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I genuinely cannot comprehend the first thing
The first seems kinds like a normal trick because of the angle. The 2nd blew my mind...
It's about the radius, outside circle is bigger.
No, it's because the short sides are slanted. Both arcs are identical, but the bottom shape is offset to the right. but because of the slanted sides, they trick your brain into thinking they are aligned right above each other. But they are aligned at the angle of the slant (thus offset)
The curve just makes it harder to understand what the illusion is. this illusion also works with straight sides, but then it's easier to see the illusion.
Source, tried this in Illustrator myself because it was breaking my brain.
No, the top arc and bottom arcs are the same, but because the outside circle is bigger it tricks your brain, you can do this with any two circles
Sorry but you're wrong. What surprised me: both the bottom and top radius of the shape are identical, or else they wouldn't fit on top of each other. So there is no "outside bigger circle". At least if I understand what you mean by "bigger circle" correctly.
It's the slant of the short sides, in combination with the offset that aligns with the angle of the slant that is causing me to think that the top shape is smaller then the bottom shape. and here's my proof, just look at the second figure on the left, curves but straight edges, and the illusion fails: https://imgur.com/a/lcWJb5P
(and yes all curvatures are identical in my image, i can export an SVG of this if you want to play around with it yourself)
Top image literally shows the bigger circle, this is two parts of two concentric circles.
Edit: The radi aren't the same, they literally can't be, they have the same length in the circumference (out and in) which is what gives the illusion when you align the edge.
No it isn't: I made this, and didn't use concentric circles :) I show that at the bottom how I made the shape, it's not 2 concentric circles, it's 2 identical circles/ovals (doesn't matter, as long as they are identical), but one circle is shifted downwards, and then you cut out the shape from that difference.
So there is no bigger circle. the bottom radius, and the top radius of the shape are identical. They have to be, or else they don't fit on top of each other, you would get a gap, or they would overlap.
I have made this too, the top and bottom wouldn't fit on top of each other if you use two equal circles.
Well then show me, i've shown proof that they fit this way. And i can prove they won't fit if you try to make the shape out of concentric circles.
Do keep in mind, that dark red and light red parts are identical, they are duplicates of each other, so they can fit on top of each other just like the video shows. (or else that illusion doesn't work).
What I think you did was create 2 different pieces (with concentric circles) but then they wouldn't fit on top of each other.
Two concentric circles is the only way two circles fit on top of each other, otherwise they wouldn't be the same size/area.
So, how do you explain my image then?
No concentric circles there, and they stack without a gap, and they are identical and fit on top of each other.
Thays exactly this ilusion. You duplicate a circle section and place it above the first, then you aligne its Start so the bottom matches with the top of the first and adjust to fit.
Same circle section with the same length, but One is stretched to match the curved behaviour of the other