Creating a gem wrapper for jbox2d in ruby-processing

The mechanism for including Daniel Shiffmans PBox2D for processing into ruby-processing is unnecessarily complicated, and I had always thought there should be an easier way. So here it is jbox2d wrapped as a gem using some jruby sugar (and avoiding the nasty reflection nonsense that Dan used, although under the hood jruby may do something similar).
The ruby-processing sketch

require 'pbox2d'
require_relative 'lib/particle_system'
attr_reader :box2d, :boundaries, :systems

def setup
  size(400,300)
  @box2d = Box2D.new(self)
  box2d.create_world
  # We are setting a custom gravity
  box2d.gravity(0, -20)

  # Create Arrays
  @systems = []
  @boundaries = []
  # Add a bunch of fixed boundaries
  boundaries << Boundary.new(box2d, 50, 100, 300, 5, -0.3)
  boundaries << Boundary.new(box2d, 250, 175, 300, 5, 0.5)
end

def draw
  background(255)
  # Run all the particle systems
  if systems.size > 0
    systems.each do |system|
      system.run
      system.add_particles(box2d, rand(0..2))
    end
  end
  # Display all the boundaries
  boundaries.each(&:display)
end

def mouse_pressed
  # Add a new Particle System whenever the mouse is clicked
  systems << ParticleSystem.new(box2d, 0, mouse_x, mouse_y)
end

The associated (local) library

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, PB
  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(PB::Vec2.new(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 > $app.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(center, r)
    # Define and create the body
    bd = PB::BodyDef.new
    bd.type = PB::BodyType::DYNAMIC
    bd.position.set(box2d.processing_to_world(center))
    @body = box2d.create_body(bd)
    # Give it some initial random velocity
    body.set_linear_velocity(PB::Vec2.new(rand(-1.0..1), rand(-1.0..1)))
    # Make the body's shape a circle
    cs = PB::CircleShape.new
    cs.m_radius = box2d.scale_to_world(r)
    fd = PB::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, PB
  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 = PB::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 = PB::BodyDef.new
    bd.type = PB::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

Ruby-Processing-2.5.1 now available from rubygems.org

If you haven't installed ruby-processing before you will need:-

1. a recent version of java (7+) 
2. an installed version of vanilla processing-2.2.1
3. a recent version of ruby (1.9.3+) preferably jruby-1.7.13 (but MRI ruby is OK)
4. preferably wget (for downloading jruby-complete)

Recommended order of installation (skip 1 if you've installed ruby-processing recently)

1. Set the processing root (you could run this sketch in processing ide)
2. gem install ruby-processing (current version 2.5.1) 
3. rp5 setup install (to download and install jruby-complete-1.7.13) 
4. rp5 setup check (to check your setup) if you haven't got a system jruby installed you should set  JRUBY: 'false' in your ~/.rp5rc file 
5. rp5 setup unpack_samples
6. cd rp_samples 
7. rake to to run contributed samples as a Demo

Choice of editor / ide

• Believe it or not it is possible to use netbeans as a development environment for ruby
• For a near to ide experience you could use jEdit
• vim is perfect for console development (examples on this blog posted using syntax/2html.vim)
• if you are a Mac you might use textmate

The simplest ArcBall sketch (under the hood sketch is translated to 300, 300 & arcball radius is set as 0.8 * height)

load_library :vecmath

############################
# Use mouse drag to rotate
# the arcball. Use mousewheel
# to zoom. Hold down x, y, z
# to constrain rotation axis.
############################

def setup
  size(600, 600, P3D)
  smooth(16)
  ArcBall.init(self, 300, 300)
  fill 180
end

def draw
  background(50)
  box(300, 300, 300)
end

See here for an example of DegLut table usage. Over 300 examples included with the gem, although there is no book for ruby-processing, code examples from Dan Shiffmans Learning Processing and his Nature of Code books have both been translated to work with ruby-processing here and here. If you are new to ruby or processing start with rp_samples/processing_app/basics, if you just want to be impressed, from the rp_samples directory run 'rake shaders', where you will see the level of performance that you might not have thought possible, given how slow ruby is supposed to be (running on gpu so it's a big cheat, but you can do this too see retained_menger.rb sketch).

Some On Line Tutorials (NB: all posted before ruby-processing-2.5.1, where install has changed)

1. Introductory level (Jeff Casimir)
2. An Introduction to Ruby Processing (Steve Kinney) 
3. Drawing with processing and Ruby (Dhaivat Pandya 

Creating a jruby extension as a gem

What's an extension:-

Many MRI gems use extensions to wrap libraries that are written in C with a ruby wrapper. Examples include nokogiri which wraps libxml2 and libxslt, pg which is an interface to the PostgreSQL database and the mysql and mysql2 gems which provide an interface to the MySQL database. This above was copied from a guide to creating a gem with extensions. In jruby java alternatives are preferred.

