# Summary. What is Computer Graphics? IRISA 2. Coordinate systems. 4. Geometric transformations for display

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1 Summary Basic Computer Graphics Kadi Bouatouch IRISA 1. Introduction 2. Coordinate systems 3. Geometric transformations 4. Geometric transformations for display 5. Choosing the camera parameters 6. Hidden surface removal 7. Lighting : Shading 8. Cast Shadows 9. Textures 1 2 Introduction Image Image = set of pixels Synthesis assign an intensity value (color) to each pixel Intensity depends on the used light sources, the location of the viewpoint and of the objects within the scene. pixel screen What is Computer Graphics? 3D models: Scene = set of different kinds polyhedron object surfaces wireframe Planar surfaces solids approximation with facets Non-planar surfaces quad parametric algebraic sphere solids primitives operations on volumes cube 3 4

2 What is Computer Graphics? 3D models: facets object = {planar facets} scenes = list of facets facet = list of vertices VERTICES Vertex parameters FACETS facet 1 facet 2 facet n Facet parameters Introduction The different processing: Geometric transformations. Clipping: Pyramidal view volume. Hidden surface removal. Cast shadows. Polygon filling. Transparent objects. Aliasing Texture mapping. 5 6 Introduction The Rendering Pipeline Illuminate Clip lp Project Rasterize ModelWorld WorldCamera Coordinate systems At least 3 coordinate systems: Word Coordinate System (o,x,y,z): in which is described the scene. y x scene Model & Camera Parameters Rendering Pipeline Framebuffer Display z 7 8

3 Coordinate systems View Coordinate System (oe,xe,ye,ze). z = depth axis. Screen Coordinate System: (os,xs,ys,zs) eye Ye Oe screen Xe Os Ys x Xs Ze, Zs depth Geometric transformations Interest: t enlarge a scene, translate t it, rotate t it for changing the viewpoint (also called camera). 2 ways for expressing a transformation: - with a matrix. - by composing transformations such as: - translation - Scaling - rotation 9 10 Geometric transformations Translation Scaling t x sx t y 0 s 0 0 y T(t x,t y,t z ) S ( s x, s y, sz ) t z 0 0 s z Geometric transformations Rotation ti One axis and one angle Expression: matrix Also composition of rotation around each axis of the coordinate system 11 12

4 Geometric transformations Rotations around axes X Y R y ( ) Z R z () cos sin 0 R x ( ) 0 sin cos cos 0 sin sin 0 cos cos sin 0 0 sin cos Geometric transformations Rotation around an axis : the sub-matrix A is orthogonal : At*A = I R = R11 R12 R13 0 R21 R22 R23 0 R31 R32 R33 0 = sub matrix A (3*3) Geometric transformations Geometric transformations Position the objects one with respect to the others the vase is on the commode Modeling without accounting for the final position Scene graph Group Node Object ation Appearance Geometry 15 16

5 Geometric transformations Geometric transformations Group Root Commode Vase Flower Group Root Commode Vase Flower Geometric transformations for rendering Projections Plane projection = transformation which associates a screen point with a scene point It is defined by: a centre of projection a projection plane Two types : perspective p and parallel screen Oe P' Perspective projection P Ze, Zs Oe Àto infinity P' Parallel projection P Ze, Zs Geometric transformations for rendering P'(xp P(xp, yp, d) = projection of P(x, y, z) d = focal distance We get: yp = d * y / z et xp = d * x / z Oe Ye d Xe P P' Ze, Zs M per = /d screen 19 20

6 Geometric transformations for rendering Homogeneous coordinates Homo. Coord. : (X, Y, Z, W) = (x, y, z, 1) * Mper We get: (X, Y, Z, W) = (x, y, z, z / d) Perspective projection of P : (X/W, Y/W, Z/W, 1) = (xp, yp, zp, 1) = (x * d / z, y * d / z, d, 1) Oe Ye d Xe P P' Ze, Zs M per = 0011/d 0 1/d screen Geometric transformations for rendering Clipping Visible objects: inside the view pyramid Made up of 6 planes Objects whose only a part lies whin the pyramid are clipped by the 6 planes Xe Ye View pyramid P' screen b b P Ze Geometric transformations for rendering Clipping Visible objects: inside the view pyramid Made up of 6 planes Objects whose only a part lies whin the pyramid are clipped by the 6 planes Ye P Ze Choosing the camera parameters Resolution : Resx x Resy (Nbr of columns) x (Nbre of rows) COP = Center Of Projection (observer). VRP = View Reference Point (targetetted point). VPN = View Point Normal (screen normal). VUP = View Up Vector d = focal distance Y 2 h Re sx h (tg / 2) d l Resy 2 Ecran VUP h h l COP Z y VRP Xe (front plane) screen (back plane) 23 x d Screen 24

