Question

I have a Rails app where users can upload images. After they upload an image, I need to apply a mask on that image, so parts of it become transparant. Is this possible?

Answer 1

RMagick (a Ruby image manipulation library) allows you to apply clipping path on objects : http://www.imagemagick.org/RMagick/doc/rvgclip.html

Basically, you’ll need to :

• create a clipping path instance
• use the drawing API to draw your mask
• apply the path to your image

A click on the keyhole image” there pops up an example of how to achieve this :

require 'rvg/rvg'

hat = Magick::Image.read('images/Flower_Hat.jpg').first
rvg = Magick::RVG.new(hat.columns, hat.rows) do |canvas|
    keyhole = Magick::RVG::ClipPath.new do |path|
        path.circle(60, canvas.width/2, 80)
        path.polygon(canvas.width/2-10, 60, 40, 230, 160, 230, canvas.width/2+10, 60)
    end
    canvas.image(hat, nil, nil, 20, 20).styles(:clip_path=>keyhole)

end

rvg.draw.write('rvg_clippath.gif')

EDIT :

If you need to apply a transparent background the simplest approach should be to crop your image, then use the Image#composite! method to add a trnasparent layer.