Intro. to Advanced Lighting, Basic Ray Tracing

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Intro. to Advanced Lighting,Basic Ray Tracing

• Glenn G. ChappellCHAPPELLG_at_member.ams.org
• U. of Alaska Fairbanks
• CS 481/681 Lecture Notes
• Monday, April 12, 2004

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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.

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Basic Ray TracingIntroduction

• 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.

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Basic Ray TracingTracing 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
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Basic Ray TracingTracing 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 objects 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
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Basic Ray TracingTwo 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.

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Basic Ray TracingWhat 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.

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Basic Ray TracingRay-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 objects 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.

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Basic Ray TracingWhat 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??

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Basic Ray TracingHits

• 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 objects normal vector at this point vec.
• Again, this is not required, but will nearly
  always speed things up by avoiding repeating a
  computation.

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Basic Ray TracingClass 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.