[heynemann]

It can be easily implemented as a filter[1][2]. If you need to use it via command-line you can save the current buffer as a tempfile[3], call your CLI with the tempfile path and read the tempfile to use as the new buffer.

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...

[3] https://docs.python.org/2/library/tempfile.html