A similar app I worked on, which focuses on extensibility: http://magickly.afeld.me/
For example, we have a ruby-based library for converting images for different types of colour-blindness[0]. It's available from ruby, or via the command line. How could we use it as a filter in thumbor?
It's even simpler if your CLI can take a stdin argument, since you can just pass the buffer as a stdin parameter.
Thumbor's filter infrastructure allows you to change the image incrementally.
[1] https://github.com/thumbor/thumbor/wiki/Brightness
[2] https://github.com/thumbor/thumbor/blob/master/thumbor/filte...