pixel accuracy, mean accuracy, mean I/U, f.w. I/U

[Jeremy Rutman]

I am looking for the meaning of these terms - pixel accuracy, mean accuracy, mean I/U, f.w. I/U in semantic segmentation.
I imagine that

pixel accuracy = Ncorrectly_classified / N
mean I/U = Sum(intersection/union for class i)/ n_classes

but the others elude me

[Evan Shelhamer]

For a summary of the metrics see https://arxiv.org/abs/1605.06211, section 5. In essence, the intersection-over-union metrics (like mean IU) are preferred since they control for the total number of pixels for a class. Accuracy can be misleading; for example the majority of pixels might be background so that a method predicting all background will still have seemingly good performance. As a ratio IU is less susceptible to this.

Evan Shelhamer

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