Drawing planets with chunky.png

planets.rb

# initial inspiration https://twitter.com/thereaIbanksy/status/670406732066111488
# pictures of output  https://twitter.com/josh_cheek/status/670460393681256448
require 'chunky_png'


Point = Struct.new :x, :y do
  def +(point)
    self.class.new x+point.x, y+point.y
  end
  def *(n)
    self.class.new x*n, y*n
  end
end

Edge  = Struct.new :from, :to do
  def x0; from.x; end
  def y0; from.y; end
  def x1; to.x;   end
  def y1; to.y;   end
  def draw_on(png, colour=WHITE)
    png.line from.x,from.y, to.x,to.y, colour
  end
end

BROWN = ChunkyPNG::Color.rgba 0x2A, 0x21, 0x1c, 255
WHITE = ChunkyPNG::Color.rgba 0xFF, 0xFF, 0xFF, 255

class Planet
  TWO_PI = Math::PI*2

  attr_reader :radius, :speed, :angle
  def initialize(radius:, speed:1, angle:0, center_x:, center_y:)
    @radius, @speed, @angle, @locations, @center_x, @center_y = radius, speed, angle, {}, center_x, center_y
  end

  def align_with(planet)
    @angle = planet.angle
  end

  def orbit!(duration=1)
    @angle += duration*speed
    @angle -= 360 while angle > 360
    self
  end

  def location
    @locations[angle] ||= Point.new(x.to_i, y.to_i)
  end

  def x
    @center_x + Math.cos(rad(angle))*radius
  end

  def y
    @center_y + Math.sin(rad(angle))*radius
  end
end

def rad(a)
  a*Math::PI/180
end

width   = height = 1000
colours = [BROWN, WHITE]
png     = ChunkyPNG::Image.new width, height, BROWN
p1      = Planet.new(radius: 200, speed:  5, center_x:width/2, center_y:height/2)
p2      = Planet.new(radius: 400, speed:  0, center_x:width/2, center_y:height/2)
p3      = Planet.new(radius: 450, speed: 13, center_x:width/2, center_y:height/2)

# To allow it to vary elegantly across the colour space, use (math.sin(angle)*127.5+128).to_i
# This will move as a sine wave, along an axis that splits it evenly
# (if you use 127 or 128, it will go out of bounds and one of the lines will be conspicuously out of place
# 361.times.map { |i|
#   r = i*Math::PI/180
#   (Math.sin(r)*127.5+128).to_i
# }.minmax # => [0, 255]

alignment = [p1,p3]
720.times do |i|
  loc = p2.location
  if i%30 == 0
    alignment.rotate! 1
    p2.align_with alignment.first
  end
  r       = rad(i)
  colour1 = ChunkyPNG::Color.rgb (Math.sin(r)*127.5+128).to_i, 0, 0xFF
  colour2 = ChunkyPNG::Color.rgb 0xFF, (Math.sin(r)*127.5+128).to_i, 0
  Edge.new(p1.location, p2.location).draw_on(png, colour1)
  Edge.new(p3.location, p2.location).draw_on(png, colour2)
  p1.orbit!
  p2.orbit!
  p3.orbit!
end

num  = Dir['orbits*.png'].map { |name| name[/\d+/].to_i }.max.next
name = "orbits#{num}.png"
png.save name, interlace: true
`which open && open #{name}`