Optical Illusion in Visualization

I was recently working on a few basic tests of MHD codes and one of the tests I did was the 2D rotor problem. In this test a circular region of high density initially starts out rotating. There is a magnetic field pointed in the +x direction. As the simulation progresses the magnetic field distorts the spinning high density region and causes magnetic waves to propagate into the surrounding low density region. Thus this test is a good one to show the basic structure of magnetic waves.

This test was done using Athena and the visualization was done in Paraview.  The 2D surface is colored according to the log10 of the density and the vector arrows show velocity and are colored according to the magnitude of the magnetic field.



One thing that came up with this visualization is that at the very beginning there is an optical illusion. Even though the gas is initially spinning in a counter-clock-wise direction because it is slowing down and begins to change direction, it appears as if it is spinning in the opposite direction. Because of the way the velocity vectors change length and direction at the beginning it gives the impression that the gas is spinning in the clock-wise direction. This is an excellent instance of an optical illusion in a visualization. Sometimes when scientists are trying to visualize their data they run the risk of having it create an optical illusion that will mess up their interpretation of the data. The way to fix this is to use a different visualizing method, in this case I could use particle tracers or stream line tracers to fix the problem.

But this is just one of the hazards of trying to use visualization to tell us something about our data. Still, visualization is better than just about any other method in giving a complete picture of what it going on. We just have to be mindful that sometimes our minds will play tricks on us.