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I've recently found the following image:

enter image description here

I can easily see the "17" in the image if I'm scrolling and (much less easy) when I move my eyes and don't look directly on it. Why is that the case?

In case you don't see it:

enter image description here

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    $\begingroup$ As an aside, I can see the 17 clearly when I (a) zoom out to make the image smaller, or (b) move my eyes away from the image. $\endgroup$ – Jeromy Anglim Nov 28 '16 at 5:30
  • $\begingroup$ Intersting! When I step away from my monitor, I can see it MUCH clearer! $\endgroup$ – Martin Thoma Nov 28 '16 at 6:14
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    $\begingroup$ In this case I suppose it is simply because the stripes are extremely high contrast which simply kicks out every other information from picture. So if you apply dithering (or move image) or defocus the eye, you reduce this contrast, and the number is better seen. $\endgroup$ – Mikhail V Nov 28 '16 at 8:59
  • $\begingroup$ @Mikhail: so if this is true, making the black stripes white avoid make the 17 clearly visible, right? $\endgroup$ – Martin Thoma Nov 28 '16 at 9:05
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    $\begingroup$ Sorry, that was autocorrect. "Avoid" should be "should" $\endgroup$ – Martin Thoma Nov 28 '16 at 9:15
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You can see the number better when you move the image or your eyes, because with this action you reduce the overall contrast of the image. The "image" in brain is a composition of many images which are acquired in small time periods, and the result can be approximately described as an average of many still images. So they are kind of blurred together. When scrolling it can also help to reduce contrast, because in short time periods the LCD pixels cannot reach the full contrast, so it appears in lowered contrast.

The contrast of stripes in your initial image is very high. Actually contrast is the synonym for information. So the underlying number 17 is simply lost in dominant information.

To demonstrate it better, one can play with different image filters which can reproduce the movement effect. A good one is Photoshop's "Motion blur" filter, here is the result for Motion blur at 45 degrees:

enter image description here

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  • $\begingroup$ Why does moving the image reduce the images contrast? $\endgroup$ – Martin Thoma Dec 1 '16 at 22:54
  • $\begingroup$ @MartinThoma added some more info $\endgroup$ – Mikhail V Dec 1 '16 at 23:30

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