Lecture16-RayTracingIntersections

# Lecture16-RayTracingIntersections - CS 455 Computer...

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CS 455 – Computer Graphics Ray-Object Intersections

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Implicit and Parametric Forms Implicit form § f(x,y) = 0 example: y – 3x + 2 = 0 is a line. § The curve/line/plane/whatever exists at values of f(x,y) that equal 0. § To find the curve/line/plane/whatever plug in values of x,y into f and see if they are 0. § f(x,y) defines a terrain and altitude = 0 is the curve. Parametric form § Looks like § Think of a particle with position at time t given by g(t), h(t). = ) ( ) ( t h t g y x
Implicit form of a ray

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Math Problem of the Day Given a line in implicit form § y = mx + b Convert it to the form § Ax + By + C = 0 Such that the equation is valid for lines that § Pass through origin § Are vertical § Are horizontal
Points and Planes Equation of a plane Plane Normal Distance from Plane to Origin Distance from point To Plane Given Points Distance between points: ( 29 ( 29 ( 29 2 1 2 2 1 2 2 1 2 2 1 ) , ( z z y y x x p p dist - + - + - = 0 = + + + D Cz By Ax ( 29 C B A , , D ( 29 1 1 1 , , z y x p = 0 = + + + D Cz By Ax ( 29 1 1 1 1 , , z y x p = ( 29 2 2 2 2 , , z y x p = 2 2 2 1 1 1 C B A D Cz By Ax dist + + + + + =

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Recap implicit vs. explicit
Many objects can be represented as implicit surfaces: § Sphere (with center at c and radius R ) : fsphere ( p ) = || p c ||2 - R 2 = 0 § Plane (with normal n and distance to origin d ) : fplane ( p ) = p · n + D = 0 To determine where a ray intersects an object: § Need to find the intersection point p of the ray and the object § The ray is represented explicitly in parametric form: r ( t ) = r o + r dt § Plug the ray equation into the surface equation and solve for t : f ( r ( t )) = 0 § Substitute t back into ray equation to find intersection point p : p = r ( t ) = r o + r dt Ray-Object Intersections

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A ray can be represented explicitly (in parametric form) as an origin (point) and a direction (vector) : § Origin: § Direction: The ray consists of all points: r ( t ) = r o + r dt Recall: Ray Representation r o = r (0) r (–1) r o + r d = r (1) r (2) r (3) = o o o o z y x r = d d d d z y x r
§ Sphere is represented as: - center (point): Sc = (xc, yc, zc) - and a radius (float): r § The surface of the sphere is the set of all points (x, y, z) such that: (x-xc)2 + (y - yc)2 + (z - zc)2 = r2 § In this form (implicit form), it is difficult to directly generate surface points § However, given a point, it is easy to see if the point lies on the surface § To solve the ray-surface intersection, substitute the ray equation into the sphere equation. Ray-Sphere Intersections

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Lecture16-RayTracingIntersections - CS 455 Computer...

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