Inthree-dimensional geometry, aparallel projection (oraxonometric projection) is aprojection of an object inthree-dimensional space onto a fixedplane, known as theprojection plane orimage plane, where therays, known aslines of sight orprojection lines, areparallel to each other. It is a basic tool indescriptive geometry. The projection is calledorthographic if the rays areperpendicular (orthogonal) to the image plane, andoblique orskew if they are not.

A parallel projection is a particular case ofprojection inmathematics andgraphical projection intechnical drawing. Parallel projections can be seen as the limit of acentral orperspective projection, in which the rays pass through a fixed point called thecenter orviewpoint, as this point is moved towards infinity. Put differently, a parallel projection corresponds to a perspective projection with an infinitefocal length (the distance between the lens and the focal point inphotography) or "zoom". Further, in parallel projections, lines that are parallel in three-dimensional space remain parallel in the two-dimensionally projected image.

A perspective projection of an object is often considered more realistic than a parallel projection, since it more closely resembleshuman vision andphotography. However, parallel projections are popular in technical applications, since the parallelism of an object's lines and faces is preserved, and direct measurements can be taken from the image. Among parallel projections,orthographic projections are seen as the most realistic, and are commonly used by engineers. On the other hand, certain types ofoblique projections (for instancecavalier projection,military projection) are very simple to implement, and are used to create quick and informal pictorials of objects.

The termparallel projection is used in the literature to describe both theprocedure itself (a mathematical mapping function) as well as the resulting imageproduced by the procedure.

Every parallel projection has the following properties:

• It is uniquely defined by its projection planeΠ and the direction${\displaystyle \overrightarrow{v}}$ of the (parallel) projection lines. The direction must not be parallel to the projection plane.
• Any point of the space has a unique image in the projection planeΠ, and the points ofΠ are fixed.
• Any line not parallel to direction${\displaystyle \overrightarrow{v}}$ is mapped onto a line; any line parallel to${\displaystyle \overrightarrow{v}}$ is mapped onto a point.
• Parallel lines are mapped on parallel lines (or on a pair of points if they are parallel to${\displaystyle \overrightarrow{v}}$).
• Theratio of the lengths of two line segments on a line or on two parallel lines stays unchanged.^[1] As a special case,midpoints are mapped on midpoints.
• Thecentroid of a set of points in space is mapped to the centroid of the image of those points
• The length of a line segment parallel to the projection plane remains unchanged. The length of any line segment is not increased if the projection isorthographic.
• Anycircle that lies in a plane parallel to the projection plane is mapped onto a circle with the same radius. Any other circle is mapped onto anellipse (or a line segment if direction${\displaystyle \overrightarrow{v}}$ is parallel to the circle's plane).
• Angles in general are not preserved. Butright angles with one line parallel to the projection plane remain unchanged.
• Anyrectangle is mapped onto aparallelogram (or a line segment if${\displaystyle \overrightarrow{v}}$ is parallel to the rectangle's plane).
• Any figure in a plane that is parallel to the image plane is congruent to its image.

Orthographic projection is derived from the principles ofdescriptive geometry, and is a type of parallel projection where the projection rays are perpendicular to the projection plane. It is the projection type of choice forworking drawings. The termorthographic is sometimes reserved specifically for depictions of objects where the principal axes or planes of the object are also parallel with the projection plane (or the paper on which the orthographic or parallel projection is drawn). However, the termprimary view is also used. Inmultiview projections, up to six pictures of an object are produced, with each projection plane perpendicular to one of the coordinate axes. However, when the principal planes or axes of an object arenot parallel with the projection plane, but are rather tilted to some degree to reveal multiple sides of the object, they are calledauxiliary views orpictorials. Sometimes, the termaxonometric projection is reserved solely for these views, and is juxtaposed with the termorthographic projection. Butaxonometric projection might be more accurately described as being synonymous withparallel projection, andorthographic projection a type ofaxonometric projection.

