Perspective Projection Mode is the specific form of projection produced by a perspective system, method or process. It describes how spatial objects, scenes, directions or visual information are geometrically or optically related to an image, view, picture surface or other representational space.
Perspective Projection Mode is therefore more specific than the general concept of Perspective Projection. Perspective Projection describes the wider process of projection, while Projection Mode identifies the particular geometrical or optical form taken by that projection.
The Dictionary of Perspective identifies central, parallel, spherical, anamorphic, line-of-sight and non-standard projection modes, together with numerous subordinate forms.
Perspective Projection and Projection Mode
Perspective Projection concerns the projection of spatial objects, scenes, outlines, light rays or images through a particular spatial arrangement.
Perspective Projection Mode identifies the form taken by that projection.
A perspective system may therefore employ a particular projection process while its Projection Mode specifies whether that process is, for example, central, parallel, spherical, anamorphic or another form.
Central Projection Mode
Central Projection organises projected directions in relation to a finite projection centre or viewpoint.
Within the Dictionary classification, the central class includes several important perspective forms:
- Linear One-Point Perspective
- Linear Two-Point Perspective
- Linear Three-Point Perspective
- Linear Unlimited-Point Perspective
- Cylindrical Perspective
- Curvilinear Perspective
The number and arrangement of vanishing directions can differ greatly, but these forms belong to the wider family of centrally organised perspective projections within the PRC classification.
Parallel Projection Mode
Parallel Projection uses projection directions that remain parallel rather than converging towards a finite projection centre.
The Dictionary places several major technical and graphical systems within the parallel family, including orthographic, multi-view, axonometric and oblique projection.
Orthographic projection includes first- and third-angle multi-view arrangements. Axonometric projection includes isometric, dimetric and trimetric forms. Oblique projection includes forms such as cavalier, cabinet and military projection.
Spherical Projection Mode
Spherical Projection organises directions or image information in relation to a spherical surface or spherical field surrounding a viewpoint.
Unlike a single flat picture plane, spherical projection can represent a much larger surrounding field and can extend both horizontally and vertically around the observer.
It is therefore particularly important to wide-field, panoramic, immersive and surrounding-view perspective systems.
Anamorphic Projection Mode
Anamorphic Projection deliberately transforms image geometry so that the represented form may appear distorted under ordinary viewing conditions but become correctly or intentionally interpretable from a prescribed viewpoint, direction, surface or optical arrangement.
Anamorphic projection demonstrates that a perspective projection need not preserve an immediately recognisable image form. The projection may instead be designed around a particular viewing condition.
Line-of-Sight Projection Mode
Line-of-Sight Projection organises projection or observation directly in relation to viewing directions extending between an observer, sensor or imaging system and the spatial points or objects being observed.
This mode is particularly relevant where spatial information is acquired through directed observation, scanning or other systems in which individual viewing directions form an important part of the projection process.
Non-Standard Projection Modes
Non-Standard Projection provides a wider category for projection systems that do not fit neatly within the conventional central, parallel, spherical, anamorphic or line-of-sight families.
Such systems may involve unusual projection geometry, complex or shaped picture surfaces, multiple projection centres, unconventional imaging arrangements or specialised scientific, technical and representational methods.
The category is important because perspective projection extends far beyond the limited group of projection methods traditionally taught in elementary perspective drawing.
Projection Mode and Picture Surface
The form of the picture or projection surface can strongly influence the resulting Projection Mode.
A projection may be formed on a planar, cylindrical, spherical, shaped, inclined, twisted or otherwise complex surface. Changing the form or orientation of that surface changes the geometrical relationship between spatial directions and their projected image positions.
Projection Mode therefore depends not only upon the direction of the projection rays, but also upon the relationship between the projection centre or direction, spatial object or scene, and the receiving image surface.
Projection Mode and Perspective Form
A Projection Mode helps determine the geometrical form of the resulting perspective image, but the two concepts are not identical.
Projection Mode describes the particular projection arrangement or operation. Perspective Form describes the resulting geometrical, optical or visual form produced within the wider perspective system.
A single broad Projection Mode can therefore contain numerous subordinate perspective forms.
Why Perspective Projection Mode Matters
The concept of Perspective Projection Mode prevents all perspective projection from being reduced to conventional flat-plane linear perspective.
Different systems may employ central, parallel, spherical, anamorphic, line-of-sight or non-standard projection relationships, each producing different spatial and image characteristics.
Perspective Projection Mode therefore provides an important analytical level between the general process of projection and the many individual perspective types and forms produced by particular systems.