QuadCropper

Robert M. Lefkowitz
3 min readDec 7, 2023

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iocn for QuadCropper app

Recently I started writing book reports at https://ockham.online/ on books that cover the “intersection of technology and social” (inspired by Monktoberfest). As part of the design, I include an image of the book spine at the top of the article (and in the article preview), and an image of the book cover at the end of the article. I might have copied the cover image from Amazon (since, as an Amazon Associate, I could then link back to Amazon). But Amazon does not have images of book spines — so I would need to take a photo of the spine myself. And as long as I’m at it, I might as well just snap a picture of the cover as well. Plus, some of the copies of the books I review have a cover which does not match the Amazon image (different edition, I guess) — and I want to be faithful to my library.

When one snaps a picture of a book cover or spine, the camera is never quite at right angles to the book. The image contains a picture of a rectangular book — but it is skewed by perspective to be non-rectangular. The Photos.app cropping tool (and most other cropping tools) provide a rectangle for cropping — but the image of the cover (or spine) is not rectangular in 2D.

Now, there are apps that will take a sheet of paper (document) and crop and de-skew a photo of it. But book covers (and especially spines) have interesting colors and aspect ratios and the apps that I tried didn’t work right. And if they didn’t work automatically, there was no manual fallback.

So I wrote some software to scratch my own itch. It’s called QuadCropper and is available as a free app in the iOS and Mac App Stores. The only reason it is an app is to host a Photo Extension Editor. Once the app is installed, one never need to open it. I just take my book photos, and go into the Photos.app editing tools (selecting the QuadCropper extension) and crop the photo. The QuadCropper extension provides four corners which can be moved independently. Then it de-skews the resulting quadrilateral and produces a rectangular picture — which I upload for the book report.

Turns out the same extension works nicely on shots of slides I’ve taken at conferences. One is never sitting directly in front of the screen — so the slides are always distorted by perspective. QuadCropper fixes those.

My other use case is sticking the phone inside appliances to capture the sticker that has serial number and model number information. I quad-crop those to have a decent album I can refer to when I’m looking for replacement parts.

Let me know if you have another use case. Also let me know if there is some way I can make it more useful. I have one such idea rattling around.

I publish on Medium (obviously, since you are reading this there) and their editor won’t let me select very thin and wide images as the default image for an article. So I’m considering adding a feature which adds some transparent space above and below an image so that it works with Medium. Alternatively I might just keep QuadCropper single-function and write another Photos Extension which “fattens” thin images so they work with Medium.

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