Simple demo of gravity in Dragonruby

main.rb

# Click the Run game! button to execute the code.

def vector_add(v1,v2)
  v1.zip(v2).map{|p1,p2| p1+p2}
end

# For demonstration purposes, we just blanket apply a terminal velocity.
# Set this low to see the effect.
def resistance(v, gravity)
  # This only applies air resistance downwards, which is lazy
  terminal_velocity = -100
  if v[1] < terminal_velocity
    return [gravity[0],-gravity[1]]
  end

  # Set f higher the more air resistance an object produces.
  # if f = 1, the object will float.
  f = 0.1
  [-f*gravity[0],-f*gravity[1]]
end

def tick args
  args.state.position ||= [400,100]
  args.state.velocity ||= [10,20]
  args.state.gravity  ||= [0,-0.4]

  # How much energy is lost in each direction on a bounce]
  args.state.attenuation = [0.9,0.7]

  args.lines << [0,100, 1280,100, 0, 255,0]
  args.labels << [100,90, "Position: #{args.state.position.inspect}"]
  args.labels << [100,70, "Velocity: #{args.state.velocity.inspect}"]
  args.outputs.solids << [*args.state.position.to_a, 8,8, 255, 0,0]

  # Update velocity based on gravity
  r = resistance(args.state.velocity, args.state.gravity)
  args.state.velocity= vector_add(args.state.velocity, args.state.gravity)
  args.state.velocity= vector_add(args.state.velocity, r)

  # Update position based on velocity
  args.state.position= vector_add(args.state.position, args.state.velocity)

  pos = args.state.position

  # Colliding...
  if pos[1] < 100 ||
     pos[1] > 720 ||
     pos[0] < 0 ||
     pos[0] > 1280

    args.state.velocity[0] = -args.state.velocity[0]*args.state.attenuation[0]
    args.state.velocity[1] = -args.state.velocity[1]*args.state.attenuation[1]
  end

  # Make sure the object doesn't "escape"
  pos[1] = 100 if pos[1] < 100
  pos[1] = 720 if pos[1] > 720
  pos[0] = 0 if pos[0] < 0
  pos[0] = 1280 if pos[0] > 1280
end