How do people that can't perceive 3D view a stereoscopic 3D image or movie? According to Mendiburu (2009), 3-15% of the people can't view a stereoscopic 3D image or movie, mostly due to poor binocular vision.

Imagine one of those is in the movie theater, trying to watch a 3D movie. How would this movie appear to this person?


Mendiburu (2009) 3D Movie making: stereoscopic digital cinema from script to screen, Oxford, England: Focal Press) (Free PDF)
People who are unable to perceive stereoscopic depth typically only perceive information from the dominant eye.

Failure to perceive stereoscopic depth is usually caused by conditions where the eyes do not converge properly (e.g., strabismus). If left untreated, it is not possible to develop the necessary correspondance between the two signals from either eye, and one eye will dominate over the other. The result is that the majority of visual cortical neurons will respond to stimuli presented to the dominant eye, whereas there will be far fewer binocular neurons (i.e., neurons that respond to stimuli from either eye) and monocular neurons that respond to stimuli from the weaker eye.