# Computer Graphics. Ray Tracing. Based on slides by Dianna Xu, Bryn Mawr College

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1 Computer Graphics Ray Tracing Based on slides by Dianna Xu, Bryn Mawr College

2 Ray Tracing Example Created by Anto Matkovic

3 Ray Tracing Example

4 Ray Tracing Example

5 Ray Tracing Most light rays do not reach the eye Start from eye point Cast one ray through each pixel Each ray either intersects or not If no intersection assign background color If intersection trace reflected, refracted, and shadow rays.

6 Simple Ray Tracing Algorithm Color trace(point p, Vector d, int step) { Color local, reflected, transmitted; Point q; Vector normal; if (step > max) return (background_color); q = intersect(p, d, status); if (status == no_intersection) return (background_color); normal = do_normal(q); Vector reflectdir = reflect(q, normal); Vector transdir = transmit(q, normal);

7 Simple Ray Tracing Algorithm // must check shadow ray local = phong(q, normal, reflectdir); reflected = trace(q, reflectdir, step+1); transmitted = trace(q, transdir, step+1); } return(local+reflected+trasmitted);

8 Ray Tracing with Several Ray Effects

9 Ray Tracing Necessary to Render Superellipsoids

10 Ray-Sphere Intersection A sphere is described by its center c and radius r. A point p on sphere: A ray is described by its origin o, and a direction vector d. A point on a ray has the form:

11 Ray-Sphere Substituting into sphere equation: Squaring both sides we get:

12 Quadratic Equation Rewrite as quadratic quation: where Number of solutions depends on discriminant:

13 The Discriminant 1. ==> no solution (ray misses) 2. ==> one solution (ray tangent) 3. ==> two solutions (ray punctures) Ray extends only in one direction (positive t) want closest intersection solution with least non-negative t.

14 The Result In case 1, report no insersection In case 2, report no intersection if t<0 ==> B>0, otherwise return solution (-B/ 2A) for t In case 3, return the least positive of the two solutions for t. If both negative, report no intersection If there is intersection, then the intersection point is o+td.

15 Ray-Triangle Intersection Compute intersection point of ray and the plane of the given triangle. If there is intersection, decide if the intersection point lies inside or outside of the triangle.

16 Ray-Plane Given vertices a1, a2, a3. Normal: A point p on the plane of triangle: Substituting Solves to

17 Point inside Triangle Check the cross product: If this cross product has the same direction as the normal then point p is on the same side of a1a2 as a3 Repeat for the other two edges.

18 Point inside Triangle boolean SameSide(Point p,point a1,point a2,point a3) { Vector cp1 = cp(a2-a1, p-a1); Vector cp2 = cp(a2-a1, a3-a1); if (dp(cp1, cp2)) >= 0 return true; else return false; } boolean PointInTriangle(Point p,point a,point b,point c) { if (SameSide(p,a,b,c)&& SameSide(p,b,a,c) && SameSide(p,c,a,b)) return true; else return false; }

19 Viewing Geometry for a Simple Ray Tracer Screen described by a point l its lower left corner. Two vectors (h and v) used to iterate over the raster positions on the target NDC space. Eye is given by e. Compute ray direction from eye to pixel (i,j)

20 Ray Direction To reach the lower left corner of desired pixel (i,j) from l : The move to the center of the pixel: Direction of ray is a vector from eye to center of desired pixel:

21 Simple Viewing Geometry P=(i/m,j/n) (1,1) U V l (0,0) H e

22 The Rays and the Outer Loops Iterate this for each pixel (integer) coordinate pair. for pixel(x,y) x = 0 to width-1 for pixel(x,y) y = 0 to height-1 { } R=(l+(i+0.5)/m*h+(j+0.5)/n*v)-e; color = raytrace(e, R, 0) pixel(x,y) = color;

23 Antialiasing Ray tracing is a sampling method, and is thus subject to aliasing errors. Over sample and then average Instead of one ray through the center of the pixel, shoot multiple rays just slightly off the original center (left, right, up, down), and average the results as the final output.

24 Antialiased Ray Tracing Adaptive Supersampling (Weighted sub-pixel averaging) D E F A B C Cast rays through pixel corners (not centers) Take average of corner values If one corner differs significantly from average, Subdivide, adding 5 corners Repeat for each sub-area, averaging to obtain area value D E F E K F E K F Value for pixel BCEF = A B C H I J B G C H I P J M N O B G C L

25 Local Reflection Model A simple model that can be computed rapidly The intensity of the reflected light as a function of: the orientation of the surface with respect to the position of a point light source and surface properties. Physical phenomena simulated Perfect specular reflection Imperfect specular reflection Perfect diffuse reflection

26 The Phong Model Has three components Diffuse Specular Ambient Uses four vectors To source (l) To viewer (v) Normal (n) Perfect reflector (r)

27 Ideal Reflector Normal is determined by local orientation Angle of incidence = angle of reflection The three vectors must be coplanar

