RMagick changing image extents with gravity

I've got an image that i'd like to 'pad' with white space and centre.

In most cases I need to resize the image from 16 or 32 pixels up to 32 pixels. If the image is 16 pixels, I want to add 8px of white space on each side开发者_如何学Python, making it a 32 pixel image (with the original floating in the middle).

If it's a 32 pixel image, then nothing changes.

I'm using RMagick to do the conversion:

image.change_geometry!("#{size}x#{size}") { |cols, rows, img|
  newimg = img.extent(cols, rows)
  newimg.write("#{RAILS_ROOT}#{path}/#{name}.png")
}

Which is working OK, but the smaller images are in the top left of the new image, not centred. I was looking at the gravity setting, it seems to be what I need, but I can't work out how to specify it in the call?

Thanks in advance.

---

Check the implementation of the following carrierwave function http://rubydoc.info/gems/carrierwave/0.5.1/CarrierWave/RMagick#resize_and_pad-instance_method

This is a version of the above method by using only RMagick dependency

require 'RMagick'
include Magick

module Converter
  def self.resize_and_pad(img, new_img_path, width, height, background=:transparent, gravity=::Magick::CenterGravity)
    img.resize_to_fit!(width, height)
    new_img = ::Magick::Image.new(width, height)
    if background == :transparent
      filled = new_img.matte_floodfill(1, 1)
    else
      filled = new_img.color_floodfill(1, 1, ::Magick::Pixel.from_color(background))
    end
    # destroy_image(new_img)
    filled.composite!(img, gravity, ::Magick::OverCompositeOp)
    # destroy_image(img)
    # filled = yield(filled) if block_given?
    # filled
    filled.write new_img_path
  end
end

---

The extent() method takes two more parameters, x & y offsets, which is where the image will be placed within the extent. If you're asking extent for a 100x100 image, for example, and your original is only 50x50, you'd do img.extent(100, 100, 25, 25) -- which would set the image to start at offset 25,25 (thus centering it).

NOTE: There's some issue with extent expecting to use negative offset values (in which case you'd want to do -25, -25) -- check this:

why is the behavior of extent (imagemagick) not uniform across my machines?