A jruby extension can act as a wrapper for a java library

The Apache commons-math library is an example of such a library, here is gem I've created as a jruby extension.

Conways Game of Life in ruby-processing (featuring MDArray)

# game_of_life.rb featuring MDArray in ruby-processing
# A Processing implementation of Game of Life
# By Joan Soler-Adillon
#
# Press SPACE BAR to pause and change the cell's values with the mouse
# On pause, click to activate/deactivate cells
# Press R to randomly reset the cells' grid
# Press C to clear the cells' grid
#
# The original Game of Life was created by John Conway in 1970.
#

require 'mdarray'

CELL_SIZE = 5
ALIVE = true
DEAD = false
ALIVE_START = 150
WIDTH = 960
HEIGHT = 640
SKIP = 10
INTERVAL = 100

attr_reader :pause, :cells, :row, :column, :last_time, :alive, :cells_buffer

# signature-specific aliases for overloaded methods
java_alias :my_color, :color, [Java::float, Java::float, Java::float]
java_alias :my_fill, :fill, [Java::int]
java_alias :my_stroke, :stroke, [Java::float, Java::float]
java_alias :my_background, :background, [Java::int]

def setup
  size WIDTH, HEIGHT
  @row = WIDTH / CELL_SIZE
  @column = HEIGHT / CELL_SIZE
  stroke_weight 2
  @last_time = 0
  @pause = false
  @cells = MDArray.boolean([row, column], random_data)
  @alive = my_color(100, 255, 100)
  my_stroke(48, 100)
  no_smooth
end

def draw
  my_background(0)
  #Draw live cells
  (0 ... row).each do |x|
    (0 ... column).each do |y|
      if (cells.get([x, y]))
        my_fill(alive)
        rect(x * CELL_SIZE, y * CELL_SIZE, CELL_SIZE, CELL_SIZE)
      end
    end
  end
  # Iterate if timer ticks
  if (millis - last_time > INTERVAL)
    if (!pause)
      tick!
      @last_time = millis
    end
  end

  # Create  new cells manually on pause
  if (pause && mouse_pressed?)
    # # Map and avoid out of bound errors
    x_cell_over = (map(mouse_x, 0, width, 0, row)).to_i
    x_cell_over = constrain(x_cell_over, 0, row - 1)
    y_cell_over = (map(mouse_y, 0, height, 0, column)).to_i
    y_cell_over = constrain(y_cell_over, 0, column - 1)

    # Check against cells in buffer
    if (cells_buffer.get([x_cell_over, y_cell_over]))  # Cell is alive
      cells.set([x_cell_over, y_cell_over], DEAD) # Kill
      my_fill(0) #reflect changed status
    else  # Cell is dead
      cells.set([x_cell_over, y_cell_over], ALIVE) # Make alive
      my_fill(alive) # Fill alive color
    end

  elsif (pause && !mouse_pressed?)  # And then save to buffer once mouse goes up
    # Save cells to buffer (so we operate with one array keeping the other intact)
    @cells_buffer = cells.copy
  end
end

def tick!  # When the clock ticks
  # Save cells to buffer (so we operate with one array keeping the other intact)
  @cells_buffer = cells.copy
  # Visit each cell:
  (0 ... row).each do |x|
    (0 ... column).each do |y|
      # And visit all the neighbours of each cell
      neighbours = 0 # We'll count the neighbours
      (x - 1 .. x + 1).each do |xx|
        (y - 1 .. y + 1).each do |yy|
          # Make sure you are not out of bounds
          if [(xx>=0), (xx<row), (yy>=0), (yy<column)].all? {|in_bounds| in_bounds == true}
            # Make sure to check against self
            if ![(xx == x), (yy == y)].all? {|is_self| is_self == true}
              if (cells_buffer.get([xx, yy])) # true == ALIVE
                neighbours += 1 # Check alive neighbours and count them
              end # alive
            end # End of if self
          end # End of if grid bounds
        end # End of yy loop
      end #End of xx loop
      # We've checked the neighbours: apply rules in one line (only in ruby)!
      cells.set([x, y], (cells_buffer.get([x, y]))?  ((2 .. 3) === neighbours) : (neighbours == 3))
    end # End of y loop
  end # End of x loop
end # End of function

def key_pressed
  case key
  when 'r', 'R'
    # Restart: reinitialization of cells 
    @cells = MDArray.boolean([row, column], random_data)
  when ' ' # On/off of pause
    @pause = !pause
  when 'c', 'C' # Clear all
    @cells = MDArray.boolean([row, column], DEAD)
  end
end

def random_data
  data = []
  (0 ... row * column).each do
    data << (rand(1000) < ALIVE_START)
  end
  return data
end