Fixing distortion for 360 degree projection
Here is a brief description of how to prepare 360 degree footage for projection in a circular theater environment. For the past three months, I've been on an internship with Clifford Ross to explore different scenarios for using footage shot in a 360 degree panorama. We've been using the Ladybug 2 from Point Grey Research, and have captured some interesting shots.
However, we had not yet projected the footage in a full-size 360 degree theater. The Narrative in the Round class has been building such a theater:
However, there was still a problem with distortion: since the projectors were set up outside the cylindrical theater and projecting in, the top edges of the projection were curved down. In order for things to look natural, a reverse correction had to be applied. Read the past the jump to see how I solved this problem.
What I did is this:
I prepared a test pattern, and made a 10 second video clip that started with an undistorted pattern, and gradually applied a more extreme correction.
So starting from this:
and ending with this:
I projected this video in the cyclorama, taking note of the timecode when the distortion appeared to be entirely corrected. I then noted what the distortion correction setting was at that timecode, and applied that same setting to all our footage. When projected, it worked out OK!
There are a couple of points to keep in mind when doing this. You should figure out ahead of time whether you are going to correct your footage in the projectors (by keystoning) or by rendering the footage. Pick one or the other. Managing two sets of corrections is complicated. It is best to have the projectors in as neutral a position as possible.
Another point: when setting up projectors like this, make sure that the center of the projected image fits on the screen - it does not matter if the corners spill over, since the distortion correction will blank them out anyway.