Why is this an illusion at all?

And why can’t I see it correctly even when I know what it “really” should look like?

There’s no context there, but I assume the horizontal lines are actually parallel, right? I’m not sure I understand the “why is this an illusion at all?” question. It’s an illusion because your brain is telling you something not objectively true about the image.

Your brain is doing some pre-processing, based on its (usually accurate enough) assumptions about the real world. It is assuming that the figure is a distorted version of a more regular one, where the horizontal-ish lines are accurately horizontal in the same way the vertical lines are accurately vertical.

In the real world, if you had a grid of horizontal and vertical lines, connected to form one shape, and you tried to pull the horizontal bars out to form that pattern, you actually WOULD cause the vertical bars to deform so that they curved to the left. Therefore your brain is trying to suggest to your conscious mind that this is in fact the case.

The pre-processing is at a layer under the conscious one, which is why you can’t get to it or reprogram it consciously

ALL the lines are parallel (or perpendicular). So what’s causing it to not look like a grid?

I think I may be seeing it differently from Aspidistra. To me, the horizontal lines look tilted/tapered, sort of like in Donkey Kong.

I think the majority of the effect is caused by the little diamond-shaped structure.
It’s slightly tilted, but your brain is attempting to see it as perfectly horizontal and vertical. That makes the entire line of medium-blue shapes appear slanted.

The mechanism that creates the (very powerful) illusion is not clear to me either. The little “diamonds with whiskers at their corners” are given a slight tilt — possibly that what’s leading the brain astray.

If I had some time I’d start altering elements in a retouching program to see what’s critical to the illusion. The tiny “four square diamonds” that have alternating orientation may play a role… but right now the “diamonds with whiskers” are my prime suspect.

Wow. That is a pretty impressive illusion. My brain refuses to believe those are parallel. Looking at the left side and right side, I could see that it is so, but I had to import it into photoshop and trace the edges to completely prove it to myself. This is cool.

ETA: I have noticed that if I squint or just casually glance at the drawing such that the individual details are out of focus, I can see the parallel lines. But as soon as my brain registers what it’s looking it, it’s snaps into a Donkey Kong type of layout.

ETA2: Or if I change my viewing angle to the monitor. If I go around so I’m almost parallel to it, it stops being tapered and looks parallel.

I think the key is the horizontal-ish sausage-link shapes (dark blue with a white center). The whisker on the left is at a different height than the whisker on the right, and we interpret the line from one whisker to the other as being horizontal when it isn’t, so the thick lines (which don’t match that line) look angled.

Do we? Those lines look irregular to me.

But you could be right! I got nothin’.

How did the artist ever figure out how to do this? It’s not at all an Escher-type illusion.

ETA: I want this in a wallpaper!

I played around with it a bit in Photoshop. The key (to my eye, anyway) is the rows of black squarish shapes. The rest of it is either obfuscation to hide this fact or a very minimal enhancement to the illusion. You can get rid of everything except those black squares and their little corner projections and the lines still look distinctly tilted. Get rid of those black squares and the lines look horizontal, as they indeed are.

The squares are not tilted, BTW. It’s the asymmetry of those corner projections that does the trick.

I very quickly edited out this middle parts…I think it made it worse.
I’m really confused When I put a perfectly horizontal or vertical line on it, it shows that everything is straight and perpendicular, including the diamonds.
Excuse the bad photoshopping, I spent, literally, 2 minutes on this just to see for myself what would happen…

Nice to have it narrowed down. Any idea why this is happening? Something I find remarkable is that it is so stable. Other illusions flip back and forth depending on how you look at them, think about them, or move your eyes around the image. Not here!

Yup, you’re on to something. It’s the diamonds. They alternate orientation and they’re not black and white, they’re black and the same shade of light blue is the big light blue squares. That’s what’s making it look all angled.

Look what happens if I cover them up with a giant black splotch.

It is an impressive illusion! One way to prove the major lines really are straight and parallel is to squint. If you’re like me, when you squint 'til your eyes are almost shut, the image is blurred and all detail is lost. Then all you see are the clearly parallel horizontal blue bands and the clearly parallel vertical black bands.

Not quite ALL the lines are parallel. The thin dark blue lines inside the horizontal blue bands zig zag. That helps the illusion, but wolfpup is probably right that the small 2x2 black/white diamonds at each black square corner are more important for the illusion.

I suspect that all of the elements of the image are relevant. The composer of the image combined a bunch of other known illusions, that all reinforce each other. That’s why this one is so powerful, and seems so consistent.

Squinting made them parallel for me too.

Check out this link.

Based on it, I’d say it’s a combination of several things. They have an animation that blurs out different aspects one at a time and you can see the illusion become less intense with each one until it reaches parallel.

Interesting link, but I didn’t see the animation. I did wonder what was illusory about the “twisted braid” though.

To my eye, this is definitive about the dominant factor in the illusion:

Here, instead of wiping out the entire area between the rows, I explicitly just eliminated the black squarish diamond-like things and left absolutely everything else. Now the lines look parallel to me.

The other things may be very minor contributors, but it’s the black squares with the asymmetrical projections that overwhelmingly seem to create the illusion.

The “animation” in the Daily Mail article was unpersuasive as they just blurred pretty much the whole thing, obfuscating what was really contributing to the illusion.

If you simply hold a ruler to the screen, or a printed image, in the right place, it will come into perfect focus.