7 Introduction: Rasterization Rasterizing: Polygons Model & Camera Parameters Illuminate Clip lp Project Rasterize Rendering Pipeline ModelWorld WorldCamera Framebuffer Display In interactive ti graphics, polygons rule the world Two main reasons: Lowest common denominator for surfaces Can represent any surface with arbitrary accuracy Splines, mathematical functions, volumetric isosurfaces Mathematical simplicity lends itself to simple, regular rendering algorithms Like those we re about to discuss Such algorithms embed well in hardware Rasterizing: Polygons Triangle is the minimal unit of a polygon All polygons can be broken up into triangles Triangles are guaranteed to be: Planar Convex Rasterizing: Triangulation Convex polygons easily triangulated (Delaunay) Concave polygons present a challenge 27 28

8 Rasterizing Triangles Interactive graphics hardware commonly uses edge walking or edge equation techniques for rasterizing triangles Rasterization: Edge Walking Basic idea: Draw edges vertically Interpolate colors down edges Fill in horizontal spans for each scanline At each scanline, interpolate edge colors across span Rasterization: Edge Walking Order three triangle vertices in x and y Find middle point in y dimension and compute if it is to the left or right of polygon. Also could be flat top or flat bottom triangle We know where left and right edges are. Proceed from top scanline downwards Fill each span Until breakpoint or bottom vertex is reached Rasterization: Edge Equations An edge equation is simply the equation of the line defining that edge Q: What is the implicit equation of a line? A: Ax + By + C = 0 Q: Given a point (x,y), what does plugging x & y into this equation tell us? A: Whether the point is: On the line: Ax + By + C = 0 Above the line: Ax + By + C > 0 Below the line: Ax + By + C <

9 Rasterization: Edge Equations Edge equations thus define two halfspaces: Rasterization: Edge Equations And a triangle can be defined as the intersection of three positive half-spaces: Rasterization: Edge Equations So simply turn on those pixels for which all edge equations evaluate to > 0: Rasterization: Using Edge Equations Which h pixels: compute min,max bounding box Edge equations: compute from vertices 35 36

10 Rasterization: Edge Equations: Code Basic structure t of code: Setup: compute edge equations, bounding box (Outer loop) For each scanline in bounding box... (Inner loop) check each pixel on scanline, evaluating edge equations and drawing the pixel if all three are positive Hidden Surface Removal Z-buffering Illuminate Clip lp Project Rasterize ModelWorld WorldCamera Z-buffering Model & Camera Parameters Rendering Pipeline Framebuffer Display Hidden Surface Removal Back Face Culling & Clipping Back Face Culling Simple test Normal: N View direction: V Dot produit: V N Clipping Real-time Hidden Surface Removal Z-buffering Z-Buffer (Catmull in 1974) Depth memory for each pixel Two 2D arrays Frame buffer for intensities (colors): FB [i] [j] Z-buffer for depths (z coordinate) ZB [] [i] [j] Facets (triangles) are processed without any ordering 39 40

11 Hidden Surface Removal Z-buffering algorithm Z-Buffer () begin for (for pixels i,j do) FB [i][j] back plane s color ; ZB [i][j] z (back plane) endfor for (all facets) do for (all pixels within the projection of the facet) do compute_intensity_and_z for all pixels using interpolation if (zfrontplane z of polygone at point i,j ZB [i][j]) then ZB [i][j] z of facet at pixel (,j) (i,j) FB [i][j] color of facet at (i,j) endif endfor endfor end Lighting Lighting Lighting: Illumination Lighting vertices Illuminate ModelWorld WorldCamera How do we compute radiance for a sample ray? Clip lp Project Rasterize Interpolative Shading Model & Camera Parameters Rendering Pipeline Framebuffer Display 43 Angel Figure

