENTER)
+    rect(x, y, w, h)
+  end
+end

samples/external_library/ruby_gem/jbox2d/lib/box.rb

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+# A Box class, note how to access class ParticleGroupDef in jruby
+# which is currently not on an included path for pbox2d
+class Box
+  attr_accessor :pg
+  def initialize(b2d, x, y)
+    w = rand(1..3)
+    h = rand(1..3)
+    shape = PolygonShape.new
+    pos = b2d.processing_to_world(x, y)
+    shape.setAsBox(w, h, pos, 0)
+    pd = Java::OrgJbox2dParticle::ParticleGroupDef.new
+    pd.shape = shape
+    @pg = b2d.world.create_particle_group(pd)
+  end
+end

samples/external_library/ruby_gem/jbox2d/lib/custom_shape.rb

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+require 'pbox2d'
+
+class CustomShape
+  include Processing::Proxy
+  # We need to keep track of a Body and a width and height
+  attr_reader :body, :box2d
+
+  # Constructor
+  def initialize(b2d, x, y)
+    # Add the box to the box2d world
+    @box2d = b2d
+    make_body(Vec2.new(x, y))
+  end
+
+  # This function removes the particle from the box2d world
+  def kill_body!
+    box2d.destroy_body(body)
+  end
+
+  # Is the particle ready for deletion?
+  def done
+    # Let's find the screen position of the particle
+    pos = box2d.body_coord(body)
+    # Is it off the bottom of the screen?
+    return false unless pos.y > box2d.height
+    kill_body!
+    true
+  end
+
+  # Drawing the box
+  def display
+    # We look at each body and get its screen position
+    pos = box2d.body_coord(body)
+    # Get its angle of rotation
+    a = body.get_angle
+    f = body.get_fixture_list
+    ps = f.get_shape
+    rect_mode(CENTER)
+    push_matrix
+    translate(pos.x, pos.y)
+    rotate(-a)
+    fill(175)
+    stroke(0)
+    begin_shape
+    # For every vertex, convert to pixel vector
+    ps.get_vertex_count.times do |i|
+      v = box2d.vector_to_processing(ps.get_vertex(i))
+      vertex(v.x, v.y)
+    end
+    end_shape(CLOSE)
+    pop_matrix
+  end
+
+  # This function adds the rectangle to the box2d world
+  def make_body(center)
+    # Define a polygon (this is what we use for a rectangle)
+    sd = PolygonShape.new
+    vertices = []
+    vertices << box2d.vector_to_world(Vec2.new(-15, 25))
+    vertices << box2d.vector_to_world(Vec2.new(15, 0))
+    vertices << box2d.vector_to_world(Vec2.new(20, -15))
+    vertices << box2d.vector_to_world(Vec2.new(-10, -10))
+    sd.set(vertices.to_java(Java::OrgJbox2dCommon::Vec2), vertices.length)
+    # Define the body and make it from the shape
+    bd = BodyDef.new
+    bd.type = BodyType::DYNAMIC
+    bd.position.set(box2d.processing_to_world(center))
+    @body = box2d.create_body(bd)
+    body.create_fixture(sd, 1.0)
+    # Give it some initial random velocity
+    body.set_linear_velocity(Vec2.new(rand(-5.0..5), rand(2.0..5)))
+    body.set_angular_velocity(rand(-5.0..5))
+  end
+end
+
+class Boundary
+  include Processing::Proxy
+  attr_reader :box2d, :b, :x, :y, :w, :h
+  def initialize(b2d, x, y, w, h, a)
+    @box2d, @x, @y, @w, @h = b2d, x, y, w, h
+    # Define the polygon
+    sd = PolygonShape.new
+    # Figure out the box2d coordinates
+    box2d_w = box2d.scale_to_world(w / 2)
+    box2d_h = box2d.scale_to_world(h / 2)
+    # We're just a box
+    sd.set_as_box(box2d_w, box2d_h)
+    # Create the body
+    bd = BodyDef.new
+    bd.type = BodyType::STATIC
+    bd.angle = a
+    bd.position.set(box2d.processing_to_world(x, y))
+    @b = box2d.create_body(bd)
+    # Attached the shape to the body using a Fixture
+    b.create_fixture(sd, 1)
+  end
+
+  # Draw the boundary, it doesn't move so we don't have to ask the Body for location
+  def display
+    fill(0)
+    stroke(0)
+    stroke_weight(1)
+    rect_mode(CENTER)
+    a = b.get_angle
+    push_matrix
+    translate(x, y)
+    rotate(-a)
+    rect(0, 0, w, h)
+    pop_matrix
+  end
+end

