# crop_with_gravity.rb

    #!/usr/bin/ruby1.9.1 -w

    #=======================================================#
    # Thanks to Robert Wagner for the idea of allowing a    #
    # GravityType instead of the x- and y-offset arguments! #
    #=======================================================#

    # Demo the use of the GravityType argument to Image#crop.

    require 'RMagick'
    include Magick

    shorts = Image.read('images/Shorts.jpg').first

    regwidth = shorts.columns/2
    regheight = shorts.rows/2

    mask = Image.new(regwidth, regheight) { self.background_color = 'white'}
    mask.opacity = 0.50 * TransparentOpacity

    black = Image.new(shorts.columns, shorts.rows) {self.background_color = 'black'}
    pairs = ImageList.new

    [NorthWestGravity, NorthGravity, NorthEastGravity,
     WestGravity, CenterGravity, EastGravity,
     SouthWestGravity, SouthGravity, SouthEastGravity].each do |gravity|
        pattern = shorts.composite(mask, gravity, OverCompositeOp)
        cropped = shorts.crop(gravity, regwidth, regheight)
        result = black.composite(cropped, gravity, OverCompositeOp)
        result.border_color = "white"
        pairs << pattern
        pairs << result
    end

    # Montage into a single image
    montage = pairs.montage {
        self.geometry = "#{pairs.columns}x#{pairs.rows}+0+0"
        self.tile = "6x3"
        self.border_width = 1
        }
    montage.write('crop_with_gravity.miff')
    #montage.display

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