A blue dot moves around, following a diamond outline. Really? To me it looks like the part is bent outwards a the tips. No problem, use the
Compensation slider. When you’re satisfied that the dot travels straight lines (for me about ¾ up), click the
Diamond check box. Oh my, no straight paths at all!
I do not really understand the mechanism of angle illusions…
This here is a kinetic version of the “Orbison Illusion” (Orbison, 1939), from the class of angle illusions like, e.g., Hering, Poggendorff, Poggendorff Arch, Zöllner etc. illusions. This kinetic version was recently discovered by Matsunaga, Ito & Kanematsu (2023). Rendered via a moving dot, the illusion is markedly stronger than statically (which you can view by switching dot off and diamond on). Matsunaga et al. have an interesting explanation for the illusion enhancement in their paper.