<p>I can't zoom in on the picture, but what I <em>see</em> is bleeding from one color into the next along the diagonals (most evident in the green to blue and purple to red transitions). Are you saying that they are not there when you zoom in? I would think that because there is a gradient in the color transition, the gradient has to be emphasized in an angled meeting, more pronounced the sharper the angle (e.g.if it were a triangle, the angle would be 120° as opposed to 90°). It's not an optical illusion.</p>

<p>In short, you see two transecting lines on the diagonal because they are strongly suggested. The mind sees this accurately and reads the suggestion.</p>

<p>It is kind of like asking, why, in the following picture, do I read all the dotted lines as lines, where only two of them are actually solid lines?</p>

<p><img src="https://i.sstatic.net/5q3c0.png" alt="enter image description here"></p>

<p>Or, if you will, why do I see curves here?</p>

<p><img src="https://i.sstatic.net/D6pUVm.jpg" alt="enter image description here"></p>

<p>Because the color differences in the offset squares are really there, suggesting a familiar curve, though in this question, it would be fair to ask <em>Why do the curves appear smooth?</em> In fact, the curves are not smooth.</p>

<p>If you do want to see interesting optical phenomenons, </p>

<ul>
<li><p>stare at the center of your image for 30 seconds, then scroll down to the blank area next to my diagrams. You will see squares of reciprocal colors not in your original (like a bright yellow.) Also interesting is that as the image fades, blinking will "refresh" it.</p></li>
<li><p>after staring directly at the center for a minute or two, switch the focus <em>very slightly</em> to one side then back; you'll be pleasantly surprised by bursts of vibrant color parallel to each other each time you change.</p></li>
</ul>

<p>Edited to add: This may be an optical illusion, but if it is, I'm unsure of what type it is. Looking at the zoomed image, clearly the impression is that there is some bleeding, but that's not the reality. Sorry I can't help more on this one. But it does get much more intriguing!</p>

CC BY-SA 3.0
2015-07-12T02:59:51.777Z
Hi, thanks! I provided a zoom in in my question as suggested.
jeromej
CC BY-SA 3.0
2015-07-12T03:59:12.607Z
+1 for the answer. Please feel free to add some stuff from my chat if you think it fits your answer :)
AliceD
CC BY-SA 3.0
2015-07-12T06:27:37.393Z
@AliceD (Your chat? I know the reply wasn't directed to me but it left me curious!)
jeromej
CC BY-SA 3.0
2015-07-13T12:09:59.650Z
@JeromeJ - I was chatting with anongoodnurse about the bleeding of green in blue as well as the parallel lines within the colored bars. The parallel lines create a physical pattern and perpendicular edges in the corners. these two factor are enough to create an edge. It is not an illusion in my opinion.
AliceD
CC BY-SA 3.0
2015-07-11T19:54:13.153Z
2015-07-12T03:05:51.610Z
2015-07-12T03:05:51.610Z
anongoodnurse
<p>I got this picture I generated myself and I don't understand how come we can see two diagonal lines when, I think, there is no actual lines.</p>

<p><img src="https://i.sstatic.net/B8Bw6.png" alt="Squarish rainbow"></p>

<p><strong>Zooming in:</strong></p>

<p><img src="https://i.sstatic.net/fVJ6k.png" alt="Zooming in"></p>

<p><strong>Center:</strong> <em>Just another 2px wide square…</em></p>

<p><img src="https://i.sstatic.net/4uyOy.png" alt="Center"></p>

<p>After thinking about it, I think it might come from the fact that, if you zoom really closely, it is only a succession of square, each time 2px smaller and of a different color.</p>

<p>Although there is no actual line directly built, we can see the corners of each square clearly and, together, our brain understand this as two diagonal lines. … I might just have answered my own question but I would appreciate your opinion/confirmation on this.</p>

<p><strong>Related:</strong> <a href="https://meta.stackexchange.com/q/260474/210483">My question on meta-SE about where to ask this question</a>.</p>

CC BY-SA 3.0
2015-07-12T03:07:24.037Z
That is so cool! I'm not sure what's going on any more...
anongoodnurse
CC BY-SA 3.0
2015-07-12T15:09:55.757Z
What is supposed to be in the center? Also what format are the pictures in and is there any compression?
StrongBad
CC BY-SA 3.0
2015-07-12T17:05:00.093Z
@StrongBad Added a picture of the center, just a 2px wide square. I generated it using `PhotoImage` in `tkinter` in Python, assigned each pixel one by one, creating the bigger square first then smaller and smaller (not very efficient, but pretty to watch it being built) then saved it as a png with `PhotoImage.write`, I don't know the compression or anything else, I left it by default.
jeromej
CC BY-SA 3.0
2015-07-12T17:25:58.587Z
My guess is it is a rendering artifact. The corner pixels have a different number of same color pixel neighbors from edge pixels, so they are in fact a different color when draw on the screen.
StrongBad
CC BY-SA 3.0
2015-07-13T04:24:19.957Z
@StrongBad I checked, it isn't. When I zoomed on the rendered picture (from a screenshot for instance) it looks exactly the same as when zoomed on the actual picture. Nice guess though, I didn't think about that.
jeromej
<perception><vision><optical-illusion>
How come we see a line when there isn't one in this picture?
jeromej
# How come we see a line when there isn't one in this picture?

