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Today I was watching a video on Youtube where I saw lines representing the light of a light source. While the image was downscaled like this:

downscaled image

There were some clear patterns on the horizontal and vertical lines going through the source. I already saw this phenomenon a few times when I received photos of computer monitors. When there were some closely packed lines I saw patterns resembling those in the screenshot when I was playing around with she scale of the image. As I was increasing or decreasing the size of the image the pattern was changing continuously.

This is what the actual image looks like when it isn't scaled down:

non-scaled image

The patterns seem to disappear.

What causes this phenomenon? I assumed it was falling under the category optics. If I am false, I am sorry.

source of the video: https://www.youtube.com/watch?v=d-o3eB9sfls

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closed as off-topic by AccidentalFourierTransform, Jon Custer, Chris, Kyle Kanos, Emilio Pisanty Mar 7 '18 at 13:19

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    $\begingroup$ Wow, that picture is migraine-inducing. $\endgroup$ – Chris Mar 3 '18 at 23:32
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    $\begingroup$ When I scroll up and down this post on my phone I get changing radial patterns in both pictures. Try it! You'll get to see the answers in action. $\endgroup$ – Asher Mar 4 '18 at 0:14
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    $\begingroup$ @Asher That's what's most headache-inducing to me. Seriously, I had to scroll the pattern off the page just to be able to finish typing this comment. $\endgroup$ – Chris Mar 4 '18 at 0:18
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    $\begingroup$ Related: physics.stackexchange.com/q/209005 $\endgroup$ – Nilay Ghosh Mar 4 '18 at 5:54
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    $\begingroup$ I'm voting to close this question as off-topic because it is not about physics. $\endgroup$ – AccidentalFourierTransform Mar 4 '18 at 16:44
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The moiré fringes are caused by undersampling, it is the difference between properly sampling a small field of view and undersampling a larger field, it causes spatial aliasing.

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This seems to be an effect of aliasing. Basically, the diagonal lines are drawn with discrete pixels, and then the width of the line varies as it is rounded back and forth between successive integer number of pixels.

This rounding occurs differently depending on the width of the line at that point and the angle of the line, but since it occurs systematically, you can sometimes see patterns appear when you look at the picture as a whole.

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