Copyright (c) 1991 Institute of Electrical and Electronics Engineers. Reprinted from Visualization '91, The Stream Polygon: A Technique for 3D Vector Field Visualization, pp. 126-132.

Schroeder, Volpe, and Lorensen developed a stream polygon to visualize tensor information in a vector field. The stream polygon is a regular n-sided polygon oriented normal to a local 3D vector in a given point. The local deformation of the vector field in that point is computed by a first order Taylor's series expansion and is given as the sum of a local strain tensor and a local deformation tensor. The stream polygon is deformed by either the whole deformation tensor, just a component of it, or an additional derived tensor (eg. the vorticity tensor for a velocity field).
The above picture shows a streamtube visualizing the air flow in a ventilated room. The streamtube is constructed by computing in every point of a streamline a streampolygon. The tube is coloured with room pressure and the radius is varied such that it represents a constant mass flow. The streamtube is therfore thin in areas of fast flow and thick where the flow is slow.