12 Lighting: Goal Must derive computer models for... Emission at light sources Scattering at surfaces Receptionatthecamera the camera Desirable features Concise Efficient to compute Accurate Lighting: Overview Direct (Local) Illumination Emission at light sources Scattering at surfaces Global l illuminationi Shadows Refractions Inter-object reflections 45 Direct Illumination 46 Lighting: Modeling Light Sources Lighting: Ambient Light Sources I L (x,y,z,)... describes the intensity of energy, leaving a light source, arriving at location(x,y,z),... from direction (),... with wavelength Light (x,y,z) 47 Objects not directly lit are typically still visible ibl e.g., the ceiling in this room, undersides of desks This is the result of indirect illumination from emitters, bouncing off intermediate surfaces Too expensive to calculate (in real time), so we use a hack called an ambient light source No spatial or directional characteristics; illuminates all surfaces equally Amount reflected depends on surface properties 48

13 Lighting: Ambient Light Sources For each sampled wavelength (R, G, B), the ambient light reflected from a surface depends on The surface properties, k ambient The intensity, I ambient, of the ambient light source (constant t for all points on all surfaces ) I reflected = k ambient I ambient Lighting: Ambient Light Sources A scene lit only with an ambient light source: Light Position Not Important Viewer Position Not Important Surface Angle Not Important Lighting: Ambient Term Represents reflection of all indirect illumination Lighting: Directional Light Sources For a directional light source we make simplifying assumptions Direction is constant for all surfaces in the scene All rays of light from the source are parallel As if the source were infinitely far away from the surfaces in the scene A good approximation to sunlight This is a total hack (avoids complexity of global illumination)! 51 The direction from a surface to the light source is important in lighting the surface 52

14 Lighting: Directional Light Sources The same scene lit with a directional and an ambient light source Lighting: Point Light Sources A point light source emits light equally in all directions from a single point The direction to the light from a point on a surface thus differs for different points: l So we need to calculate a normalized vector to the light source for every point we light: p Lighting: Other Light Sources Spotlights are point sources whose intensity falls off directionally. Requires color, point direction, falloff parameters Supported by OpenGL Lighting: Other Light Sources Area light sources define a 2-D emissive surface (usually a disc or polygon) Good example: fluorescent light panels Capable of generating soft shadows (why? ) 55 56

15 Lighting: Overview Direct (Local) Illumination Emission at light sources Scattering at surfaces Global l illuminationi Shadows Refractions Inter-object reflections Lighting: Modeling Surface Reflectance R s (,)... describes the amount of incident energy, arriving from direction (),... leaving in direction (,), with wavelength γ,) Surface () () Direct Illumination Lighting: Empirical Models Ideally measure radiant energy for all combinations of incident angles Too much storage Difficult in practice ) () Lighting: The Physics of Reflection Ideal diffuse reflection An ideal diffuse reflector, at the microscopic level, is a very rough surface (real-world example: chalk) Because of these microscopic variations, an incoming ray of light is equally likely to be reflected in any direction over the hemisphere: Surface 59 What does the reflected intensity depend on? 60

16 Lighting: Diffuse Reflection How much light is reflected? Depends on angle of incident light Surface Lighting: Lambert s Cosine Law Ideal diffuse surfaces reflect according to Lambert s cosine law: The energy reflected by a small portion of a surface from a light source in a given direction is proportional to the cosine of the angle between that direction and the surface normal These are often called Lambertian surfaces Note that the reflected intensity is independent of the viewing direction, but does depend on the surface orientation with regard to the light source Lighting: Lambert s Law Lighting: Computing Diffuse Reflection The angle between the surface normal and the incoming light is the angle of incidence: l n I diffuse = k d I light cos In practice we use vector arithmetic: I diffuse = k d I light (n l) diffuse d light 63 64

