“Archimedean spiral is the base of Spiral moving fish. The equation of Archimedean spiral is r = a + b * theta. If you just increase theta with the same amount, the distance between each segment of the moving fish would also be increased. In order to equally spaced each segment, I need to find out all angles between each segment. That’s what the find_parent_angles module does in the .scad file. For more precisely controlling models between each segment, I also divide all parent angles found by the find_parent_angles module into several sub angles which may be controlled by the children_per_step parameter.”