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CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Viewing Part I (History and Overview of Projections) 1 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © History of projection in art Geometric constructions Types of projection (parallel and perspective) Lecture Topics 2 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Painting based on mythical tale as told by Pliny the Elder: Corinthian man traces shadow of departing lover. Projection through use of shadows Detail from: The Invention of Drawing (1830) – Karl Friedrich Schinkle Drawing as Projection (Turning 3D to 2D) 3 / 45 10/2/2014 William J. Mitchell, The Reconfigured Eye, Fig 1.1
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Plan view (parallel, specifically orthographic, projection) from Mesopotamia (2150 BC): Earliest known technical drawing in existence Greek vase from the late 6 th century BC: Shows signs of attempts at perspective! Note relative sizes of thighs and lower legs of minotaur Early Forms of Projection (1/2) Ingrid Carlbom Planar Geometric Projections and Viewing Transformations Fig. 1-1 Theseus Killing the Minotaur by the Kleophrades Painter 4 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Ancient Egyptian Art: Multiple Viewpoints Parallel Projection Tomb of Nefertari, Thebes (19th Dyn, ~1270 BC), Queen Led by Isis. Mural Note how the depiction of the body implies a front view but the feet and head imply side view (early cubism!) Early Forms of Projection (2/2) 5 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Starting in the 13 th century (AD): New emphasis on importance of individual viewpoint, world interpretation, power of observation (particularly of nature: astronomy, anatomy, etc.) Masaccio, Donatello, DaVinci, Newton Universe as clockwork: rebuilding the universe more systemically and mechanically Tycho Brahe and Rudolph II in Prague (detail of clockwork), c Copernicus, Kepler, Galileo…: from earth- centric to heliocentric model of the (mechanistic) universe whose laws can be discovered and understood The Renaissance 6 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © In art, an attempt to represent 3D space more realistically Earlier works invoke a sense of 3D space but not systematically Lines converge, but no single vanishing point Early Attempts at Perspective Giotto Franciscan Rule Approved Assisi, Upper Basilica, c / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Brunelleschi invented systematic method of determining perspective projections (early 1400s). He created demonstration panels with specific viewing constraints for complete accuracy of reproduction. The perspective is accurate only from one POV Vermeer and others created perspective boxes where a picture, when viewed through viewing hole, had correct perspective Vermeer on the web: Brunelleschi and Vermeer Perspective Box Samuel van Hoogstraten National Gallery, London Perspective Box of a Dutch Interior Samuel van Hoogstraten National Gallery, London 8 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Brunelleschi was reported to have determined the accuracy of his paintings by making a hole in the vanishing point, examining the reflection in a mirror and comparing the line convergence to the real model The realism of his paintings are evidence that Brunelleschi had some systematic method for determining perspective projections, although the procedure he used was never documented His illusion inspired other artists to explore linear perspective Brunelleschi’s Method painting mirror baptistry Image credit: COGS011 (Perception, Illusion and Visual Art, William Warren) 9 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © James Burke: “Masters of Illusion” 10 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Forced Perspective Art 11 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Driving ideas behind linear perspective: Parallel lines converge (in 1, 2, or 3 axes) to vanishing point Objects farther away are more foreshortened (i.e., smaller) than closer ones Example: perspective cube Rules of Linear Perspective edges same size, with farther ones smaller parallel edges converging 12 / 45 10/2/2014 Great depth cue, but so are stereo/binocular disparity, motion parallax, shading and shadowing, etc..
