Intro. to Advanced Lighting, Basic Ray Tracing

1. Intro. to Advanced Lighting,Basic Ray Tracing Glenn G. ChappellCHAPPELLG@member.ams.org U. of Alaska Fairbanks CS 481/681 Lecture Notes Monday, April 12, 2004

2. Intro. to Advanced Lighting • In our next unit, we look at advanced techniques for lighting scenes. • Most of these do not fit well within the pipeline-based rendering model we have been discussing. • Most of these techniques are also too slow for real-time rendering. • The first technique we will look at is called “ray tracing”. CS 481/681

3. Basic Ray Tracing:Introduction • In “normal” rendering: • We deal with a series of objects, made of primitives. • For each primitive, we determine which pixels it affects, if any. • Ray tracing turns this around: • We deal with pixels, one by one. • For each pixel, we ask what we see (which primitive?) when we look at it. CS 481/681

4. Basic Ray Tracing:Tracing A Ray • The way we determine what we see when we look at a pixel is to draw an imaginary ray from the viewing position, through the pixel, into the scene. • We ask which objects in the scene the ray hits. • The first hit is the one that counts. Image Scene objects Current pixel First hit CS 481/681

5. Basic Ray Tracing:Tracing A Ray • What do we do when we have a hit? • We determine what color the object is at that point. • Light sources and the object’s normal may affect the computation. • We can also do true specular reflection: • Reflect the ray and do the ray tracing computation again. • We can also do true refraction, for translucent objects. Original ray Reflected ray Normal CS 481/681

6. Basic Ray Tracing:Two Questions • When we do ray tracing, there are two basic questions that need to be answered repeatedly: • Given a ray, does it hit an object in the scene, and, if so, which one does it hit first? • If a ray hits an object, what color do we see when we look along the ray? • Designing code to answer these is an excellent application of object-oriented design principles. CS 481/681

7. Basic Ray Tracing:What is a Ray? • A ray is half of a line. It has a starting point and a direction. • To store a ray, we need: • A starting point: pos. • A direction vector (unit vector): vec. • It is reasonable to implement a ray as a class. • For convenience, rays can know how to reflect and refract themselves. CS 481/681

8. Basic Ray Tracing:Ray-Object Intersection • The first question that needs to be answered is whether a ray hits any object in the scene. • To answer this, test the ray against each object in turn. • This test is called ray-object intersection. • What class knows how to do this? • Answer: The object’s class. • Since every object class needs to be able to do ray-object intersection: • Write an abstract base class for objects. • Each object is derived from the base class. • Ray-object intersection is a virtual function. CS 481/681

9. Basic Ray Tracing:What Do Objects Do? • More generally, what do objects need to be able to do? • Answer: They need to be able to answer the two questions, for themselves. • So, an object has two member functions: • First, given a ray, test whether the ray hits the object, and, if so, how far from the start of the ray the hit lies. • How far, so we can tell which hit comes first. • Second, given a ray that hits the object, tell what color is seen along the ray. • And that is all! • Except for administrative stuff: constructors, etc. • Right?? CS 481/681

10. Basic Ray Tracing:Hits • In practice, to determine the color (question 2), an object needs to know where the ray hit and what the normal is. • These are almost always calculated during the ray-object intersection test (question 1). • Therefore, it is convenient to have a “hit” class. This holds: • Whether the ray hit the object: bool. • If so: • How far along the ray: double. • Where the hit is: pos. • The object’s normal vector at this point: vec. • Again, this is not required, but will nearly always speed things up by avoiding repeating a computation. CS 481/681

11. Basic Ray Tracing:Class Summary • So, in a simple ray tracer, we have: • A ray class. • An object base class. • Various objects are derived from it. • Virtual functions for ray-object intersection and color computation. • A hit class. • This design is easily extended. • Adding new objects works without changing other code. • We can add features like multiple rays per pixel, etc. CS 481/681