sips: Scriptable image processing system#

I wanted to convert some .webp images to .png on my Mac. I asked ChatGPT:

On MacOS use CLI to convert webp images to PNG

And it told me about sips:

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sips -s format png image.webp --out image.png

Or to run it against all PNGs in a folder:

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for file in *.webp; do sips -s format png "$file" --out "${file%.*}.png"; done

I had never heard of sips before - but apparently it’s been a default command on macOS for a very long time.

It stands for “scriptable image processing system”. man sips starts like this:

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NAME
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     sips – scriptable image processing system.
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SYNOPSIS
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     sips [image-functions] imagefile ...
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     sips [profile-functions] profile ...
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DESCRIPTION
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     This tool is used to query or modify raster image files and ColorSync ICC
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     profiles.  Its functionality can also be used through the "Image Events"
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     AppleScript suite.  It also supports executing JavaScript to either
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     modify or generate images.

It can run JavaScript!

I asked ChatGPT for an example, but it halucinated something that didn’t actually work.

After some searching I found manicmaniac/sips-js-api which is the only place online I could see that documented how to use the sips --js option. Here’s their example - save this in smile.js:

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const canvas = new Canvas(150, 150)
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canvas.beginPath()
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canvas.arc(75, 75, 50, 0, Math.PI * 2, true)
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canvas.moveTo(110, 75)
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canvas.arc(75, 75, 35, 0, Math.PI, false)
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canvas.moveTo(65, 65)
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canvas.arc(60, 65, 5, 0, Math.PI * 2, true)
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canvas.moveTo(95, 65)
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canvas.arc(90, 65, 5, 0, Math.PI * 2, true)
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canvas.stroke()
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const output = new Output(canvas, sips.outputPath)
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output.addToQueue()

Then run:

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sips -j smile.js -o smile.png

This produces an alpha-transparent PNG of a smiling face.

A smiling face

The only other relevant documentation I could find was this section of the man sips page:

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JavaScript
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     HTML Canvas objects can be created and used to create a 2D drawing context.  The commands for drawing into the context are well documented elsewhere.  This
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     section will describe the sips global object and other interesting classes.
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     Global variable (sips) properties
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     images
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           Valid images passed as arguments converted into an array of Image objects
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     arguments
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           Arguments passed into the program as an array of strings
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     size  Recommended size for output. Setting the crop or resample flags will set this value.
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     longestEdge
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           If specified, the value of the -Z/--resampleHeightWidthMax option. [default: 0]
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     outputPath
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           Output directory [default: current directory]
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     Image Object
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     name  Name of image
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     size  Size of image (pixels)
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     properties
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           Image properties
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     getProperty(name)
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           Return the image property for name, if any.
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     sizeToFitLongestEdge(length)
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           Return the size that will contain the image with the longest edge set to length. Maintains aspect ratio.
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     Output Object
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     new Output(context, name[, type])
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           Output the context to disk with name and optional type (extension or UTI).
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     addToQueue()
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           Adds the output to the queue to be written to disk.
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     Functions
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     print(str)
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           Output to standard output. Equivalent to console.log(str).