A Guide to Monocular Cues: Mastering Depth Perception

What is a Monocular Cue?

In our everyday lives, humans need both eyes to truly interpret a wide range of spatial relationships. When looking at something, our two eyes provide a slightly different view of the object – this is referred to as binocular vision. Our brains use this “binocular disparity” to calculate depth via a process known as stereopsis by merging the two images, giving us a more accurate perception of both distance and depth.

Herein lies the problem. If you look through a monocular, unlike with binoculars, you do so using one eye only. You effectively deprive the brain of its ability to compute how far an object is. To compensate for this, you can cue off the information that you do see.

A monocular cue is a visual signal that allows an individual to perceive depth and distance using only one eye.

Interpreting monocular cues is something you have to train your brain to do. In doing so you are creating the illusion of three-dimensional depth from two-dimensional images. Some cues are more reliable than others. Some are easy to master while others require much practice and understanding to use meaningfully.

Below, we look at the eight most common monocular cues, what they are, how reliable they are, and how difficult they are to master. We’ve also included a reference about how useful or effective a cue is when viewing an object via a night vision monocular. This applies to both infrared and thermal monoculars. Understanding and applying these monocular cues can significantly improve your depth perception with one eye.

Summary Table:

“Paris Street, Rainy Day” by Gustave Caillebotte, Art Institute of Chicago – There are at least six monocular cues visible in this painting. Can you identify them?

Relative Size

• Reliability: High. This cue is consistently reliable, especially in familiar contexts where the actual size of objects is known.
• Ease of Mastery: Easy. Beginners can quickly understand and apply this cue, as it is straightforward and commonly encountered in daily life.
• Effectiveness in Night Vision: Effective because objects closer appear larger due to consistent size relationships, aiding depth perception.

Relative size is a monocular cue that helps you perceive depth by comparing the size of objects you expect to be the same size. If you expect two objects to be the same size, then you will assume that the one that looks smaller is further away than the other. This cue taps into how your brain understands the physical world, and that your brain has been trained to understand that the same object will appear to be larger or smaller based on how far it is from you. For example, if you look at a person standing next to a truck, and the person appears to be larger than the truck, you will infer that the truck is farther away than the person.

Interposition (Overlap)

• Reliability: High. This is a highly reliable cue as overlapping consistently indicates relative distances.
• Ease of Mastery: Easy. This cue is intuitive and easily understood by beginners since it occurs frequently in visual scenes.
• Effectiveness in Night Vision: Effective as overlapping objects indicate depth, even when displayed in thermal or infrared contrast.

Interposition, sometimes referred to as overlap, is a monocular cue that helps you perceive depth based on the positioning of objects in a scene.

When you look at two objects and the one partially overlaps the other, you know that the object that is being overlapped is behind the other and therefore farther away. This cue is particularly useful when you are looking at a complex scene with a lot of different objects – think of a busy sidewalk for example. The interposition of people to others enables your brain to form a picture of where everyone is positioned and how you may want to plan your walk to navigate the sidewalk. This cue helps you to construct a layered understanding of the scene’s depth.

Linear Perspective

• Reliability: High. It is a very reliable indicator of depth in a scene with clear lines.
• Ease of Mastery: Moderate. While the concept is straightforward, applying it correctly in drawings or recognizing it in complex scenes might take practice.
• Effectiveness in Night Vision: Effective because the convergence of parallel lines still provides depth cues in both thermal and infrared images.

Linear perspective is a powerful monocular cue that involves the convergence of parallel lines as they recede into the distance, creating an illusion of depth. Parallel lines appear to get closer to each other as they move further away from us. They will eventually converge at a vanishing point on the horizon. A lot of man-made objects like roads and buildings are designed in fairly straight lines (by comparison to nature) and this cue becomes particularly useful when these are part of the scene. architects and artists often use this linear perspective to model and create the perception of a three-dimensional area on a two-dimensional surface. It is a common cue, yet mastering it may require some practice especially when there is interference from objects in more complex scenes.

Texture Gradient

• Reliability: High. This cue reliably indicates depth, especially in natural settings where textures are prominent.
• Ease of Mastery: Moderate. Beginners might need time to recognize and use this cue effectively, especially in less textured environments.
• Effectiveness in Night Vision: Less effective because night vision images often lack detailed textures, making gradients harder to discern.

