Study Guide 4 Fall 2013 1 What is projective geometry Explain how foreshortening and size scaling help us interpret scenes in 3 dimensions a Projective Geometry the geometry that describes the transformations that occur when the three dimensional world is projected onto a two dimensional surface It investigates the mathematical relationships between objects in the environment and their optical projections on the retina or a picture b Foreshortening The changes in surface orientation cause projected shapes to change It helps us interpret 3D by the use of perspective to represent in art the apparent visual contraction of an object that extends back in space at an angle to the perpendicular plane of sigh c Size scaling changes is surface depth cause projected size to change 2 What are the 4 perceptual biases the visual system relies on to resolve ambiguities in mapping 2 D visual information into 3 D perception Give an explicit example of each a Unless there is information to the contrary objects will be perceived as resting on the ground Example Information from shadows or indirect illumination can override the bias to perceive objects to be in contact with the ground b There is a strong bias to interpret scenes such that depth increases with height in the viewing plane Example c Convex interpretations are preferred over concave ones d Example Circles with shadow on bottom convex Interpretations involving generic views are preferred over those that require a specific vantage point Example Rock sculpture in Blackhawk CO 3 What is an Ames Room and what is the underlying principle that causes the objects within it to have illusory sizes a A trapezoidal room that creates an optical illusion with one large and one small person Specially designed room that is trapezoidal in shape but appears to be rectangular from a viewpoint This room gives misleading visual cues that lead people to believe that two similar sized objects are of different sized depending on their location in the room 4 What are generic views Accidental views Non accidental properties What relevance do they have in the study of 3D structure Give a specific example a Generic views Stipulates that the interpretation made by an observer of a distal phenomenon should be such as to not require that the observer be in a special position to or relationship with the phenomenon in question b Accidental views If you look straight on at a box you see a square If you look at a bottle you see a circle There are 26 combinations of ways you can define objects Require a specific vantage point c Non accidental properties properties of an images such a co linearity co termination or parallelism that seldom occur by accident within optical projections Thus is lines in an image are parallel or co terminate they will be interpreted perceptually as if they are parallel or co terminating in the 3D environment Example Parallel lines and vertices implicit knowledge of environment regularities facilitates viewpoint invariant recognition 5 What was Brunelleschi s Panel What is the depth cue on which it was based a Brunelleschi s Panel is a two panel painting illustrating geometric optical linear perspective b Brunelleschi painted a picture of a baptistery on silver He drilled a hole into the painting and help up a mirror and the Italians couldn t tell the difference between the real and painted building through the pinhole c Depth cue he exploited linear perspective the idea being there is a vanishing point where al the parallel lines meet up 6 What is anamorphic art Give three examples a Distorted projection or perspective requiring the viewer to use special devices or occupy a specific vantage point to reconstruct the image Anamorphosis means to form again b Three examples are Julian Beever English chalk artist Orosz mirror cylinder on top of finger painting Varlini yellow square on picture continuous contours Cinemaschope Panavision technirama 7 How in principle could the visual system use ocular motor cues to deduce the relative distances of objects from the viewer a Visual systems in principle could use ocular motor cues to deduce the relative distances of objects from the viewer by convergence and accommodation b There are cues based on the ability to sense the position of our eyes and the tension in the eye muscles 8 What is linear perspective and how can the visual system use this cue to deduce relative depth from the observer What is the horizon ratio a Linear perspective is a depth cue based on the fact that lines that are parallel in the three dimensional world will appear to converge in a two dimensional image a type of perspective used by artists in which the relative size shape and position of objects are determined by drawn or imagined lines converging at a point on the horizon Convergence on a vanishing point b Horizon ratio All objects of the same height whatever their distance from the observer have 1 3 above the horizon and 2 3 below If the position on the horizon is the same then we perceive objects as the same size principle states that if a person is standing on flat terrain the place where the horizon intersects the object will be one eye height above the ground 9 How do special effects editors create the perception of giant or diminutive people in movies a Forced perspective employs the optical illusion to make an object appear on a different scale b Manipulates human visual perception through the use of scaled objects and the correlation between them and the vantage point of the camera c Displacement of objects for one to be closer to the camera and 10 What is meant by surface texture and how can it be used to therefore look bigger define 3 D structure a Surface texture is the local deviations of surface from a perfectly flat plane The measure of the surface texture is generally determined in terms of its roughness waviness and form 11 What is chiaroscuro What is bas relief a Chiaroscuro light dark renaissance shading shading with orientation of surface to light source not viewer Consistent lighting direction Use of light to unify the scene and give 3D appearance to object creates emotional effect Usually the use of strong contrasts between light and dark usually bold contrasts affecting a whole composition b Bas relief a type of sculpture in which the relief of the depicted objects is much flatter than would ordinarily be the case Note however this has relatively little effect on the perceived 3D structure Usually a kind