Thanks, RaananW!
This was a big help!
There are still some subtle issues.
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The result was flipped left and right – writing was mirror-image, as was stereoscopy, so I used scaling.x = -1;
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My original video was compressed and needed to be expanded to the right ratio, so I set the x:y ratio to 16:9.
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The “window” into the video was slanted – I’m not sure why – so I rotated the image 30 degrees around the y axis.
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In order to have the speaker in the video look directly at the viewer I tried to put the speaker’s eyes directly in front of a seated viewer (still working on that).
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I think the image (which should be a rectangle) is now slightly, but not seriously, distorted by being treated as a segment of a sphere (as if it was projected on to the surface of a sphere, as opposed to being treated as a flat surface viewed in front of the sphere).
Thanks again! (I can now go to the next step of importing lots of media!)
Eric