Depth perception gives humans the ability to see in three dimensions. To achieve this feat, we use monocular and binocular depth cues to gain information about the relative size, distance, shape and orientation of objects and images we look at. These cues allow us to create a more accurate interpretation of the image or object and the actions and behaviors of objects in motion. Monocular depth cues refer to those cues occurring in the individual eye.
Motion Parallax and Kinetic Depth Perception
Motion Parallax occurs when an object travels across the retina of a moving person. The moving person focuses on one object while noting the relative movement of other objects. Parallax causes objects in the distance to appear to move more slowly than objects that are closer.
Kinetic depth perception allows us to gauge the velocity of moving objects. When an object moves away, it appears to grow smaller. When an object approaches, it appears to become larger. We constantly judge changes in our positions to others using kinetic depth perception cues.
Linear and Aerial Perspectives
Linear perspective frequently occurs with the observation of parallel lines. Such lines will appear to recede and converge at the horizon. The horizon will appear to rise. This cue is related to the relative size and texture gradient, and often all three work together.
Aerial perspective is also referred to as relative height. This cue notes that objects closer to the line of horizon will appear farther away. This phenomenon relates to the way light scatters in the air, causing objects on the horizon to appear in faded colors or washed in lower light luminance and contrast, while objects that are near will appear to have vibrant or intense colors with strong contrast.
Texture Gradient
Most objects have a textured surface. When an object is farther away from the viewing point, the texture appears finer and smoother, and may appear to be closer.
Familiar and Relative Size
Previous experience with objects allows us to know many objects' relative size. This experience informs our interpretation of distance. The familiar size cue tells us that the visual angle of objects becomes smaller with distance, allowing us to calculate the probable depth or distance of objects. Known size, together with perspective and texture effects, are strong depth cues.
When we know one object is similar in size to another object and both objects are within our plane of vision, the relative size cue allows us to understand that the object with the larger visual angle on the retina is closer than the other object.
Shadow and Occlusion
The shadow monocular depth cue has several rules: If an object is solid, it will cast a shadow. If there is only one light source, then all shadows will fall in the same direction and the shadow will be opposite from the source of light. Objects with shadows falling on them are farther away than objects casting the shadow. If the object is lower than the "ground plane" (like a well), the shadow will appear on the same side as the source of light.
Occlusion, also referred to as overlapping or interposition, occurs when one object blocks another object. The object that is blocked is understood, by this cue, to be farther away than the object blocking it.
Peripheral Vision and Accommodation
The peripheral vision cue occurs due to the curvature of the eyeball. This curvature causes the visual field to distort or appear to bend at its extreme edges. This visual distortion is accommodated for when we interpret an image or scene. Often we will ignore that the lines of objects, which our prior knowledge understands are straight, will appear to be curved. The effect of this curvature can be seen in some photographs where no accommodation has taken place, which is partially why photographs often do not capture the image we think we have seen.
The accommodation cue occurs when the dioptic power of the lens increases and allows close objects to be focused clearly on the retina. How this cue informs the understanding of distance is not yet clearly understood by science.
We perceive monocular depth cues just as easily with one eye as with two, reflecting how powerful and important these depth cues are to our vision.
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