17 Lighting: Diffuse Lighting Examples We need only consider angles from 0 to 90 (Why?) y A Lambertian sphere seen at several different lighting angles: Lighting: Specular Reflection Shiny surfaces exhibit specular reflection Polished metal Glossy car finish A light shining on a specular surface causes a bright spot known as a specular highlight 65 Where these highlights hli ht appear is a function of the viewer s position, so specular reflectance is view dependent 66 Lighting: The Physics of Reflection Lighting: The Optics of Reflection At the microscopic level a specular reflecting surface is very smooth Thus rays of light are likely l to bounce off the microgeometry in a mirror-like fashion The smoother the surface, the closer it becomes to a perfect mirror Reflection follows Snell s Laws: The incoming ray and reflected ray lie in a plane with the surface normal The angle that the reflected ray forms with the surface normal equals the angle formed by the incoming ray and the surface normal: (l)ight = (r)eflection 67 68

18 Lighting: Specular Reflection Lighting: Geometry of Reflection Reflection is strongest near mirror angle Examples: mirrors, metals N R N L L R N (L) L R 69 L = R 70 Lighting: Geometry of Reflection Lighting: Geometry of Reflection 2(N. L)N N (N. L)N N L cos( i )N R N (L) L R N (L) L R L R L = R 71 L = R 72

19 Lighting: Geometry of Reflection Lighting: Geometry of Reflection 2(N. L)N N L 2(N. L)N N L L R N (L) L R N (L) L R L R L = R 73 L = R 74 Lighting: Non-Ideal Specular Reflectance Snell s law applies to perfect mirror-like surfaces, but aside from mirrors (and chrome) few surfaces exhibit perfect specularity How can we capture the softer reflections of surface that are glossy rather than mirror-like? One option: model the microgeometry of the surface and explicitly bounce rays off of it Or 75 Lighting: Non-Ideal Specular Reflectance An Empirical Approximation Hypothesis: most light reflects according to Snell s Law But because of microscopic i surface variations, some light may be reflected in a direction slightly off the ideal reflected ray Hypothesis: s as we move from the ideal reflected ray, some light is still reflected 76

20 Lighting: Non-Ideal Specular Reflectance An illustration of this angular falloff: How might we model this falloff? 77 Lighting: Phong Lighting The most common lighting model in computer graphics was suggested by Phong: I specular ksilight cos n shiny The n shiny term is a purely empirical constant that varies the rate of falloff Though this model has no physical basis, it works (sort of) in practice v 78 Lighting: Calculating Phong Lighting The cos term of Phong lighting can be computed using vector arithmetic: I specular ksi light v r n shiny v is the unit vector towards the viewer r is the ideal reflectance direction v Lighting: Phong Examples These spheres illustrate the Phong model as l and n shiny are varied: shiny 79 80

21 Lighting: Combining Everything Simple analytic model: diffuse reflection + specular reflection + emission + ambient Lighting: Combining Everything Simple analytic model: diffuse reflection + specular reflection + emission + ambient Surface Surface Lighting: OpenGL Reflectance Model Sum diffuse, specular, emission, and ambient Lighting: The Final Combined Equation Single light source: Viewer R N L V 83 I I K I K ( N L ) I K ( V R ) E A AL D L S n I L 84

22 Lighting: Final Combined Equation Lighting: Examples Multiple light sources: N Viewer L 1 L2 V Paramètres : K d =0.25 K s =0.75 n=50.0 I I E K A I AL i ( K D ( N L ) I i i K S ( V R ) i n I ) i Lighting: Examples Lighting: Examples Paramètres : K d =0.25 K s =0.75 n=200.0 Paramètres : K d =0.25 K s =0.75 n=

23 Lighting: Examples Lighting: Examples Paramètres : K d =0.25 K s =0.25 n=50.0 Paramètres : K d =0.75 K s =0.25 n= Lighting: interpolative shading Lighting Smooth D object representations: Quadrics superquadrics splines Computing surface normals can be very expensive Interpolative shading: approximate curved objects by polygonal meshes, compute a surface normal that varies smoothly from one face to the next computation can be very cheap Many objects are well approximated by ypolygonal meshes silhouettes are still polygonal Done: rasterization step Lighting vertices Model & Camera Parameters Illuminate Clip lp Project Rasterize Rendering Pipeline ModelWorld WorldCamera Interpolative Shading Framebuffer Display 91 92

