How fast does a field vary? Guess its effective spatial frequency feff by eye.
Checking the server…Sums of ten radial sines on the unit disk, each field in a frequency band of its own. The amplitude carries no hint.
feff of the whole disk as one partition, computed from exact derivatives: how fast the slope changes relative to the slope, in the units of a sine’s frequency.
feff decides how well the backbone reconstructs a partition, and more partitions lower it, which is why any field becomes easy once its partitions are small enough.