samples/external_library/ruby_gem/jbox2d/lib/particle_system.rb

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+require 'forwardable'
+
+module Runnable
+  def run
+    reject! { |item| item.done }
+    each { |item| item.display }
+  end
+end
+
+class ParticleSystem
+  include Enumerable, Runnable
+  extend Forwardable
+  def_delegators(:@particles, :each, :reject!, :<<, :empty?)
+  def_delegator(:@particles, :empty?, :dead?)
+
+  attr_reader :x, :y
+
+  def initialize(bd, num, x, y)
+    @particles = []          # Initialize the Array
+    @x, @y = x, y            # Store the origin point  
+    num.times do
+      self << Particle.new(bd, x, y)
+    end
+  end
+
+  def add_particles(bd, n)
+    n.times do
+      self << Particle.new(bd, x, y)
+    end
+  end
+end
+
+# A Particle
+require 'pbox2d'
+
+class Particle
+  include Processing::Proxy
+  TRAIL_SIZE = 6
+  # We need to keep track of a Body
+
+  attr_reader :trail, :body, :box2d
+
+  # Constructor
+  def initialize(b2d, x, y)
+    @box2d = b2d
+    @trail = Array.new(TRAIL_SIZE, [x, y])
+    # Add the box to the box2d world
+    # Here's a little trick, let's make a tiny tiny radius
+    # This way we have collisions, but they don't overwhelm the system
+    make_body(x, y, 0.2)
+  end
+
+  # This function removes the particle from the box2d world
+  def kill_body
+    box2d.destroy_body(body)
+  end
+
+  # Is the particle ready for deletion?
+  def done
+    # Let's find the screen position of the particle
+    pos = box2d.body_coord(body)
+    # Is it off the bottom of the screen?
+    return false unless (pos.y > box2d.height + 20)
+    kill_body
+    true
+  end
+
+  # Drawing the box
+  def display
+    # We look at each body and get its screen position
+    pos = box2d.body_coord(body)
+    # Keep track of a history of screen positions in an array
+    (TRAIL_SIZE - 1).times do |i|
+      trail[i] = trail[i + 1]
+    end
+    trail[TRAIL_SIZE - 1] = [pos.x, pos.y]
+    # Draw particle as a trail
+    begin_shape
+    no_fill
+    stroke_weight(2)
+    stroke(0, 150)
+    trail.each do |v|
+      vertex(v[0], v[1])
+    end
+    end_shape
+  end
+
+  # This function adds the rectangle to the box2d world
+  def make_body(x, y, r)
+    # Define and create the body
+    bd = BodyDef.new
+    bd.type = BodyType::DYNAMIC
+    bd.position.set(box2d.processing_to_world(x, y))
+    @body = box2d.create_body(bd)
+    # Give it some initial random velocity
+    body.set_linear_velocity(Vec2.new(rand(-1.0..1), rand(-1.0..1)))
+    # Make the body's shape a circle
+    cs = CircleShape.new
+    cs.m_radius = box2d.scale_to_world(r)
+    fd = FixtureDef.new
+    fd.shape = cs
+    fd.density = 1
+    fd.friction = 0  # Slippery when wet!
+    fd.restitution = 0.5
+    # We could use this if we want to turn collisions off
+    # cd.filter.groupIndex = -10
+    # Attach fixture to body
+    body.create_fixture(fd)
+  end
+end
+
+class Boundary
+  include Processing::Proxy
+  attr_reader :box2d, :b, :x, :y, :w, :h
+
+  def initialize(b2d, x, y, w, h, a)
+    @box2d, @x, @y, @w, @h = b2d, x, y, w, h
+    # Define the polygon
+    sd = PolygonShape.new
+    # Figure out the box2d coordinates
+    box2d_w = box2d.scale_to_world(w / 2)
+    box2d_h = box2d.scale_to_world(h / 2)
+    # We're just a box
+    sd.set_as_box(box2d_w, box2d_h)
+    # Create the body
+    bd = BodyDef.new
+    bd.type = BodyType::STATIC
+    bd.angle = a
+    bd.position.set(box2d.processing_to_world(x, y))
+    @b = box2d.create_body(bd)
+    # Attached the shape to the body using a Fixture
+    b.create_fixture(sd, 1)
+  end
+
+  # Draw the boundary, it doesn't move so we don't have to ask the Body for location
+  def display
+    fill(0)
+    stroke(0)
+    stroke_weight(1)
+    rect_mode(CENTER)
+    a = b.get_angle
+    push_matrix
+    translate(x, y)
+    rotate(-a)
+    rect(0, 0, w, h)
+    pop_matrix
+  end
+end