24 Lighting: interpolative shading Two kinds of interpolative shading Gouraud Shading: cheap but gives poor highlights. Phong Shading: slightly more expensive but gives excellent highlights. hli ht Lighting: interpolative shading Vertex Normals All interpolative shading methods rely on vertex normals. A vertex normal is the average of the normals of all of the faces sharing that vertex Lighting: g Gouraud Shading Lighting Poor Highlights from Gouraud Sading 95 96

25 Lighting: Phong Shading Interpolate the normals instead of the RGB values. Compute normals at each vertex A B and C. Compute the normals at P 1 and P 2 By interpolation using the normals from A and B and C and B. Compute the normal at P By interpolating the normals from P 1 and P 2. Compute RGB values at P Using Phong s rule. Lighting: Phong Shading Interpolating the normals Cast Shadows: Shadow Map Two Z-buffers: one from the light source and another from the viewer Cast Shadows Source eye Front Plane Light B Zl Z Back plane Light

26 Cast Shadows: Shadow Map Compute an image as seen by the point light source Compute an image as seen by the eye Let (X,Y,Z) be a point seen through a pixel (x,y) of the camera screen Let t(xlylzl)b (Xl,Yl,Zl) be the coordinates of fthis point tin the source coordinate system and (xl,yl) the coordinates of the associated pixel on the source s s screen If Zl > Z_Buffer_Ecl[xl][yl] then the point is shadowed Else the point is lit and we compute its intensity. Texture Mapping Vertex texture coordinates Model & Camera Parameters Introduction The Rendering Pipeline Illuminate Clip lp Project Rasterize Rendering Pipeline ModelWorld WorldCamera Texture mapping Framebuffer Display 103 Texture Mapping Texture = 2D Image (we do not consider 3D textures) Texture : represented by a 2D array of RGB triplets Triplet: Color, Normal or any other thing Normalized texture space: (u,v), u et v ranging from 0 to 1 For an intersection point P: compute its texture coordinates either directly or by interpolation y D x 3D space C P A B x v (0,0) (0,1) (1,0) (1,1) Texture space u 104

27 Texture Mapping Texture Mapping Polygon: Triangle or quadrilateral l Each vertex of the 3D polygon is assigned texture coordinates For an intersection point P: compute its texture by bilinear interpolation of the texture coordinates of the polygon s vertices y D x 3D space C P A B x v (0,0) (0,1) (1,0) (1,1) Texture space u Texture Mapping Texture Mapping: Interpolation Specify a texture coordinate (u,v) at each vertex Canonical texture coordinates (0,0) (1,1) Specify a texture coordinate (u,v) at each vertex Interpolate the texture values of intersection points lying on the polygon using those of its vertices

28 Texture Mapping: Interpolation Texture Mapping s Texture -> Object ation (u 0,v 0 ) (u 1,v 1 ) Common texture coordinate mapping planar Cylindrical Spherical s = [0 1] t t = [0 1] u = [0 1] v = [0 1] (u 3,v 3 ) (u 2,v 2 ) Planar: Triangle Texture Mapping Barycentric coordinates: a way to parameterize a triangle with P 0, P 1 and P 2 as vertices Let P be a point on the triangle and 0, 1, 2 its barycentric coordinates -Thus: P 0P0 1P1 2P2 P Since: ( 1 1 2) P0 1P1 2P2 Planar: Triangle Texture Mapping Given an intersection P lying on a triangle Compute its texture coordinates (s,t), say (β 1,ββ 2 ) by solving the following linear system: P P P Unknowns: ( 1 1 2) P0 1P1 2P2 0 2 P2 P1, P1 P0 1 1 and

