Solipsism is based on actual research surrounding the science of sleep, string theory, and the debate between philosophers and scientists called the mind-body problem, or problem of consciousness.
The title, Solipsism, refers to the state of being out of touch with reality and believing that reality is not real. This is a common theme throughout the movie as Stan tires to tell what is real and what is not.
This film used practical effects and camera tricks to pull off most of the dream sequences, but sometimes there was no way around using special effects and measures were taken to ensure realism.
To produce the 5-dimensional doorway in the film, a prop door frame was built from wood and painted the same color as the doorway on the set. This prop doorway was placed at a 45-degree angle under the set doorway, propped up by bricks, while the camera was aimed at this set-up on a tripod. After 10 seconds of recording, the prop door was turned 45-degrees in the opposite direction and the set-up was filmed for another 10 seconds. In post production, these clips were superimposed and masked to produce the illusion of a 5-dimensional doorway.