Texture gradient can help you perceive depth by observing the change in texture density as it recedes into the distance. If you look at a gravel path or a field of flowers that extends away from us, the texture will appear smoother and more closely packed the farther away they are. For example, in a photograph of a pebble beach, the pebbles in the foreground appear larger and more distinct, while those in the background appear smaller and more compressed, giving a sense of depth and distance.

Aerial Perspective (Atmospheric Perspective)

• Reliability: Moderate to High. This cue is reliable but can be influenced by weather conditions and lighting.
• Ease of Mastery: Moderate. Understanding the effect of the atmosphere on distant objects can take some observation and practice.
• Effectiveness in Night Vision: Less effective because thermal and infrared images do not mimic atmospheric haze, making distant objects less distinguishable.

Aerial perspective, sometimes referred to as atmospheric perspective, is a monocular cue that relies on the effect of the atmosphere on the appearance of objects as they recede into the distance. As objects get farther away, The scattering of light by particles of objects that are further away causes them to appear hazier, less detailed, and often with a bluish tint. This is why the phenomenon is particularly noticeable in landscapes, where distant mountains or buildings appear less distinct than those closer to the observer. For example, in a photograph of a mountain range, the closest peaks are sharp and clear, while those farther away appear softer and bluer. This cue helps us gauge the relative distance of objects based on their clarity and color.

Shading and Lighting

• Reliability: High. This is a very reliable cue as lighting and shadows consistently indicate shape and position.
• Ease of Mastery: Difficult. Mastering shading and lighting requires understanding light sources and how they interact with objects, which can be complex.
• Effectiveness in Night Vision: Not effective because night vision images do not depict traditional light and shadow; they show temperature differences or light amplification.

Description: Shading and lighting are crucial monocular cues for perceiving the three-dimensional form and depth of objects. The way light falls on an object and the shadows it casts can provide significant information about its shape, texture, and position in space. For instance, a sphere illuminated from one side will have a bright area where the light hits and a gradually darker area on the opposite side, with a shadow cast on the surface it rests on. This gradient of light and shadow helps our brain interpret the sphere as a three-dimensional object. Artists use shading and lighting techniques to create realistic images, emphasizing highlights, midtones, and shadows to convey depth and form.

Motion Parallax

• Reliability: High. This cue is very reliable and directly linked to movement.
• Ease of Mastery: Moderate. Recognizing and interpreting motion parallax requires an understanding of relative motion, which can take some practice.
• Effectiveness in Night Vision: Effective because relative movement of objects at different distances remains discernible, similar to visible light.

Description: Motion parallax is a monocular cue that involves the relative movement of objects as we move. When we change our position, objects at different distances move across our field of view at different rates. Objects that are closer to us appear to move faster and in the opposite direction to our own movement. Objects that are further away seem to move more slowly and in the same direction. It’s easy to identify this cue from a moving car or train: nearby trees and road signs whiz by quickly, while distant mountains move slowly across the horizon. Motion parallax provides critical information about the relative distances of objects, helping us navigate and understand our environment.

Accommodation

• Reliability: Moderate. This cue is reliable but less effective for very distant objects.
• Ease of Mastery: Difficult. This is a physiological process that is not consciously controlled or easily observable by beginners.
• Effectiveness in Night Vision: Not effective because the displays do not change focus like natural vision, remaining at a fixed focus distance.

Description: Accommodation is a physiological monocular cue that involves the eye’s lens changing shape to focus on objects at different distances. When we look at a close object, the lens becomes more rounded to focus the light properly on the retina. For distant objects, the lens flattens. The brain uses the degree of strain on the ciliary muscles controlling the lens as a cue to determine the distance of the object. While accommodation is a subtle and unconscious process, it provides important depth information, especially for objects within arm’s reach. For instance, when reading a book, the lens’s accommodation helps keep the text in focus while providing a sense of the book’s proximity.

Conclusion

Understanding and applying these monocular cues can significantly enhance depth perception, even with one eye. These cues are also essential in art, photography, and other visual fields to create a sense of depth and realism.