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Artist David Hockney proposed that many Renaissance artists, including Vermeer, might have been aided by camera obscura, raising a big controversy David Stork, a Stanford optics expert, refuted Hockney’s claim in the heated 2001 debate about the subject. Also wrote “Optics and Realism in Renaissance Art” to disprove Hockney’s theory More recently, in “Tim’s Vermeer” Inventor Tim Jenison paints a Vermeer using mirrors Directed by Teller, written by Penn Jillette and Teller A Similar Idea: Camera Obscura Hockney, D. (2001) Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters. New York: Viking Studio. Stork, D. (2004) Optics and Realism in Renaissance Art. Scientific American 12, / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Both Da Vinci and Alberti created accurate geometric ways of incorporating linear perspective into a drawing using the concept vanishing points Linear Perspective (Vanishing Points) viewing distance Distance Point transversals “Little Space” diagonals perpendicular CP baseline Da Vinci’s Method Alberti’s Method Image credit: COGS011 (Perception, Illusion and Visual Art, William Warren) 14 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Published first treatise on perspective, Della Pittura, in 1435 “A painting [the projection plane] is the intersection of a visual pyramid [view volume] at a given distance, with a fixed center [center of projection] and a defined position of light, represented by art with lines and colors on a given surface [the rendering].” (Leono Battista Alberti ( ), On Painting, pp ) A different way of thinking about perspective from the vanishing point Alberti on Linear Perspective (View Points) 15 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © object picture plane projected object Idea of “visual pyramid” implies use of geometry of similar triangles Easy to project object onto an image plane based on: height of object (||AB||) distance from eye to object (||CB||) distance from eye to picture (projection) plane (||CD||) relationship ||CB|| : ||CD|| as ||AB|| : ||ED||; solve for ||ED|| Triangles and Geometry (1/2) 16 / 45 10/2/2014 ||CB|| : ||CD|| as ||AB|| : ||ED||
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Triangles and Geometry (2/2) The general case: the object we’re considering is not parallel to the picture plane Use the projection of CA’ onto the unit vector CB/||CB|| to determine the vector CB’, then use prior similar triangle technique Remember, the dot product of a vector a with a unit vector b is the projection of a onto b (scalar) ||CB’|| : ||CD|| as ||A’B’|| : ||ED|| 17 / 45 10/2/2014 So if U is the unit vector in the direction of CB (i.e. U = CB/||CB||), we get: CB’ = ||CB’|| * U = (CA’ U) * U U: direction, ||CB’||: magnitude
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Concept of similar triangles described both geometrically and mechanically in widely read treatise by Albrecht Dürer ( ). Refer to chapter 3 of the book for more details. Dürer Woodcut Albrecht Dürer Artist Drawing a Lute Woodcut from Dürer’s work about the Art of Measurement. ‘Underweysung der messung’, Nurenberg, / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Art of Perspective (1/5) Robert Campin - The Annunciation Triptych (ca. 1425) 19 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Point of view influences content and meaning of what is seen Are royal couple in mirror about to enter room? Or is their image a reflection of painting on far left? Analysis through computer reconstruction of the painted space: royal couple in mirror is reflection from canvas in foreground, not reflection of actual people (Kemp pp ) Art of Perspective (2/5) 20 / 45 10/2/2014 Diego Velázquez, Las Meninas (1656)
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Art of Perspective (3/5) Piero della Francesca, The Resurrection (1460) Perspective can be used in unnatural ways to control perception Use of two viewpoints concentrates viewer’s attention alternately on Christ and sarcophagus 21 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Art of Perspective (4/5) Leonardo da Vinci, The Last Supper (1495) “Mr. King provides a lively account of Leonardo’s continual hunt for faces he might sketch, and speculates about the identity of the models (including himself) that he might have used to create the faces of Jesus and the apostles. He also writes about how Leonardo presumably started the painting by hammering a nail into the plaster to mark “the very center of the mural, the point on which all lines and all attention would converge: the face of Christ,” and how he used perspective and his knowledge of geometry and architecture to map out the rest of the painting.” Ross King, ‘Leonardo and “The Last Supper” ’ 22 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Art of Perspective (5/5) Vredeman de Vries, Perspective 23 (1619) Kemp p.117 Several vanishing points, two point perspective 23 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Types of Projection 24 / 45 10/2/2014 Different methods of projecting objects to the screen can have a large impact on the viewer’s interpretation of the scene Here, two objects are displayed in very different ways to highlight certain features
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Main Classes of Planar Geometrical Projections (a) Perspective: determined by center of projection (in our diagrams, the “eye”) Simulates what our eyes or a camera sees (b) Parallel: determined by direction of projection (projectors are parallel—do not converge to “eye” or COP). Alternatively, COP is at ∞ Used in engineering and architecture for measurement purposes In general, a projection is determined by where you place the projection plane relative to principal axes of object (relative angle and position), and what angle the projectors make with the projection plane 25 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Logical Relationship Between Types of Projections 26 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Overview of Parallel Projections Assume object face of interest lies in principal plane, i.e. parallel to xy, yz, or xz planes. (DOP = direction of projection, VPN = view plane normal) 27 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Used for: Engineering drawings of machines, machine parts Working architectural drawings Pros: Accurate measurement possible All views are at same scale Cons: Does not provide “realistic” view or sense of 3D form Usually need multiple views to get a three-dimensional feeling for object Multiview Orthographic (Parallel) 28 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Same method as multiview orthographic projections, except projection plane not parallel to any of coordinate planes; parallel lines equally foreshortened Isometric: Angles between all three principal axes equal (120 o ). Same scale ratio applies along each axis Dimetric: Angles between two of the principal axes equal; need two scale ratios Trimetric: Angles different between three principal axes; need three scale ratios Axonometric (Parallel) 29 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Used