Optic flow refers to the coherent motion of visual features such as edges, textures, and colors in your environment across your retina that signal to your brain that you are moving through that environment. As an example, seeing a field of stars expanding radially outward from the center of your field of view can tell your brain that you are moving forward through space. The amount of vection you experience, which is the potential reason for discomfort, is correlated with the amount and speed of optic flow you see in VR. The techniques in this section offer suggestions for reducing optic flow to improve user comfort.
While it is common for console games to have avatars that walk and run at unrealistically high speeds, the resulting optic flow is more intense than what we are accustomed to experiencing in real life. The average human being walks at a rate of about three miles per hour (1.4 meters per second), and runs at about twice that speed. Keep this in mind when designing your environments.
Vignettes, sometimes referred to as Tunnel Vision, darken or completely occlude the edges of the screen when movement occurs. They serve to limit the amount of visible optic flow, which can help reduce vection during acceleration.
The tradeoff with vignettes is that a diminished field of view can potentially be disorienting or claustrophobic for the user. Simple implementations can also feel mechanical or intrusive. More advanced vignetting systems analyze the scene geometry to selectively occlude sections of the visual scene that contain the most optic flow information. For example, if you’re moving forward with a textured wall to your left and open space to your right, the vignette would occlude the part of your visual field containing the wall where there are strong optic flow cues, but leave more of the open space visible because it generates less motion on the retina. Ubisoft’s Eagle Flight demonstrates this technique masterfully.
Some designs may allow for geometry that occludes some optic flow in the environment or in regions of the user’s visual field. For example, vehicle cabins, cockpits, helmets, headgear, etc. can help the user maintain a feeling of immersion while also relegating visible optic flow to windows or apertures. This provides the benefits of an aggressively tuned Vignette effect where much of the optic flow is obscured, while still enabling people to see and interact with the virtual environment created by the occluding geometry.
This technique affords creative opportunities for immersion and discomfort mitigation, as well. For example, DiRT Rally places a stabilized camera inside the driver’s seat of a race car. To indicate the car is going over bumpy ground, the car geometry jumps and shakes around the user, but the camera maintains a stable view of the outside environment that is consistent with how the user’s head is (not) moving. This creates less visual-vestibular mismatch than if the camera shook along with the car, while still conveying information to the user about their movement on the virtual terrain.
The purpose of temporal occlusion is to briefly obscure part of the display so that there is a reduction of optic flow. The effect is visible for only a short amount of time so that it doesn’t interfere with our awareness of the environment. There are a number of ways to apply temporal occlusion, for example, as a dynamic pattern on the edge of the vignette or as ribbons of solid color that appear across the entire display, quickly disappearing when rapid movements occur. Eagle Flight demonstrates both of these effects very well.
In some designs, it may be possible to have geometry that becomes opaque depending on how far from the center of the field of view it is at any given moment. This can result in significant reduction in optic flow when detailed environments are hidden behind relatively solid surfaces. This is particularly effective in vehicles where most of the optic flow takes place in the windows which display the environment moving past the user. In this example, if the window you are currently viewing is always transparent, then you can always see what you want to see, even if the rest of the vehicle windows are opaque. This provides the benefits of an aggressively tuned vignette effect, where much of the optic flow is eliminated, while still allowing people to see and interact with the interior of the vehicle. Ultrawings demonstrates this technique effectively.
The more details that are visible on the surfaces of objects, the more optic flow that will occur when a user moves past them. Reduce optic flow with art styles that make heavy use of more solid textures, and minimize the number of visible edges and noisy textures. Advanced techniques include leveraging pixel shaders to reduce texture detail for surfaces in relation to their speed of movement as projected on the screen.
See below for the sections outlining the many design techniques and best practices to help inspire and inform your next VR locomotion system.