Principle : the pixel values along the field lines are strongly correlated, but orthogonally to them almost uncorrelated. As a result, the field lines stand out visually from the background and become visible.
Intuition:
* throwing paint in a river
* visualisation of magnetic field lines using randomly distributed iron filings
* "emulates what happens when a rectangular area of massless fine sand is blown by strong wind"
Intuitively, the flow of a vector field in some domain is visualized by adding a static random pattern of dark and light paint sources. As the flow passes by the sources, each parcel of fluid picks up some of the source color, averaging it with the color it has already acquired in a manner similar to throwing paint in a river. The result is a random striped texture where points along the same streamline tend to have similar color.