Theprimary views includeplans,elevations andsections; and theisometric,dimetric andtrimetric projections could be consideredauxiliary views. A typical (but non-obligatory) characteristic of multiview orthographic projections is that one axis of space usually is displayed as vertical.

When the viewing direction is perpendicular to the surface of the depicted object, regardless of the object's orientation, it is referred to as anormal projection. Thus, in the case of a cube oriented with a space's coordinate system, theprimary views of the cube would be considerednormal projections.

In anoblique projection, the parallel projection rays are not perpendicular to the viewing plane, but strike the projection plane at an angle other than ninety degrees.^[2] In both orthographic and oblique projection, parallel lines in space appear parallel on the projected image. Because of its simplicity, oblique projection is used exclusively for pictorial purposes rather than for formal, working drawings. In an oblique pictorial drawing, the displayed angles separating the coordinate axes as well as the foreshortening factors (scaling) are arbitrary. The distortion created thereby is usually attenuated by aligning one plane of the imaged object to be parallel with the plane of projection, creating a truly-formed, full-size image of the chosen plane. Special types of oblique projections includemilitary,cavalier andcabinet projection.^[3]

If the image plane is given by equation${\displaystyle \mathrm{\Pi }:\overrightarrow{n}\cdot \overrightarrow{x}-d=0}$ and the direction of projection by${\displaystyle \overrightarrow{v}}$, then the projection line through the point${\displaystyle P:\overrightarrow{p}}$ is parametrized by

${\displaystyle g:\overrightarrow{x}=\overrightarrow{p}+t\overrightarrow{v}}$ with${\displaystyle t\in \mathbb{R}}$.

The image${\displaystyle P^{\prime }}$ of${\displaystyle P}$ is the intersection of line${\displaystyle g}$ with plane${\displaystyle \mathrm{\Pi }}$; it is given by the equation

${\displaystyle P^{\prime }:{\overrightarrow{p}}^{\prime }=\overrightarrow{p}+\frac{d-\overrightarrow{p}\cdot \overrightarrow{n}}{\overrightarrow{n}\cdot \overrightarrow{v}}\overrightarrow{v}.}$

In several cases, these formulas can be simplified.

(S1) If one can choose the vectors${\displaystyle \overrightarrow{n}}$ and${\displaystyle \overrightarrow{v}}$ such that${\displaystyle \overrightarrow{n}\cdot \overrightarrow{v}=1}$, the formula for the image simplifies to

${\displaystyle {\overrightarrow{p}}^{\prime }=\overrightarrow{p}+(d-\overrightarrow{p}\cdot \overrightarrow{n})\overrightarrow{v}.}$

(S2) In an orthographic projection, the vectors${\displaystyle \overrightarrow{n}}$ and${\displaystyle \overrightarrow{v}}$ are parallel. In this case, one can choose${\displaystyle \overrightarrow{v}=\overrightarrow{n},|\overrightarrow{n}|=1}$ and one gets

${\displaystyle {\overrightarrow{p}}^{\prime }=\overrightarrow{p}+(d-\overrightarrow{p}\cdot \overrightarrow{n})\overrightarrow{n}.}$

(S3) If one can choose the vectors${\displaystyle \overrightarrow{n}}$ and${\displaystyle \overrightarrow{v}}$ such that${\displaystyle \overrightarrow{n}\cdot \overrightarrow{v}=1}$, and if the image plane contains the origin, one has${\displaystyle d=0}$ and the parallel projection is alinear mapping:

${\displaystyle {\overrightarrow{p}}^{\prime }=\overrightarrow{p}-(\overrightarrow{p}\cdot \overrightarrow{n})\overrightarrow{v}=\overrightarrow{p}-(\overrightarrow{v}\otimes \overrightarrow{n})\overrightarrow{p}=(I_3-\overrightarrow{v}\otimes \overrightarrow{n})\overrightarrow{p}.}$

(Here${\displaystyle I_3}$ is theidentity matrix and${\displaystyle \otimes }$ theouter product.)

