# Ray Casting. Outline. Similar to glulookat derivation. Foundations of Computer Graphics

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1 Foundations of omputer Graphics Online Lecture 10: Ray Tracing 2 Nuts and olts amera Ray asting Outline amera Ray asting (choose ray directions) Ravi Ramamoorthi Outline in ode Image Raytrace (amera cam, Scene scene, int width, int height) { Image image = new Image (width, height) ; for (int i = 0 ; i < height ; i++) for (int j = 0 ; j < width ; j++) { Ray ray = RayThruPixel (cam, i, j) ; Intersection hit = Intersect (ray, scene) ; image[i][j] = Findolor (hit) ; return image ; Virtual Viewpoint Ray asting Virtual Screen Objects Ray Multiple misses intersects intersections: all object: objects: shade Pixel Use using colored closest color, black one lights, (as does materials OpenGL) Finding Ray Direction Goal is to find ray direction for given pixel i and j Many ways to approach problem Objects in world coord, find dirn of each ray (we do this) amera in canonical frame, transform objects (OpenGL) asic idea Ray has origin (camera center) and direction Find direction given camera params and i and j Similar to glulookt derivation glulookt(eyex, eyey, eyez, centerx, centery, centerz, upx, upy, upz) amera at eye, looking at center, with up direction being up Eye Up vector amera params as in glulookt Lookfrom[3], Lookt[3], up[3], fov From earlier lecture on deriving glulookt enter 1

2 onstructing a coordinate frame? We want to associate w with a, and v with b ut a and b are neither orthogonal nor unit norm nd we also need to find u onstructing a coordinate frame We want to position camera at origin, looking down Z dirn Hence, vector a is given by eye center The vector b is simply the up vector Up vector Eye From basic math lecture - Vectors: Orthonormal asis Frames enter anonical viewing geometry anonical viewing geometry -w αu βv -w αu βv anonical viewing geometry -w αu βv Foundations of omputer Graphics Online Lecture 10: Ray Tracing 2 Nuts and olts Ray-Object Intersections Ravi Ramamoorthi 2

3 Outline amera Ray asting (choosing ray directions) Outline in ode Image Raytrace (amera cam, Scene scene, int width, int height) { Image image = new Image (width, height) ; for (int i = 0 ; i < height ; i++) for (int j = 0 ; j < width ; j++) { Ray ray = RayThruPixel (cam, i, j) ; Intersection hit = Intersect (ray, scene) ; image[i][j] = Findolor (hit) ; return image ; Substitute P 0 Substitute Substitute Simplify Simplify 3

4 Solve quadratic equations for t 2 real positive roots: pick smaller root oth roots same: tangent to sphere One positive, one negative root: ray origin inside sphere (pick + root) omplex roots: no intersection (check discriminant of equation first) Intersection point: Normal (for sphere, this is same as coordinates in sphere frame of reference, useful other tasks) Ray-Triangle Intersection One approach: Ray-Plane intersection, then check if inside triangle Plane equation: Ray-Triangle Intersection One approach: Ray-Plane intersection, then check if inside triangle Plane equation: Ray-Triangle Intersection One approach: Ray-Plane intersection, then check if inside triangle Plane equation: Ray-Triangle Intersection One approach: Ray-Plane intersection, then check if inside triangle Plane equation: ombine with ray equation 4

5 Ray inside Triangle Once intersect with plane, need to find if in triangle Many possibilities for triangles, general polygons We find parametrically [barycentric coordinates]. lso useful for other applications (texture mapping) α β P γ Ray inside Triangle β α P γ Other primitives Much early work in ray tracing focused on ray-primitive intersection tests ones, cylinders, ellipsoids oxes (especially useful for bounding boxes) General planar polygons Many more Ray Scene Intersection Intersection (ray, scene) { mindist = infinity; hitobject = NULL ; For each object in scene { // Find closest intersection; test all objects t = Intersect (ray, object) ; if (t > 0 && t < mindist) // closer than previous closest object mindist = t ; hitobject = object ; return IntersectionInfo(mindist, hitobject) ; // may already be in Intersect() Outline amera Ray asting (choosing ray directions) Ray-Tracing Transformed Objects We have an optimized ray-sphere test ut we want to ray trace an ellipsoid Solution: Ellipsoid transforms sphere pply inverse transform to ray, use ray-sphere llows for instancing (traffic jam of cars) Same idea for other primitives 5

6 Transformed Objects onsider a general 4x4 transform M (matrix stacks) pply inverse transform M -1 to ray Locations stored and transform in homogeneous coordinates Vectors (ray directions) have homogeneous coordinate set to 0 [so there is no action because of translations] Do standard ray-surface intersection as modified Transform intersection back to actual coordinates Intersection point p transforms as Mp Normals n transform as M -t n. Do all this before lighting Foundations of omputer Graphics Online Lecture 10: Ray Tracing 2 Nuts and olts Lighting alculations Ravi Ramamoorthi Outline amera Ray asting (choosing ray directions) Outline in ode Image Raytrace (amera cam, Scene scene, int width, int height) { Image image = new Image (width, height) ; for (int i = 0 ; i < height ; i++) for (int j = 0 ; j < width ; j++) { Ray ray = RayThruPixel (cam, i, j) ; Intersection hit = Intersect (ray, scene) ; image[i][j] = Findolor (hit) ; return image ; Shadows Light Source Shadows: Numerical Issues Numerical inaccuracy may cause intersection to be below surface (effect exaggerated in figure) ausing surface to incorrectly shadow itself Move a little towards light before shooting shadow ray Virtual Viewpoint Virtual Screen Objects Shadow ray to to light is is unblocked: object visible in shadow 6

7 Similar to OpenGL Lighting Model Lighting model parameters (global) mbient r g b ttenuation const linear quadratic Per light model parameters Directional light (direction, RG parameters) Point light (location, RG parameters) Some differences from HW 2 syntax Material Model Diffuse reflectance (r g b) Specular reflectance (r g b) Shininess s Emission (r g b) ll as in OpenGL Shading Model Global ambient term, emission from material For each light, diffuse specular terms Note visibility/shadowing for each light (not in OpenGL) Evaluated per pixel per light (not per vertex) Foundations of omputer Graphics Online Lecture 10: Ray Tracing 2 Nuts and olts Recursive Ray Tracing Ravi Ramamoorthi Outline amera Ray asting (choosing ray directions) Mirror Reflections/Refractions Virtual Viewpoint Virtual Screen Generate reflected ray in mirror direction, Get reflections and refractions of objects Objects 7

8 asic idea For each pixel Trace Primary Eye Ray, find intersection Trace Secondary Shadow Ray(s) to all light(s) olor = Visible? Illumination Model : 0 ; Trace Reflected Ray olor += reflectivity * olor of reflected ray Recursive Shading Model Highlighted terms are recursive specularities [mirror reflections] and transmission (latter is extra) Trace secondary rays for mirror reflections and refractions, include contribution in lighting model Getolor calls RayTrace recursively (the I values in equation above of secondary rays are obtained by recursive calls) Problems with Recursion Reflection rays may be traced forever Generally, set maximum recursion depth Same for transmitted rays (take refraction into account) Some basic add ons rea light sources and soft shadows: break into grid of n x n point lights Use jittering: Randomize direction of shadow ray within small box for given light source direction Jittering also useful for antialiasing shadows when shooting primary rays More complex reflectance models Simply update shading model ut at present, we can handle only mirror global illumination calculations 8

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