If you need to process huge images, GraphicsMagick does some things that ImageMagick can't do. A few years ago when I was making poster sized art prints, I would routinely process 60k^2 pixel (4 gigapixel) images. Just a resize with a Gaussian filter would thrash and never finish overnight in ImageMagick, and only took a minute or two in GraphicsMagick.

I'm not in love with the CLI for either, but GM's is better and I like that it's all collected under one roof rather than a whole bunch of separate commands.

I do wish the projects could merge back into one and have the best of both. I also wish they were better at color handling, resizing needs to be done at a higher bit depth, better support for converting between modern color spaces, etc.

I love sips on my mac because it's fast, but I'm not sure if anything's a better cross-platform choice for basic batch operations on images, or for the backend of a web service, than GraphicsMagick.

I'm curious, as I want to make some poster-sized art prints soon:

1) What resolution were the source files?

2) What upscaling options did you use?

3) What multiple of the original file size did you go up to? Were you limited by the quality of the upscaling, or did you just decide on the size of print and dpi, and multiply?

Oh, I was downscaling, mainly for filtering/anti-aliasing. My source files were the 60k^2 images, and the output was more like 20k^2... I shoot for at least 300 dpi and I was printing 60-inch prints.

I had enough trouble downscaling such large images that I ended up writing my own downscaling in my renderer so that I wouldn't have to use GraphicsMagick or anything else, but of everything I ever tried, GraphicsMagick was the best at handling enormous images.

FWIW, I've never been happy with any upscaling. If you do have the option to render at your target res and not use upscaling, that's ideal. If you must upscale, it's hard to find anything that can do more that 2-3x without noticeable artifacts or visible blurriness.