There is room for disagreement in some cases. “Look how the black text becomes invisible when it goes in front of a black background” is not what I would call an optical illusion, for instance.
I would describe an illusion as something where my (say) visual cortex is lying to me. This results in an image looking very odd: I _know_ that one thing is happening, but it looks like something else is happening.
Black text going in front of a black background doesn't have that effect. My knowledge of what I'm seeing agrees with what I see. My knowledge is "there is black text in front of that black text", and my visual cortex says "sure, I can't see it but that's perfectly plausible". This pigeon video does have that effect. My knowledge says that the pigeon image is moving smoothly, but I can clearly see its head bobbing; there's a disagreement.
Do other people define "illusion" differently? Or perceive something differently?
The color stepping feet version[1] I think definitely qualifies. The color contrast is high enough that if you focus on the leading or trailing edge of the "feet" it's trivial to see its moving the whole time, but if you look at the entire thing as a whole, the "stepping" effect appears.
Black text going in front of a black background doesn't have that effect. My knowledge of what I'm seeing agrees with what I see. My knowledge is "there is black text in front of that black text", and my visual cortex says "sure, I can't see it but that's perfectly plausible". This pigeon video does have that effect. My knowledge says that the pigeon image is moving smoothly, but I can clearly see its head bobbing; there's a disagreement.
Do other people define "illusion" differently? Or perceive something differently?