Monocular Cues are visual cues used for depth perception that are dependent on one eye. Several different types of monocular cues help us to estimate the distance of objects: interposition, motion parallax, relative size and clarity, texture gradient, linear perspective, and light and shadow. In this blog, I’ve gone through and described each monocular cue in detail, which are all very important to understand the way we perceive Virtual Reality experiences.
Description of Monocular Depth Cues
Relative Size: The relative size of an object serves as an important monocular cue for depth perception that applies to three dimensional and two dimensional images. If two objects are roughly the same size, the object that looks the largest will be judged as being the closest to the observer. Two objects on a piece of paper are the same distance away, yet size difference can make the larger object appear closer and the smaller object appear farther away.
Texture Gradient: Another essential monocular cue is the use of texture to gauge depth and distance. When you’re looking at an object that extends into the distance, such as a grassy field, the texture becomes less and less apparent the farther it goes into the distance. As you look out over a scene, the objects in the foreground have a much more apparent texture. As the scene recedes into the distance, these texture cues become less and less apparent. These texture differences serve as important monocular cues for gauging the depth of objects that are both near and far.
Linear perspective: Linear perspective is a visual cue that explains how parallel lines created in the three-dimensional world, are seen as lines that merge in a two-dimensional picture. An example of this would be seen when driving on a long highway, and it looks as if the northbound and southbound lanes converge in the far distance, when in reality they remain parallel.
Occlusion: This particular depth cue is reliable and present in almost all visuals. Occlusion helps us judge objects’ relation to one another, so when an object blocks the view of a portion of a different object, you will perceive the object being blocked as being further away or behind the object doing the blocking. So if you are playing pool, and you are trying to hit the 8 ball in, but part of it is being blocked by the 5 ball, you would perceive the occluded 8 ball to be behind the 5 ball.
Motion Parallax: This monocular cue refers to the way in which the viewer perceives objects relative to each other as they fixate on a single point, while they are moving. Objects that are closer in distance to the viewer appear to be moving faster than those that are further away from the viewer. A good example of this in everyday life is observing the objects outside of a car window as it drives past. The street lights closer to the moving car appear to be moving faster than the mountains in the distance.
Aerial Perspective: This cue is a result of light being scattered by particles in the atmosphere, causing objects farther in the distance to appear blurrier, less saturated, and almost “hazy”. Sky light usually contains more light of short wavelength than other wavelengths, so objects in the distance also appear to look more blue. A common example is the blue-ish tone that the mountains take on as they get farther and farther away from you.
Rendering a virtual reality scene makes use of all these monocular cues in a very important way. Essentially, our headsets trick our eyes using polarized lenses and monocular cues to completely immerse us in their world, making us feel as though we are exploring an endless environment, when in reality the screen is only an inch or so from our face. VR usually consists of polarized lenses with two images and thus, each eye uses monocular cues to perceive the size, depth etc of the objects within the experience (Fulvio). Furthermore, adding motion parallax as a cue causes observers to rely almost exclusively on monocular cues. Thus, it is monocular cues that help us enjoy VR! While people may use both monocular cues such as relative size as well binocular cues such as changing disparity when looking at 3D motion, research suggests that binocular cues actually have very little or no effect on the perceptual experience for 3D motion or virtual reality.