3D Lightning Reconstruction (2013) | Hacker News Reader

3D Lightning Reconstruction (2013)

calculatedimages.blogspot.com

76 pointsnkron10 comments

Eric Bruning gave a good talk on this kind of stuff last year at SciPy [1]. His primary work seems to be focused on 3D lightning mapping using VHF antenna arrays - a slightly different approach than trying to take simultaneous pictures from multiple locations, but you get many more datapoints, especially in west Texas [2]. The downside is, you aren't getting a "true" 3D model of a single strike - rather, a 3D model of a storm based on the samples generated from the mapping data (IIRC)

[1] https://www.youtube.com/watch?v=0Z17Q22HEMI

[2] http://pogo.tosm.ttu.edu/about/

"It is immediately clear that they are taken from about the same direction but different heights: the second bolt looks squashed vertically."

Isn't it also possible that the photos were taken at slightly different times and the path of the lightning bolt shifted slightly over that period?

For example, if you look at this 9-second video of a high voltage electrical arc[1], you'll see that its path shifts around quite a bit over time.

[1] https://www.youtube.com/watch?v=euW4NerLAPg

The lighting discharge happens very quickly (a few microseconds [1]) so it's unlikely you'll get the same shifting you see in that video. The exposure time of the camera in that dim light was probably much longer than the time of the lightning discharge.

[1] http://en.wikipedia.org/wiki/Lightning#Downward_leader_forma...

>Isn't it also possible that the photos were taken at slightly different times and the path of the lightning bolt shifted...

In the time it takes for the light from a lightning strike to fade, the bolt basically doesn't shift at all.

There's a guy called Tom A. Warner who shoots 7207 fps video of lightning; if you watch the first video on his page[1] you'll see that once the bolt reaches the ground, it's path doesn't change.

[1] http://www.ztresearch.com/

I'm still not sure how this was done. Here's what I have gathered so far from the post: The author has two images taken unknown location. He scaled them and then marked points on each. Then he matched up points on two images manually. Now he has dx and dy for each point on one image, relative to other. Now he asserts that bigger dx means nearer to camera. So I'm thinking he takes some proportionality constant to get z = c * dx.

But wouldn't that produce pretty arbitrary shape depending on value of c?

I think he used something like z = c * dy since earlier in the post he mentioned the photos were from a similar direction but different elevations. He compared this to your how your eyes compute distance but in this case the difference is vertical.

And don't forget "In this case because the precise location and elevation of the photographers isn't known this is slightly more art than science, but it is still fun!"

A question I had was how he made the correspondences between points in the two bolts. I have a hunch that he just traversed the two lightning bolts separately and said point 1 in A corresponds to point 1 in B, ... Point N in A corresponds to point N in B, etc. if this is the case, you would expect dx and dy to grow as from top to bottom and hence his reconstructed depth to become closer from top to bottom, which is what happens.

Why would you expect the bottom of the lightning bolt pictures to be more different, just because he started labeling points from the top?

I think he just assumed two orthographic projections of the same points. The usual framework for thinking precisely about this kind of situation is https://en.wikipedia.org/wiki/Epipolar_geometry

I like how it has a shadow.

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