The GEOMETRY Workflow - 3D Postproduction: Stereoscopic Workflows and Techniques - page 132

adjusting. Now is also a good time to view the imagery in “wiggle” mode
(alternate L/R rapidly) to see how it feels. If the geometry is correct, it will
feel like we are moving horizontally between one camera and the other
and everything in the image will react accordingly. If we feel something is
wrong, re-check the shot from step one to i ne-tune or use the automatic
or semi-automatic tools now that the images have been pre-adjusted. We
will most likely get better results than if we used these tools directly at the
beginning. As a i nal step, put your 3D glasses back on and watch the shot.
Is it comfortable? You can always compare it with the original uncorrected
to feel the difference.
4.7.7 Additional Issues
The above steps will help you resolve some of the most common geometric
issues. However, there are other more subtle issues that may make your
i ve-step process, as well as your automatic alignment, even harder. The
most common issues are:
• Vertical camera position mismatch. This one can potentially make your
scale mismatch hunt a nightmare. It happens when one camera is slightly
higher than the other. It behaves similar to a “normal” vertical or scale
misalignment, with the exception that it only happens in objects close to
camera. The closer, the worse the error. The best way to i x the mismatch
is using advanced optical l ow image distortion (if you have a tool with
that capability) just on the target areas using one eye as reference.
• Lens distortion. This one can make the resolution of keystoning especially
challenging because you will not just have perspective distortion but also
barrel distortion from the lens. If the distortion is so extreme that it becomes
an issue, the best solution is to linearize or “map” the offending lens in a
similar process that you would do for a VFX shot prior to all the geometry
correction. Then correct the images and restore the distortion at the very
end, in case you want it back. This will ensure both eyes are distorted in
a similar way and with no vertical disparities.