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3079/GC31 3D Computer Graphics and Image Processing

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Lecturers David Swapp, d.swapp@cs.ucl.ac.ukd.swapp@cs.ucl.ac.uk Simon Arridge, s.arridge@cs.ucl.ac.uks.arridge@cs.ucl.ac.uk Simon Prince, s.prince@cs.ucl.ac.uks.prince@cs.ucl.ac.uk

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Course information http://www.cs.ucl.ac.uk/teaching/3079 Mailing lists –You need to register to one of the mailing lists: 3079@cs.ucl.ac.uk gc31@cs.ucl.ac.uk –Information on how to register http://www.cs.ucl.ac.uk/teaching/coursemail.htm

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Assessment Written Examination (2.5 hours, 80%) Coursework Section (4 pieces, 20%)

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Timetable Lecture Times – Tuesday10:00-11:00, MPEB 1.13 – Tuesday16:00-17:00, Foster Court 215a – Thursday11:00-12:00, MPEB 1.13 Lab Times – Thursday 09:00-10:00, MPEB 1.21 – Thursday 16:00-17:00, MPEB 1.21

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Course Book The book supporting the lectures is –Computer Graphics And Virtual Environments - From Realism to Real-Time. Mel Slater, Yiorgos Chrysanthou, Anthony Steed, ISBN 0201-62420-6, Addison-Wesley, 2002. http://www.cs.ucl.ac.uk/staff/a.steed/book_tmp/CGVE/

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Course content (1) Introduction –The painter's method Creating an image using ray tracing –Ray casting using a simple camera –Local illumination –Global illumination with recursive ray tracing Specifying a general camera –World / image coordinates –Creation of an arbitrary camera –Ray tracing with an arbitrary camera

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Course content (2) Constructing a scene –Definition of polyhedra –Scene hierarchy –Transformations of objects / rays –Other modelling techniques From ray tracing to projecting polygons –Transforming the polygons to image space –Sutherland-Hodgman clipping –Weiler-Atherton clipping

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Course content (3) Polygon rasterization /Visible surface determination –Scan conversion –Z-buffer –Interpolated shading –Texture mapping

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Introduction to 3D Graphics Lecture 1: Illusions and the Fine Art of Approximation

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Outline Anatomy of an Illusion –Environment –Light transport and interaction –Reception at the eye The Painters Method –Ray-casting –Approximations

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Environment A description of a space consisting of objects Objects have description and state Description consists of behaviour, geometry and appearance Geometry must be described relative to a co- ordinate frame State defines the object at a particular moment in time

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Radiometry - How does light propagate in the real world? 700nm 400nm pure light spectral distribution white light

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Life and Death of a Photon Emission Reflection Absorption

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Lighting is a Global Problem That is, if you consider any point in the environment, it receives light from all around

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Surfaces are Rarely Mirrors Specular surface Diffuse Surface

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Some Simplifying Assumptions Wavelength independence –No fluorescence Time invariance –No phosphorescence Light transport in a vacuum –No participating media Objects are isotropic –Reflectance characteristics are constant over the surface

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Photometry - How do we see light?

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Physiology of Eye Response 6 million cones in the fovea –cones sense red green or blue light –colour perception region is very small 120 million rods over the whole eye –peripheral vision –motion sensitive

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Colour Response Cones A = Red B = Green C = Blue

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Assumption for Real-Time Graphics Ignore real spectral distributions Instead calculate at three wavelengths, Red, Green and Blue that monitors provide Obviously this is a gross approximation –Really should find the spectrum for each point calculate the closest RGB value

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RGB Colour Model WHITE MAGENTA (1,0,1) R ED (1,0,0) BLACK G REEN (0,1,0) CYAN (0,1,1) B LUE (0,0,1) YELLOW (1,1,0)

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Colour Matching How much R,G,B do you need to make a particular pure colour?

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Outline About the Course Anatomy of an Illusion –Environment –Light transport and interaction –Reception at the eye The Painters Method –Ray-casting –Approximations

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Painting Through a Window COP = Centre of Projection

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Major Concepts of Graphics Separation of Scene Specification, Viewing and Rendering –Scene is modelled independent of any view –Views are unconstrained –There are many possible rendering methods given a scene and a view

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Major Concepts of Graphics View volume –The extent of the pixels on the screen and the COP define a pyramid –Clipping is the process of removing anything from the scene that is not in the view volume

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Major Concepts of Graphics Aliasing –Pixels are square and only sample the actual light

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Combating Aliasing Send several rays through each pixel –Stochastic sample –Regular sample (full-screen anti-aliasing) Stochastic sample is correct since it removes regularity But only regular sample is easy with the rendering pipeline

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Major Concepts of Graphics Perspective Projection –Image size depends on distance COP

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Major Concepts of Graphics Lighting –Ray-casting is the simple part –Determining the colour of the pixel is hard for all the reasons described earlier –Theoretically we have to calculate all incoming light –In practice we will consider only local illumination - light received directly from light sources

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Summary Taken a brief look at the general problem of doing visual simulation Reviewed the limits of human response Given an over-view of the simulation process and the concepts of –Scene, view, rendering –Aliasing –Projection –Lighting

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Future Work To Develop –Mathematics of scene description –Geometric descriptions –Lighting models –Move from ray-cast to forward projection Stages in the graphics pipeline

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