module ImageTools
  class < < self
    def dimensions(source)
      img = Image.read(source).first
      img.rows, img.columns
    end
  end
end

Then in your model, use a before_validation hook:

class MyModel < ActiveRecord::Base
  before_validation :set_dimensions!
  def set_dimensions!
    rows, cols = ImageTools.dimensions(self.file_path)
    self.width = cols
    self.height = rows
    self.aspect_ratio = cols.to_f / rows
  end
end

Hope that helps.

This is a great article, and something that I’ve been searching for, for a while now.

However, you say …

”It helps if your model includes an aspect_ratio field, which is generated from the original image on upload”

… and I was just wondering how you achieve this ?

(I’m new to image processing, but it’s kinda key to an app I’m developing)

Thanks

-Mic