29 Planar: Quadrilateral Texture Mapping Given an intersection P lying on a quadrilateral P(u,v)=(1-u)(1-v)P 0 + u(1-v)p1 + uvp 2 + (1-u)vP 3 Compute its texture coordinates (s,t) by solving the following linear system (for each coordinate): P=(1-u)(1-v)P 0 + u(1-v)p1 + uvp 2 + (1-u)vP 3 Unknowns: u and v Texture Mapping Cylinder P(x,y,z): point on the cylinder (s,t): texture coordinates [ A, B ], z [ z A, z B ] xrcos, yrsin, z [ za, z A s B A z t zb z za A ] B Texture Mapping Cylinder How to computetexture coordinates Given an intersection P lying on a cylinder Compute its texture coordinates (s,t) as: s ( a cos( x / r ) A ) /( B A ) t ( z z A ) /( z B z A ) Texture Mapping Sphere P=(x (x,y,z) on the sphere (s,t): texture coordinates x r sin cos, y r sin sin, [ 0, ], [0,2 ] z rcos s, t

30 Sphere Texture Mapping How to compute Texture coordinates Given an intersection P lying on a sphere Compute its texture coordinates (s,t) as: s a cos ( z / r ) / t a cos (x/( r sin ( s))) / 2 Texture Mapping & Illumination Texture mapping can be used to alter some or all of the constants in the illumination equation: pixel color, diffuse color, alter the normal,. Classical texturing: diffuse color k d changes over a surface and is given by a 2D texture which is an image I nblum local Ii 2 i0 d i vis( i) k d d N L k R V i s i n Bump Mapping Use textures to alter the surface normal Does not change the actual shape of the surface Just shaded as if it was a different shape Bump Mapping Add more realism to synthetic images without adding a lot of geometry

31 Bump Mapping Bump Mapping Normal of bumped surface, so-called perturbed normal: Derivation can be found in Simulation of Wrinkled Surfaces James F. Blinn SIGGRAPH 78 Proceedings, pp , 1978 (Pioneering paper...) Use texture to store either: perturbed normal map bump map itself Bump Mapping The light at each point depends on the normal at that point. Take a smooth surface and perturb it with a function B. But we don t really yp perturb that surface (that is not displacement mapping). We modify the normals with the function B(u,v), measuring the displacement of the irregular surface compared to the ideal one. we are only shading it as if it was a different shape! This technique is called bump mapping. The texture map is treated as a single-valued height function. The value of the function is not actually used, just its partial derivatives. Bump Mapping The partial derivatives tell how to alter the true surface normal at each point on the surface to make the object appear as if it was deformed by the height function

32 Bump Mapping Choice of function B(u,v) Blinn has proposed various techniques: B(u,v) defined analytically as a polynomial with 2 variables or a Fourier serie (very expensive approach) B(u,v) defined by 2-entry table (poor results, requires large memory) B(u,v) defined by 2-entry table smaller and an interpolation ti is performed to find in-between values Bump Mapping Treat the texture as a single- valued height function Compute the normal from the partial derivatives in the texture

33 Bump Mapping There are no bumps on the silhouette of a bump-mapped object Bump maps don t allow self-occlusion or self- shadowing Problem solved with Displacement Mapping Displacement Mapping Use the texture t map to actually move the surface point along the normal to the intersected point. The geometry must be displaced before visibility is determined, which is different from bump mapping Displacement Mapping Compute intersection ti between ray and bounding volume Result: points A and B Height (or depth) is stored in a texture Use a search technique for the first intersection point: here point 3 Flat surface Bounding volume 131 Displacement Mapping A has a depth vale of 0 and B has a depth value of 1.0. At each step, compute the midpoint of the current interval and assign it the average depth and texture coordinates of the end points. (used to access the depth map). If the stored depth is smaller than the computed value, the point along the ray is inside id the height-field surface (point 1). In this case it takes three iterations to find the intersection between the height-field and the ray However, the binary search may lead to incorrect results if the viewing ray intersects the height-field surface more than once 132

34 Displacement Mapping Displacement Mapping However, the binary search may lead to incorrect results if the viewing ray intersects the height-field surface more than once: In this situation, since the value computed at 1 is less than the value taken from the height-field, the search will continue down. In order to remedy this, the algorithm starts t with a linear search The technique can also handle surface self-shadowing: We must decide if the light ray intersects the height-field surface between point C and the point where the viewing ray first hits the surface Displacement Mapping Image from: Geometry Caching for Ray-Tracing Displacement Maps by Matt Pharr and Pat Hanrahan. note the detailed shadows cast by the stones Displacement Mapping Bump Mapping combined with texture

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