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Firstly, we may use a three-dimensional extension of the hedgehog plot, that is, we place arrows pointing towards the direction of the flow. However, the perception of depth is worse in case of separate arrows than in case of surfaces, and the image may easily be overloaded.
A similar technique is the use of streamlets or streamlines. However, using lighting, transparency and a good selection of starting points it is possible to improve depth perception. It is primarily the lighting of the streamlines that makes this technique applicable in three dimensions. However, a large number of streamlets are usually needed in order to follow flow around divergence nodes and to give a surface-like impression necessary for a good perception of depth. This means that modelling streamlines with thin tubes made up of polygons does not allow for rendering fast enough. Therefore, streamlets should be drawn as lines, but unfortunately, graphic libraries do not directly support shading lines.