Lambertus Hesselink and Thierry Delmarcelle visualize a 2D symmetric
second-order tensor field by extracting its topological sceleton.
The topological analysis of a tensor field is build from the concept
of degenerated points, which play the role of critical points in
vector fields (see image 4).

A point is degenerated if the tensor field in the point has
two identical eigenvalues.
In degenerated points hyperstreamlines
(image 1) do cross.
The authors use the tensor gradient in the degenerated points to
classify the points into trisectors and widgets and connect
them by hyperstreamlines.

Above image visualizes a 2D stress tensor in a periodic flow
past a cylinder.
The texture represents the most compressive eigenvector and color
encodes the magnitude of the compressive force (red most, blue least),
The topological sceleton of the tensor field is superimposed, where
**W** symbolises a wedge and **T** a trisector.
And degenerated points (\ie trisectors and widgets) the viscous stresses
vanish and both eigenvalues are equal to the pressure;
degenerated points in above image are hence points of pure pressure.