From this analytic representation of a parallel projection one can deduce most of the properties stated in the previous sections.

Axonometry originated inChina.^{[4][unreliable source?]} Its function in Chinese art was unlike thelinear perspective in European art since its perspective was not objective, or looking from the outside. Instead, its patterns used parallel projections within the painting that allowed the viewer to consider both the space and the ongoing progression of time in one scroll.^[5] According to science author andMedium journalist Jan Krikke, axonometry, and the pictorial grammar that goes with it, had taken on a new significance with the introduction ofvisual computing andengineering drawing.^[5][4][6][7]

The concept ofisometry had existed in a rough empirical form for centuries, well before ProfessorWilliam Farish (1759–1837) ofCambridge University was the first to provide detailed rules for isometric drawing.^[8][9]

Farish published his ideas in the 1822 paper "On Isometric Perspective", in which he recognized the "need for accurate technical working drawings free of optical distortion. This would lead him to formulate isometry. Isometry means "equal measures" because the same scale is used for height, width, and depth".^[10]

From the middle of the 19th century, according to Jan Krikke (2006)^[10] isometry became an "invaluable tool for engineers, and soon thereafter axonometry and isometry were incorporated in the curriculum of architectural training courses inEurope and theU.S. The popular acceptance of axonometry came in the 1920s, whenmodernist architects from theBauhaus andDe Stijl embraced it".^[10] De Stijl architects likeTheo van Doesburg used axonometry for theirarchitectural designs, which caused a sensation when exhibited inParis in 1923".^[10]

Since the 1920s axonometry, or parallel perspective, has provided an important graphic technique for artists, architects, and engineers. Like linear perspective, axonometry helps depict three-dimensional space on a two-dimensional picture plane. It usually comes as a standard feature ofCAD systems and other visual computing tools.^[5]

• 
  Optical-grinding engine model (1822), drawn in 30° isometric perspective^[11]
• 
  Example of a dimetric perspective drawing from a US Patent (1874)
• 
  Example of a trimetric projection showing the shape of theBank of China Tower inHong Kong.
• 
  Example of dimetric projection in Chinese art in an illustrated edition of theRomance of the Three Kingdoms, China, c. 15th centuryCE.
• 
  Detail of the original version ofAlong the River During the Qingming Festival attributed to Zhang Zeduan (1085–1145). Note that the picture switches back and forth between axonometric and perspective projection in different parts of the image, and is thus inconsistent.

In this drawing, the blue sphere is two units higher than the red one. However, this difference in elevation is not apparent if one covers the right half of the picture.

ThePenrose stairs depicts a staircase which seems to ascend (anticlockwise) or descend (clockwise) yet forms a continuous loop.

Objects drawn with parallel projection do not appear larger or smaller as they lie closer to or farther away from the viewer. While advantageous forarchitectural drawings, where measurements must be taken directly from the image, the result is a perceived distortion, since unlikeperspective projection, this is not how human vision or photography normally works. It also can easily result in situations where depth and altitude are difficult to gauge, as is shown in the illustration to the right.

This visual ambiguity has been exploited inop art, as well as "impossible object" drawings. Though not strictly parallel,M. C. Escher'sWaterfall (1961) is a well-known image, in which a channel of water seems to travel unaided along a downward path, only to then paradoxically fall once again as it returns to its source. The water thus appears to disobey thelaw of conservation of energy.Oscar Reutersvard is credited with discovery of the impossible object, an example of the impossible triangle (top) shown in this mural byPaul Kuniholm.

• Projection (linear algebra)

• Schaum's Outline:Descriptive Geometry, McGraw-Hill, (June 1, 1962),ISBN 978-0070272903
• Joseph Malkevitch (April 2003),"Mathematics and Art",Feature Column Archive,American Mathematical Society
• Ingrid Carlbom, Joseph Paciorek (December 1978), "Planar Geometric Projections and Viewing Transformations",ACM Computing Surveys,10 (4):465–502,doi:10.1145/356744.356750,S2CID 708008

• Graphical projections

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