I prefer the PHP version of ImageMagick as PHP is a better scripting tool than 'cli'. The manual is a bit hard to follow as things are essentially undocumented. However, if you want to carefully prepare a batch of images, going to PHP is taking it to the next level.
Particularly interesting are features such as being able to save images in different colour spaces, e.g. 4:2:0, going lo-res on the chroma for a lower ultimate file size in jpg.
If I had a lifetime spare I would complete the PHP Imagick documentation and write a version of Fred's scripts in PHP, so they can be chained together in a nifty object oriented way...
The GitHub wiki for VIPS has a page [1] very relevant to GP's question. It lists two dozen alternatives to VIPS (both command line tools and libraries) and compares how they performed on a load-crop-scale-apply convolution-save task. The source code for each implementation of the task is right there on the page.
I second that.
G'MIC has an impressive bunch of built-in commands to manipulate images, see e.g. the examples shown in the reference documentation (click the first magnifying glass, then use right arrow on the keyboard):
And as the commands are built-in, no needs for external scripts, those effects are available from any interface using the `libgmic` API (including C/C++ programs).
