The human eye’s primary function(s) are perception of form, space, and colour.
Our visual system evolved to correctly interpret apparent changes to the visual features of a scene/object as viewed from a particular location.The theory, processes, and technical procedures involved in determining how and why these features change is named visual optics, or more generally, visual perspective (2nd or retinal type).
Visual Perception
What is of primary importance for human vision to operate is that the human eye achieves a ‘point-to-point’ correspondence between the object and its image on the nervous layer that is receptive to light (the retina).The eye must produce distinct or visibly sharp representations –or images– of a three-dimensional scene. The pattern image is largely in focus, without undue optical blurring, optical aberrations, or shape distortions.
Ergo, the image produced by the eye is a good approximation of how an object looks from a particular vantage point. Still, the perceptual processes necessary to achieve sufficient clarity of vision are complex and involve trade-offs among image sharpness, field of view, depth of focus, and the perception of three dimensions, among other factors.
It is essential to realise that many ostensibly purely optical processes occurring within the eye are often complemented by the human perceptual system, including visual processing procedures and psychological processes. We can summarise human vision by stating that visual perception is complex (in operation), and further that whilst much has been learned, certainly even more remains hidden, undiscovered, or unknown to science.
Decoding Spatial Reality
While looking at the blue sky without any clouds in sight, we see a formless extent of colour, without any feeling or judgement of depth whatsoever; apart from perhaps the impression of looking into an immensity of open space. No visible geometrical framework/structure means no depth is discernible (space itself is not intrinsically visible).
Contrast that situation with looking down/ along a set of parallel railway tracks, stretching into the distance and apparently converging to a vanishing point. Here, we get a real sense of depth, size, angle, and shape for large regions of the depicted space. Such an image is the overt recognition of the railway-track Form, plus standard perspective phenomena, including viewing aspect and the diminution of size, which result in converging parallels and a vanishing point on the horizon line.
However, physical reality is not composed solely of railway-track-type structures! But rather it contains an infinite variety of different Forms. How then is it possible to correctly ‘decode’ images? First, we apply knowledge of common spatial structures, how they are sized and shaped/located plus are visually transformed into perspective-image facets (and phenomena) by a category such as linear perspective. Second, we must have knowledge of other perspective-related elements: optical assembly (scene + method optics/geometry), plus projection and observation mode(s); and further how these affect perspective phenomena.
Overall, a category such as linear perspective embodies standard mathematical relationships for image transformation factors. But in a real-world situation, such as using a camera, the apparent distance, size, and shape features may differ significantly from expected results. Such differences increase towards the edges of an eye/lens image, where wide-field perspective distortions can come into play, leading to curvilinear/spherical perspective effects, etc
Patently, for cartographic, astronomical, engineering, and technical drawing purposes, etc., it is desirable to employ accurate image analysis techniques; and that explains the use of parallel perspective and/or other counter-distortion perspective types/forms and associated methods.
Linear perspective typically provides a rectilinear structure (often a metric grid) for the depiction on a surface of the apparent shape, size, and relative position of the objects constituting a 3-D spatial scene; that is, for the representation of Form, or what is sometimes called the representation of space. This is a form of perspective image/view that most people are familiar with and learned about, or at least learned to recognise and name in school.
In fact, this ‘rectilinear’ geometry, with converging parallels, is so basic to how humans perceive space that a pre-processing physical image detector for converging parallels has been discovered to exist on the retina of the human eye.
Beholder’s Share
Humans see form/structure from a deeply formless world, one beset by vast amounts of disorder or complexity in the geometry of spatial structures.To say nothing of the fact that light rays are fundamentally mixed up and reflected in all kinds of different directions.
Within this disordered context, visual perspective produces a structured image space that emanates partly from spatial reality, partly from the perspective method/observer, and partly from the visual imager—for example, the human eye and perhaps in combination with a perspective instrument such as a camera.Whereby such a process happens by the application of perspective principles/methods/ theory (whether overtly realised or not).
Perspective remains a complex topic because it typically involves comparison of fundamentally different categories of space. For example, the (attempted) reconciliation of the 3-D space of the physical world (object space), with the 2-D space of graphical or photographic perspective (perspective space). Often, despite a strong desire to achieve equality/ alignment, this reconciliation is difficult (if not impossible) to achieve with perfect correlation, because one is dealing with dimensions that must map without any physically based 1:1 correspondence or identical mapping.
Patently, information may be lost in this process due to the inherent optical limitations of a single point-of-view, plus aspect-of-form shape changes and scale/shape/size relations, etc., resulting in reduced (or concealed/confused) visual information/details that are to be interpreted in a single visual snapshot (very hopefully). Overcoming aspects of this geometric correspondence—or equivalence—problem is a key ‘goal’ of optical/technical perspective.
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