28 Perfectly Diffuse Reflector Matte/Lambertian surface Light scattered equally in all directions Amount of light reflected is proportional to the vertical component of incoming light reflected light depends on cos θ cos θ = l n if vectors normalized

29 Specular Surfaces Most surfaces are neither ideal diffusers nor perfectly specular (ideal reflectors) Smooth surfaces show specular highlights due to incoming light being reflected in directions concentrated close to the direction of a perfect reflection specular highlight

30 Modeling Specular Reflections Phong proposed using a term that dropped off as the angle φ between the viewer and the ideal reflection increased I r ~ k s I s cos α φ reflected shininess coef intensity incoming intensity reflection coef φ

31 Specular Bump Adds to Diffuse Component Diffuse component (Viewer independent) Specular component (Viewer dependent)

32 The Shininess Coefficient As α increases, the reflected light is concentrated over a narrower region. Values of α between 100 and 200 correspond to metals, smaller values give surface that look like plastic cos α φ -90 φ 90

33 Ambient Light Ambient light is the result of multiple interactions between (large) light sources and the objects in the environment Appears to be uniform all over. Add k a I a to diffuse and specular terms reflection coef amount of ambient light

34 Distance Terms The amount of light from a point source that reaches a surface is inversely proportional to the square of the distance between them We can add a factor to the diffuse and specular terms The constant and linear terms soften the effect of the point source

35 Light Sources In the Phong Model, we add the results from each light source Each light source has separate diffuse, specular, and ambient terms Separate red, green and blue components Hence, 9 coefficients for each point light source

36 Material Properties Material properties match light source properties 9 reflection coefficients Shininess coefficient α

37 Adding up the Components For each light source and each color component, the Phong model can be written (without the distance terms) as For each color component we add contributions from all sources

38 Shadow Ray Trace ray from point of intersection back towards light source. If blocked by another surface before reaching light, then original point is in shadow only lit with ambient term If some surfaces are highly reflective (mirrors) Shadow ray will bounce off and continue This is usually done recursively

39 Reflection and Transmission If a ray hits a surface not in shadow Do reflection calculation according to Phong model Cast ray in direction of perfect reflection Cast ray in direction of transmitted ray

40 Transmission Vector

41 Transmission Vector (cont.) 1 We now have two equations and two unknowns! Some algebra and a look at the geometry gives us:

42 Ray Tracing with Recursion Add shadow rays to light sources. Stop at diffuse surface, or when maximum number of recursive steps exceeded. Color at visible point is established by recursively combining the shades computed at the end of each ray which depends of course, on what, if anything, it hits.

43 Recursive Ray Tracing If an eye ray intersects an object: Cast shadow rays to each light source. diffuse illumination shadows Cast reflection ray, search for object intersection. ambient illumination Cast transmission ray, search for object intersection. Transparency For each new reflection and transmission ray, repeat. Until no intersection Until N intersections (recursion depth) (3 N 7)

44 Ray Tracing Diagram (Diffuse) Yellow light Eye and pixel P Diffuse surface Solid surface Translucent surface Red light P is visible; illuminated by yellow light; but not by red light

45 Ray Tracing Diagram (Reflective) Yellow light Eye and pixel P Reflective surface Solid surface Translucent surface Red light P is visible; illuminated by yellow light; but not by red light

46 Typical Ray Traced Image

47 Refracted Ray Geometry Transmitting surface I N = Index of refraction of substance 1 Snell s Law: (I and T are coplanar) -N T = Index of refraction of substance 2

48 Refraction and the Critical Angle Different effect at the critical angle N Transmitting surface I T -N

49 Refraction and the Critical Angle Internal reflection effect beyond the critical angle N Transmitting surface I T -N

50 Index of Refraction Medium Water 1.33 Ethyl Alcohol 1.36 Carbon Bisulfide 1.63 Air Methylene Iodide 1.74 Fused Quartz 1.46 Glass, Crown 1.52 Glass, Dense Flint 1.66 Sodium Chloride 1.53 Index of Refraction Note: These are approximations. The index of refraction actually differs with each wavelength of light.

51 Reflection and Refraction

52 Closer to Real Glass

53 Ray Tracing Cost Considerations COST = X Y 2 a (m + 1) (2 n - 1) P X = number of pixels horizontally Y = number of pixels vertically a = anti-aliasing super-sampling factor m = number of (point) light sources n = tree (recursion) depth P = number of primitives tested for ray intersections if X=Y=1000, a=0, m=1, n=10, P=1000 then COST ~ 2,046,000,000,000 ray-primitive intersection tests!

54 Problems With Ray Tracing Ray/object intersection calculations become expensive with complex objects and more objects. Light from other objects (other than pure reflection) is not modeled. Still use the constant ambient light component. Shadows often appear too sharp Surfaces appear too glossy or smooth Images take a long time to render, must recalculate for each new view Have to tweak illumination model constants to get desired effect

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