for: Catalogue illustrations Patent office records Furniture design Structural design 3D Modeling in real time (Maya, AutoCad, etc.) Pros: Don’t need multiple views Illustrates 3D nature of object Measurements can be made to scale along principal axes Cons: Lack of foreshortening creates distorted appearance More useful for rectangular than curved shapes Isometric Projection Construction of an isometric projection: projection plane cuts each principal axis by 45° 30 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Video games have been using isometric projection for ages. It all started in 1982 with Q*Bert and Zaxxon which were made possible by advances in raster graphics hardware. Still in use today when you want to see things in distance as well as things close up (e.g. strategy, simulation games). SimCity IV, StarCraft II While many games technically use axonometric views, the general style is still referred to isometric or, inappropriately, “2.5D”/ “three quarter”. Axonometric Projection in Games 31 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Projectors at oblique angle to projection plane; view cameras have accordion housing, can adjust the angle of the lens relative to the projection plane Pros: Can present exact shape of one face of an object (can take accurate measurements): better for elliptical shapes than axonometric projections, better for “mechanical” viewing Lack of perspective foreshortening makes comparison of sizes easier Displays some of object’s 3D appearance Cons: Objects can look distorted if careful choice not made about position of projection plane (e.g., circles become ellipses) Lack of foreshortening (not realistic looking) Oblique Projection (Parallel) 32 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Examples of Oblique Projections Construction of oblique parallel projection Plan oblique projection of city Front oblique projection of radio (Carlbom Fig. 2-4) 33 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Rules for placing projection plane for oblique views: projection plane should be chosen according to one or several of following: Parallel to most irregular of principal faces, or to one which contains circular or curved surfaces Parallel to longest principal face of object Parallel to face of interest Rules for Constructing Oblique Views Projection plane parallel to circular face Projection plane not parallel to circular face 34 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Cavalier: Angle between projectors and projection plane is 45. Perpendicular faces projected at full scale. Cabinet: Angle between projectors and projection plane: tan -1 (2) = 63.4 o. Perpendicular faces projected at 50% scale Main Types of Oblique Projections VPN DOP VPN DOP 35 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © A Desk in Parallel Multiview Orthographic CavalierCabinet 36 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Three main types of parallel projections: Orthographic: projectors orthogonal to projection plane, single face shown Axonometric: projection plane rotated relative to principle axes, reveals multiple faces Oblique: projectors intersect projection plane at oblique angle, revealing multiple faces, often more skewed representation, with a plane of interest undistorted 10/2/ / 45 Summary
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Used for: Fine Art Human visual system… Pros: Gives a realistic view and feeling for 3D form of object Cons: Does not preserve shape of object or scale (except where object intersects projection plane) Different from a parallel projection because Parallel lines not parallel to the projection plane converge Size of object is diminished with distance Foreshortening is not uniform Two understandings: Vanishing Point and View Point There are also oblique perspective projections (same idea as parallel oblique), we’ll see an example next lecture Perspective Projections If we were viewing this scene using parallel projection, the tracks would not converge 38 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Lines extending from edges converge to common vanishing point(s) For right-angled forms whose face normals are perpendicular to the x, y, z coordinate axes, number of vanishing points equals number of principal coordinate axes intersected by projection plane Vanishing Points (1/2) Three Point Perspective Two Point Perspective (z and x-axis vanishing points) One Point Perspective (z-axis vanishing point) z (z, x, and y-axis vanishing points) 39 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © What happens if same form is turned so its face normals are not perpendicular to x, y, z coordinate axes? New viewing situation: cube is rotated, face normals no longer perpendicular to any principal axes. Although projection plane only intersects one axis (z), three vanishing points created. Can still achieve final results identical to previous situation in which projection plane intersected all three axes. Vanishing Points (2/2) Unprojected cube depicted here with parallel projection Perspective drawing of the rotated cube 40 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Art employs the vanishing point idea while computer graphics uses the view point concept, where your view point is the location of the virtual camera (eye) Rays of light reflecting off of an object converge to the point of the viewer’s eye Lines representing light intersect the picture plane thus allowing points in a scene to be projected along the path of light to the picture plane (basis for ray tracing…stay tuned!) Concept of similar triangles described earlier applies here The Single Viewpoint 41 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © We’ve seen two pyramid geometries for understanding perspective projection: Vanishing Points and the View Point (1/4) 1.Perspective image is result of foreshortening due to convergence of some parallel lines toward vanishing points. 2.Perspective image is intersection of a plane with light rays from object to eye (COP) 42 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © We can combine the two: Vanishing Points and the View Point (2/4) 43 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Project parallel lines AB, CD on xy plane Projectors from eye to AB and CD define two planes, which meet in a line that contains the view point, or eye This line does not intersect projection plane (XY) because it’s parallel to it. Therefore, there is no vanishing point Vanishing Points and the View Point (3/4) 44 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Lines AB and CD (this time with A and C behind the projection plane) projected on xy plane: A’B and C’D Note: A’B not parallel to C’D Projectors from eye to A’B and C’D define two planes which meet in a line which contains the view point This line does intersect projection plane Point of intersection is vanishing point Vanishing Points and the View Point (4/4) 45 / 45 10/2/2014
CS123 | INTRODUCTION TO COMPUTER GRAPHICS Andries van Dam © Next Time: Projection in Computer Graphics 46 / 45 10/2/2014
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