What is edge feature in snake example in PyStruct EdgeGraphCRF demo?

[chen ma]

Hello :

        I am a PhD student research on structure learning, I find your pystruct project.

There is an snake inference demo example in GraphCRF that http://pystruct.github.io/auto_examples/plot_snakes.html#sphx-glr-auto-examples-plot-snakes-py

What is edge feature ? I can't understand.

Please help me, my research is stuck , your pystruct is crucial for me.