Accidental optical illusions

I have a fun little side-project to make a 3D model of the Milky Way using all-sky HI data. By measuring how fast the gas is moving and doing some trigonometry, it's possible to convert velocity into distance. The equations are a bit awkward, and if you get things a bit off, the end result looks very strange. They also have a limitation that they give a meaningless double solution for any point closer to the centre of the Galaxy than the Sun.

This meant there was quite a bit of trial and error involved until I got the correct result (more on that in a future post). To check I where things were going wrong, I had the code output the calculated galactic coordinates (latitude and longitude across the sky, measured from the galactic centre, as well as velocity along the line of sight), with the data set to zero inside the solar circle where the solutions would be garbage. Actually I'm pretty sure I got the position of the solar circle wrong, so this is just a complete mistake.

But it did produce a couple of fun little optical illusions :

Raw image here.

Both are quite similar. The colour in the grey circle looks like its varies, but it doesn't : it's completely uniform.

The effect is strongest with the left figure, which is velocity. The right side of the circle appears significantly brighter and the left significantly darker, a bit like looking at a crater in partial shadow (or, if you take the inverse perspective, a dome). Cover everything except the circle with your hands and you'll see this is entirely the result of your brain inventing stuff.

The figure on the right (galactic longitude) can be subtle at first, but once you see it, it's very hard indeed to make it stop. This time the right side of the circle appears darker and the left brighter, especially when you focus on the edges. I find that I can more-or-less control how strong this appears by concentrating on different parts of the circle, but sometimes it becomes so strong that I can barely make it stop even by covering the edges.

If we apply an animated mask to the regions outside the circles then things get even more fun (apologies for the small radial artifacts caused by gif compression) :

Raw image here.

It really is quite hard to graphically prove that the circles are always of constant colour. The only way to show it for sure is to download the images and an examine them in extreme close-up for yourself.