From: Gavin Kistner <gavin.kistner@...>
Date: 2006-09-10T00:44:47+09:00
Subject: Re: Overlapping circles - modelling living populations

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Here's a slightly better version that uses the overlapping status of a =
circle to change its color. (It doesn't do anything with pairs of =
circles, though that'd also be easy...you just have to know how you'd =
want to visualize it.)

# http://downloads.sourceforge.jp/ruby-svg/2288/ruby-svg-1.0.3.tar.gz
require 'svg/svg'

class Array
  def each_unique_pair
    self.each_with_index{ |a,i|
      self[(i+1)..-1].each{ |b| yield a,b }
    }
  end
end

class SVG::Circle
  # test (and save) result
  def overlaps?( other_circle )
    center_distance =3D Math.sqrt(
      ( cx - other_circle.cx ) ** 2 +
      ( cy - other_circle.cy ) ** 2
    )

    intersects =3D center_distance < ( r + other_circle.r )
   =20
    # Save the result on both circles   =20
    other_circle.overlaps[ self ] =3D intersects
    self.overlaps[ other_circle ] =3D intersects
  end
 =20
  def overlaps
    # Make sure we have a hash to save a set of overlappers
    @overlapping_circles ||=3D {}
  end
 =20
  # An array of all circles this one overlaps
  def overlapping_list
    # Make sure we have a hash to save a set of overlappers
    @overlapping_circles ||=3D {}
    @overlapping_circles.select{ |circle,status|
      status =3D=3D true
    }.map{ |circle,status|
      circle
    }
  end
end

# Create/manage your populations
populations =3D []
20.times{
  # Randomly add populations
  x =3D rand 800 + 100
  y =3D rand 800 + 100
  radius =3D rand 100 + 10
  populations << SVG::Circle.new( x, y, radius )
}

# Make sure the overlapping list is valid
populations.each_unique_pair{ |a,b|
  a.overlaps?( b )
}

# Make the picture
svg =3D SVG.new('100%', '100%', '0 0 1000 1000')

# Your grid here
0.step( 1000, 20 ){ |i|
  svg << SVG::Line.new( i, 0, i, 1000 )
  svg << SVG::Line.new( 0, i, 1000, i )
}

# Make all circles have a common style
svg << g =3D SVG::Group.new{
  self.style =3D SVG::Style.new(
    :fill =3D> '#ffe', :stroke =3D> 'blue', :stroke_width =3D> 1.5
  )
}

# Need opacity per element, not on the group
low_opacity =3D SVG::Style.new( :opacity =3D> 0.6 )

# Override style for circles that overlap
olap_style =3D SVG::Style.new( :fill=3D>'#ff9', :stroke=3D>'red', =
:opacity=3D>0.4 )

populations.each{ |circle|
  if circle.overlapping_list.empty?
    circle.style =3D low_opacity
  else
    circle.style =3D olap_style
  end
  g << circle
}

File.open( 'populations.svg', 'w' ){ |file|
  file << svg.to_s
}

# view in your favorite SVG viewer, like FF 1.5
# for an example, see http://phrogz.net/svg/populations.svg

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