Modify images in Swift using ImageMagick

When you are programming for iOS, and you need to modify images, you can use the UIKit framework. If you are working on a macOS scripting program, you do not have access to it.

You can use third party libraries, like jpeg and png, or you can use an external program, like ImageMagick.

We will use ImageMagick, first with Process, and then with ShellOut.

Setup your machine with ImageMagick

ImageMagick is a free software used to modify, resize, and convert image in many different formats. You can download it directly from the official website, but the preferred installation method is using Homebrew:

brew install imagemagick

which will install version 7. You can install version 6 if you experience problems. Version 6 is still maintained.

Together with magick, ImageMagick installs other command line tools. After the installation, in terminal run

which magick

to output the location of the ImageMagick binary, which is probably /usr/local/bin/magick. You will need it later.

Now we can resize an image with

magick mogrify -resize 50% path/to/image.jpg

Run the command line tool from Swift using Process

To run the command line tools from Swift, we will use Process, a class available in the Foundation framework.

let task = Process()

task.executableURL = URL(fileURLWithPath: "/usr/local/bin/magick")

task.arguments = [
    "mogrify",
    "-resize",
    "50%",
    "path/to/image.jpg"
]

try task.run()
task.waitUntilExit()

let status = task.terminationStatus

if status == 0 {
    fputs("Resizing succeeded.\n", stdout)
} else {
    fputs("Resizing failed.\n", stdout)
}

You can see how this is all implemented in version 0.1.0 of a plugin for Publish, a static site generator for Swift developers.

Run the command line tool from Swift using ShellOut

Instead of making use of Process directly, we can take advantage of existing packages, like ShellOut, which uses Process under the hood.

After adding ShellOut as a dependency, you can replicate the script above with

let arguments = [
    "mogrify",
    "-resize",
    "50%",
    "path/to/image.jpg"
]

do {
    try shellOut(to: "/usr/local/bin/magick \(arguments.joined(separator: " "))")
} catch {
    let error = error as! ShellOutError
    print(error.message)
    print(error.output)
}

This is implemented in version 0.2.0 of the plugin.