Tests demonstrating how `#_calculateProjectiveTransform` works

[sashadev-sky]

Problem

LDI allows perspectival distortions of aerial images over a flat, projected map. It is key for georectification.

Projective transformation

We correct for camera perspective distortion by using a projective transformation; specifically we calculate a 4*4 projective transformation matrix mapping each corner of the aerial photograph to a geographic coordinate on the map.

Georectification

Ultimately, this transformation allows us to neatly fit images taken from multiple perspectives into a common map coordinate system, a process called georectification.

Below is a great visual example of an image being rectified over a plane:

Set of 2D images. The original images are taken from different perspectives (row 1). Using systematic transformations (rows 2 and 3), we are able to transform both images such that corresponding points are on the same horizontal scan lines (row 4).

Since the projective transformation functionality is so core to LDI, we would like to document it better.

Solution

You can find our #_calculateProjectiveTransform method here:

https://github.com/publiclab/Leaflet.DistortableImage/blob/af4bea824707298cc47739b14c5f0d7f57228910/src/DistortableImageOverlay.js#L453-L485

You will probably want to check out L.MatrixUtil as well: https://github.com/publiclab/Leaflet.DistortableImage/blob/main/src/util/MatrixUtil.js

A shell for the test can be found here - please overrwrite this test to be useful and remove the it.skip so that it runs.

https://github.com/publiclab/Leaflet.DistortableImage/blob/af4bea824707298cc47739b14c5f0d7f57228910/test/src/DistortableImageOverlaySpec.js#L33-L44

Test whatever aspect(s) of this functionality you'd like. The goal is for it to be explanatory, as a form of documentation that might allow us to clean the comments in the function itself.

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[sashadev-sky]

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[SidharthBansal]

Published Thanks Sasha you are super helpful