(Asked by: username_0 on 2015-07-11)

I got this picture I generated myself and I don't understand how come we can see two diagonal lines when, I think, there is no actual lines.

![Squarish rainbow](https://i.sstatic.net/B8Bw6.png)

**Zooming in:**

![Zooming in](https://i.sstatic.net/fVJ6k.png)

**Center:** *Just another 2px wide square…*

![Center](https://i.sstatic.net/4uyOy.png)

After thinking about it, I think it might come from the fact that, if you zoom really closely, it is only a succession of square, each time 2px smaller and of a different color.

Although there is no actual line directly built, we can see the corners of each square clearly and, together, our brain understand this as two diagonal lines. … I might just have answered my own question but I would appreciate your opinion/confirmation on this.

**Related:** [My question on meta-SE about where to ask this question](https://meta.stackexchange.com/q/260474/210483).

**Tags:** perception, vision, optical-illusion

> **Comments:**
> **username_1**: That is so cool! I'm not sure what's going on any more...  
> **username_2**: What is supposed to be in the center? Also what format are the pictures in and is there any compression?  
> **username_0**: @username_2 Added a picture of the center, just a 2px wide square. I generated it using `PhotoImage` in `tkinter` in Python, assigned each pixel one by one, creating the bigger square first then smaller and smaller (not very efficient, but pretty to watch it being built) then saved it as a png with `PhotoImage.write`, I don't know the compression or anything else, I left it by default.  
> **username_2**: My guess is it is a rendering artifact. The corner pixels have a different number of same color pixel neighbors from edge pixels, so they are in fact a different color when draw on the screen.  
> **username_0**: @username_2 I checked, it isn't. When I zoomed on the rendered picture (from a screenshot for instance) it looks exactly the same as when zoomed on the actual picture. Nice guess though, I didn't think about that.  

---

## 1 answers

### Answer by username_1 on 2015-07-11. Score: 3

I can't zoom in on the picture, but what I *see* is bleeding from one color into the next along the diagonals (most evident in the green to blue and purple to red transitions). Are you saying that they are not there when you zoom in? I would think that because there is a gradient in the color transition, the gradient has to be emphasized in an angled meeting, more pronounced the sharper the angle (e.g.if it were a triangle, the angle would be 120° as opposed to 90°). It's not an optical illusion.

In short, you see two transecting lines on the diagonal because they are strongly suggested. The mind sees this accurately and reads the suggestion.

It is kind of like asking, why, in the following picture, do I read all the dotted lines as lines, where only two of them are actually solid lines?

![enter image description here](https://i.sstatic.net/5q3c0.png)

Or, if you will, why do I see curves here?

![enter image description here](https://i.sstatic.net/D6pUVm.jpg)

Because the color differences in the offset squares are really there, suggesting a familiar curve, though in this question, it would be fair to ask *Why do the curves appear smooth?* In fact, the curves are not smooth.

If you do want to see interesting optical phenomenons,

* stare at the center of your image for 30 seconds, then scroll down to the blank area next to my diagrams. You will see squares of reciprocal colors not in your original (like a bright yellow.) Also interesting is that as the image fades, blinking will "refresh" it.
* after staring directly at the center for a minute or two, switch the focus *very slightly* to one side then back; you'll be pleasantly surprised by bursts of vibrant color parallel to each other each time you change.

Edited to add: This may be an optical illusion, but if it is, I'm unsure of what type it is. Looking at the zoomed image, clearly the impression is that there is some bleeding, but that's not the reality. Sorry I can't help more on this one. But it does get much more intriguing!

> **Comments:**
> **username_0**: Hi, thanks! I provided a zoom in in my question as suggested.  
> **username_3**: +1 for the answer. Please feel free to add some stuff from my chat if you think it fits your answer :)  
> **username_0**: @username_3 (Your chat? I know the reply wasn't directed to me but it left me curious!)  
> **username_3**: @JeromeJ - I was chatting with username_1 about the bleeding of green in blue as well as the parallel lines within the colored bars. The parallel lines create a physical pattern and perpendicular edges in the corners. these two factor are enough to create an edge. It is not an illusion in my opinion.  

---

