I don’t feel this is much of an illusion. I think you just can’t see the movement of gray on black. Or maybe this is what illusions are in a very simple form?
Play with the color of the pigeons. Slide it all the way to black, and it's no longer an illusion - the edges really do move in steps. Slide it all the way to white and the same effect appears in the inverse.
Somewhere in between, there are shades of gray where the smooth movement of the pigeons is visible. And there are shades of gray where your brain can't separate it from the underlying black motion. That's interesting, no?
That's what makes all optical illusions interesting - exploring the thresholds where our brain's perceptual machinery takes shortcuts.
I've noticed there's a tendency, especially among smart people, to be dismissive of optical illusions. 'I didn't fall for it' - because to a certain personality, it's important to feel like your mind can understand things and you can't be 'fooled'.
But good optical illusions aren't fooling you. You don't need to feel defensive about whether you were 'tricked'. They're hacks that exploit edge cases in your visual cortex, and cause your brain to be fed erroneous data. They're interesting and useful because they help you calibrate the instruments your brain uses to collect data!
On the other hand, with black-on-black, the idea that there is something representing a moving pigeon is a sort of illusion in itself, as you infer a boundary that you never see in full.
On the third hand, movement is an illusion in all cases: what's really happening is that static pixels are changing color and brightness (the pixels may jitter slightly in changing color, but that's not the motion we perceive.)
All these views seem to me to be reasonable, they are just different perspectives.
For me, just with the color slider, there was nearly always some sort of illusory motion. Transparent perception of the actual motion of the pigeons was either impossible or required precise conditions, which really affirms the power of this effect.
Let's say you recreate this effect by painting the pigeons onto a transparency sheet, and moving the sheet above a paper with a striped background. The pigeon shapes would appear to bend, when in fact they do not... so in physical form, it clearly is an illusion.
As a pixelated image, I dunno: our insistence on viewing groups of pixels as shapes is an illusion to start with.
The image is a grid of pixels. When a group of the pixels that the human viewer perceives as a worm appears to grow, we cannot claim that the pixels in the overlap actually belong to the background. It is equally valid to claim they belong to the worm, in which case... the worm actually has lengthened.
In the physical version, if the worm/pigeon and the grid are exactly the same color, we still consider them separate entities.
In the digital version, if a group of pixels appears to be part of some object, that's as good a reason as any to argue it is part of that object (though strictly speaking, it's a meaningless question since it's just a bunch of pixels).
One is an illusion, or the set of all illusions, and the other is the set of all images generated by pixels.
Not all images generated by pixels are an illusion. Otherwise you would say that all images generated by, say, acrylic paint, are an illusion because they're made of molecules.
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.
When your brain fudges the results of what you're seeing because of some hidden processing heuristic. If the bird were actually just colored black, then what we would see would be accurate. But its presented as if we should percieve continuous movement when we really shouldn't.
I’m just saying upon initial thought that it seemed too simple to be considered an “illusion” because it doesn’t rely on some mental trickery, just blindness. But I’m willing to accept that optical illusions may all rely on some form of blindness. Glad to spawn the conversation!
With the pigeons it seemed illusory, but with just worms you can tell that's exactly whats happening. Dunno if it is the single-row shape or just less action on the screen allowing you to focus on one cell
Somewhere in between, there are shades of gray where the smooth movement of the pigeons is visible. And there are shades of gray where your brain can't separate it from the underlying black motion. That's interesting, no?
That's what makes all optical illusions interesting - exploring the thresholds where our brain's perceptual machinery takes shortcuts.
I've noticed there's a tendency, especially among smart people, to be dismissive of optical illusions. 'I didn't fall for it' - because to a certain personality, it's important to feel like your mind can understand things and you can't be 'fooled'.
But good optical illusions aren't fooling you. You don't need to feel defensive about whether you were 'tricked'. They're hacks that exploit edge cases in your visual cortex, and cause your brain to be fed erroneous data. They're interesting and useful because they help you calibrate the instruments your brain uses to collect data!