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If you assemble two cameras onto a rig—at its simplest a piece of metal with cameras bolted to it—you’ll rapidly discover that the two cameras are looking in different directions, with different image sizes, and usually with quite different roll angles.
Using a wide field of view is important to achieving a good stereoscopic effect— sense of depth—especially since some of the view will be sacrificed in calibration. The wide field of view frequently means substantial lens distortion, and removing the distortion will eliminate some of the field of view also.
So an important initial goal of stereo image processing is to make the two images conform to one another (match geometrically). (Color and brightness should also be equalized.)
There are three basic methods:
1) Mechanical calibration before shooting, using physical adjustments on the rig to align the cameras, in conjunction with a monitor that can superimpose the two images
2) Electronic calibration, by shooting images of reference grids, then analyzing that footage to determine corrections to apply to the real footage to cause it to match up.
3) Take as-shot footage with no calibration, track and analyze it to determine the stereo parameters, use them to correct the footage to match properly.
Of these choices, the first is the best, because the as-shot images will already be correct and will not need resampling to correct them. The downside is that a suitably adjustable rig and monitor are more complex and expensive.
The second choice is reasonable for most home-built rigs, where two cameras are lashed together. We recommend that you set up the shoot of the reference grid, mechanically adjust the cameras as best you are able, then lock them down and use electronic correction (applied by the image preprocessor) to correct the remaining mismatch. With a little care, the remaining mismatch should only require a zoom of a few percent, with minimal impact on image quality.
The third case is riskiest, and is subject to the details of each shot: it may not always be possible to determine the camera parameters accurately. We recommend this approach only for “rescuing” un-calibrated shots at present.
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