On 2D plane positive u values are displayed by the red color
(negative "tail" is not shown). v values are displayed by the green
color. This script uses simple explicit algorithm and
1024×1024 float32 textures with the Neumann (CLAMP_TO_EDGE)
boundary conditions. It makes 2 it time steps per frame.
For t > 200 (after spiral formation)
the script begins to plot the path of the spiral wave tip (the white curve).
The center of rotation path is the yellow curve.
It is determined as intersection of ∂tu = 0
and ∂tv = 0 curves (i.e. the point where spiral
is constant). Set b = 0.3 to get a plain spiral.