Pixelmator Pro is one of the many Mac apps for editing photos. It stands out for many features, notably its ability to work with Apple’s Photos app as an extension and retain editing history, so you can go back and make changes to your edits later; something not many other Mac apps offer currently. (I know that Raw Power offers this; I haven’t encountered any other apps that work this way.)
It’s most recent update includes a feature called MS Super Resolution, which allows you to upscale images and not get the jaggies. That’s what you often see when low-resolution images are upscaled, because apps generally just interpolate the pixels, creating, for example, four pixels where there is one in the original.
I tried this out on a few photos, and it’s quite impressive. Let me show an example of how this works. Here is a photo of Titus the Cat; the original is 5608 x 3739 px. A cat photo is a difficult one for a feature like this, because of the hair and whiskers.
(Note that all photos below have been downsized for display; click them to see them in full size. If not, you won’t really be able to see the differences.)
I downsampled the photo to 1000 px wide using Acorn. I’m guessing that it uses the built-in image framework in macOS.
I then upsampled the image, to its original resolution, with Acorn. Here is a close-up at 100% resolution of that upsampled image in Preview, showing lots of jaggies, especially around the whiskers.
And here’s a close up, around the same size, of the downspampled photo upsampled to the original resolution with Pixelmator Pro. You can see that the whiskers, and other lines, are straight.
And here’s that same photo zoomed out to fit the Preview window.
And here’s the Acorn upsample zoomed out.
Finally, here are two 100% images of a close up of Titus the Cat’s nose, mouth, and whiskers. Above, Pixelmator; below, Acorn.
What is obvious is that the Pixelmator witchcraft really is much better than what Acorn does. I’ve not tried with other photo editors, and would be interested to know if their upscaling comes close to what Pixelmator offers.