GRASS tool highlight: i.segment does object-based image segmentation using region growing with merging or mean shift. Supports hierarchical segmentation: run at threshold=0.02, feed the output as seeds into the next pass at 0.08, 0.20, 0.40. As the threshold increases, more spectrally similar neighbors merge into fewer, larger segments.
By Eric Momsen and Markus Metz.

Docs: https://grass.osgeo.org/grass-stable/manuals/i.segment.html
8.5: https://grass.osgeo.org/grass85/manuals/i.segment.html