From Ohm

The gradient magnitude technique modulates the emission and absorption coefficients with the absolute value of the gradient vector (gradient magnitude).

Occuring effect: If the volume function is constant, the gradient is zero, so that the volume is rendered transparently. The gradient magnitude technique only shows interesting parts of the volume, that is the material boundaries between materials with different scalar values.

• Automatic iso surface extraction at material boundaries.
• No manual tweaking of the TF, a linear one already does the job.
• No explicit extraction of geometry required (compared to iso surfacing).