# matte_replace.rb

    #!/usr/local/bin/ruby18 -w

    require 'RMagick'
    include Magick

    img = Image.new(200,200)
    img.compression = LZWCompression

    bg = Image.read('plasma:fractal') { self.size = '200x200' }
    bg[0].matte = false

    gc = Draw.new
    gc.stroke_width(2)
    gc.stroke('black')
    gc.fill('white')
    gc.roundrectangle(0, 0, 199, 199, 8, 8)

    gc.fill('black')
    gc.circle(100,  45, 100,  25)
    gc.circle( 45, 100,  25, 100)
    gc.circle(100, 155, 100, 175)
    gc.circle(155, 100, 175, 100)

    gc.draw(img)

    img.write('matte_replace_before.gif')

    # Set the border color. Set the fuzz attribute so that
    # the matte fill will fill the aliased pixels around
    # the edges of the black circles.
    img.border_color = 'black'
    img.fuzz = 10
    img = img.matte_replace(100, 45)

    # Composite the image over a nice bright background
    # so that the transparent pixels will be obvious.
    img = bg[0].composite(img, CenterGravity, OverCompositeOp)

    img.write('matte_replace_after.gif')
    exit

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