The GEOMETRY Workflow - 3D Postproduction: Stereoscopic Workflows and Techniques - page 130

Figure 4.25 After Rotation Fix.
4.7.5 Keystoning
Now we have the images matched in position, rotation and scale. Now,
look for a feature as close to the corner of the image as possible. If you
see a vertical offset between L and R in the corner, you most likely have
keystoning. Compensate for keystoning (usually both eyes at the same time)
until the features at the corners are aligned vertically (they don't have to line
up horizontally). Note: Regardless if you shot converged or parallel, it is a
good idea to check on this. When converging, keystoning happens naturally,
but when shooting parallel it happens as a misalignment (when the cameras
are not perfectly parallel). See Figures 4.26 and 4.27.
4.7.6 Convergence, Clean-up and Review
Now we have hopefully corrected most of the geometry issues, it's time
to dial in whatever convergence (depth) we want on the shot and also
scale and position everything a bit in case we created black edges while