I’ve learned a whole lot more about ImageMagick commands last week than I ever really wanted to know. The problem was that our uploaded images were having content cropped off the top, bottom, and sides.  Like many folks in the Rails world, we pass our attachments through Paperclip to handle all of the nitty gritty resizing operations.

I was interested in understanding how we could prevent our content from being cropped off when I came across an interesting idiom in the geometry settings:

has_croppable_attachment :image,
styles: {
:'630x315' => { geometry: "630x315#", format: :jpg },
}


Well take a look at that. There is a “#” symbol suffixed to my image geometry. I went to ImageMagick to lookup what this flag meant. Spoiler alert: It doesn’t exist there. After some digging around, I discovered that this idiom is provided by the Paperclip gem, and translates to the following convert command:

convert '/path/to/source.jpg' -resize "630x" -crop "630x315+0+0" '/path/to/output.jpg'

You can see the resize + crop combination of commands being built by Paperclip according to their documentation:

Paperclip also adds the “#” option (e.g. “50×50#”), which will resize the image to fit maximally inside the dimensions and then crop the rest off (weighted at the center).

Well, that is no good! If an image is not a 2:1 aspect ratio as per my dimensions 630×315 (or whatever aspect ratio you have) YOU WILL LOSE CONTENT! Time to rethink this…

 

Instead of resizing maximally (so that an image is a minimum of 630 width AND a minimum of 315 height) lets resize minimally (so that an image is a maximum of 630 width OR 315 height). The aspect ratio is preserved in both scenarios.

We want to resize while preserving aspect ratio , but we also need to make our canvas 630×315. The canvas dimensions are referred to as the extent command. When we do this, we will likely have space on the top and bottom, or sides we need to fill to have exactly these dimensions. What you fill this background with can be a color (in my case white). We also likely want to center the minimally resized image on this canvas. You can pass these convert options into Paperclip like so:

convert_options: {
:'630x315' => " -background white -gravity center -extent 630x315",
}

The resulting command will look something like this:

convert '/path/to/source.jpg' -resize "630x315" -background white -gravity center -extent 630x315 '/path/to/output.jpg'

Notice that our lossy crop flag has been replaced with a nicer extent flag. We can see the results this has on a similarly sized image:

 

As a final note, this works for both images larger and smaller than the target outcome dimensions. Looking at the ImageMagick documentation for flags can be helpful, but daunting as the real power lies in chaining multiple flags for a desired effect.

With a little effort I was able to get what is (in my opinion) a better image resize with just a few custom flags.