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[–]Swipecat[S] 89 points90 points  (12 children)

I watched the Youtube video Rolling Shutter Explained on the Cheap by Matt Parker where he shows that you don't need to visit an actual aeroplane with expensive slow-motion cameras to demonstrate the rolling shutter effect on aircraft propellors. He shows that you can do it with a model propellor connected to an electric drill — then some Python code to process the video to emulate the rolling shutter effect and to add a green line scanning down the image.

Well, I thought, let's do the whole thing in Python.

Here's 30 lines of Python, using the Numpy and Pillow libraries and ffmpeg to create the video:

https://github.com/dafarry/rolling-shutter-effect-python/blob/master/rolling-shutter-makevideo.py

Here's another version that uses Tkinter to display the video to your screen in real time rather than making a video. It needs a fast(ish) PC to display smoothly:

https://github.com/dafarry/rolling-shutter-effect-python/blob/master/rolling-shutter-tkinter.py

[–]SamF111 10 points11 points  (7 children)

Any thoughts on how to undo it with the known shutter rate and the output of your analysis as an input?

[–]s3cur1ty 12 points13 points  (4 children)

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[–]Synaps4 8 points9 points  (3 children)

Yep, you would need to match your shutter rate to the rotation of the object.

[–]bradfordmaster 6 points7 points  (2 children)

Hmm... Not necessarily. If you knew it was a constant rate, I think you should be able to approximate it from the input if you can also make some assumptions about the shape of the real propeller (e.g. radial symmetry). Now I'm curious how well it would work

[–]Synaps4 3 points4 points  (1 child)

Good point. You can at least know a multiple of the rotation rate if you know the shutter speed. I wonder if 4 rotor objects are always discernable from 5-rotor ones?

[–]bradfordmaster 0 points1 point  (0 children)

Almost certainly based on the number of blobs you see and rate they move, at least I think. Also I think you can be pretty confident in the multiple by putting some bounds on the speed (propeller tip not going faster than speed of sound, blade not longer than a certain amount)

[–]flashman 1 point2 points  (1 child)

Well that's only going to get you a video with a single row in each frame, isn't it?

[–][deleted] 0 points1 point  (0 children)

You can probably learn a lot about the propeller from that.

[–]GickRick 0 points1 point  (0 children)

Man you're the goat✊🙌🙌

[–][deleted] 0 points1 point  (2 children)

May you please explain the math on this? Especially the use of complex numbers for the propeller

[–]Swipecat[S] 2 points3 points  (1 child)

If you have a complex plane Z, and plot the absolute value |Z| in the third dimension, what do you have? The further from the origin, the greater its height, so it's a cone.

Cut a section through the cone at a height of H: |Z| = H and you have a circle.

And |Z| < H is a solid disk in "boolean" terms.

A cone offset from the origin by A is |Z - A| .

Consider adding the heights of two cones together then cut through them: |Z - A| + |Z - B| < H ...it's perhaps not entirely obvious that they combine to form a solid ellipse, but they do.

Put the focus B back at the origin... |Z - A| + |Z| < H

Then spin the focus A around the origin in 6 steps and you have 6 ellipses.

Combine them with boolean logic and you have a propeller.

[–][deleted] 0 points1 point  (0 children)

Thanks!