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imagemagickruby-on-rails-5carrierwaveminimagick

Cropping transparent pixels with carrierwave

Using Carrierwave on Ruby 5 with MiniMagick, is it possible to trim transparent pixels ?

Suppose a user uploads a 500x500 image but only the inner 250x250 pixels are indeed filled, the rest is transparent. Is there a processing command that would help detect and trim the image to 250x250 before additional processing ?

I found https://www.imagemagick.org/discourse-server/viewtopic.php?t=12127 and it seems there is a trim transparent command on Imagemagick but I'm not sure how to use it with the Ruby wrapper Minimagick ?

Solution

  • The MiniMagick::Image.trim is all that's needed. Without a pixel-iterator, it would be simplest to apply trim on a clone image, and act on the smallest result.

    require 'mini_magick'

def trimed_image(path)
  image = MiniMagick::Image.open(path)
  test_image = image.clone
  test_image.trim
  if test_image.width < image.width || test_image.height < image.height
    test_image
  else
    image
  end
end

    Test case with convert rose: -resize x100 rose.png

    rose = trimed_image("rose.png")
rose.write("rose_output.png")

    No change expected.

    rose_output.png

    Test transparent image with convert -size 100x100 gradient: -background black -extent 200x200-50-50 -alpha copy trim.png

    trim = trimed_image("trim.png")
trim.write("trim_output.png")

    Trim expected.

    trim_output.png