Ray tracing
New Concepts
• The recursive ray tracing algorithm
• Generating eye rays
• Non Real-time rendering
Ray tracing
• Using ray tracing you can achieve photorealistic effects
• You can achieve many complex lighting effects, such as shadows,
refraction and reflection.
• It is incredibly hard, if not impossible to achieve these accurate
effects with the standard rendering pipeline.
• While you get a huge advance in visual quality, it comes with the
price of very long rendering times.
Camera Rays
• First you shoot rays from the camera out into the scene.
• Pixels can be rendered in any order, but in this lab we
will go from top to bottom left to right.
• We loop over the pixels and generate an initial primary
ray (eye ray)
• The ray origin is just the camera’s position
• The direction is computed by first finding the 4 corners of
a virtual image in world space, then interpolating to the
correct spot, and finally computing a normalized direction
from the camera to the virtual pixel.
Ray Intersection
• The eye ray is then tested for intersection against every
object in the scene.
• If there is no intersection then we just color that pixel the
specified ‘background’ color.
• If there is a hit, then we want to find out what the closest
object hit was.
• At the intersection, we need to know the position,
normal, color, texture coordinates, material etc…about
that exact location.
• If the hit is inside a triangle, then all that information is
just interpolated from the vertex data.
Lighting
• Once we have the key intersection information, we can
apply any lighting model we want.
• This can include procedural shaders, lighting
computations, texture lookups, bump mapping etc…
• Many of the most interesting forms of lighting involve
spawning additional rays and tracing them recursively.
(Which we will be doing in this lab for shadows)
• The result of the lighting equation is a color which is
used to color the pixel.
Shadows
• Shadows are really easy to achieve
• Just trace another ray from a hit point to every light in the
scene, if you hit an object before you hit the light, then
that light cannot contribute to the color.
• Shadow rays only need to know if something is hit,
where as normal rays need to know position, color,
normal, etc.
• When we spawn new rays off of a surface, it is a good
idea to add a bit of an offset to the origin. That is to push
it up slightly (0.0001) along the normal of the surface.
– This prevents the ray from intersecting with the object
it spawned off of.
Reflection Rays
• Reflections are another awesome and easy to do effect
with ray tracing.
• Instead of calculating the lighting equation at a particular
hit point, you just trace a secondary ray, called the
reflection ray, and trace it into the scene.
• Reflection rays, like shadow rays, should also be nudged
up a bit.
Reflection Rays
• If the reflection ray hits a normal material, we compute
the illumination and use that as the final color.
• But if the reflection hits another mirror, then we just
recursively generate a new reflection ray and trace it.
• To prevent the system from getting trapped in an infinite
loop, you can set a limit to the depth of the recursion. 10
is a good number…
Intersection Test
• For this lab, everything has been created by using triangles, so you
will only have to write an intersection test for ray-triangle
intersections.
• Once we know a ray hits a triangle at a point q we must verify that q
lies inside the 3 edges of the triangle.
• We do this by calculating the barycentric coordinates.
– q’ = q-v0
– e1 = v1 - v0
– e2 = v2- v0
– b1 = (q’ * e2)/(e1 * e2)
– b2 = (q’ * e1)/(e1 * e2)
– Reject if b1 <0 , b2 <0 or b1 + b2 > 1
File Overview
• Camera.[cpp h] : You will do most of the work in these files, including
writing the actual RayTrace algorithm, and all the code for producing
eye rays.
• Image.[cpp h] : All the functions for drawing, and setting the screen
pixels. You don’t have to write any code here
• Light.[cpp h] : All the information for the lights. You don’t have to
write any code.
• Model.[cpp h] : You will have to write the triangle and model
intersection code here
• Ray.h : Has the structures for ray’s and intersections defined here
• Vector.h : All of the vector manipulation stuff.
• Scene.[cpp h] : The scene is defined in here
• Raytracer.cpp : this is the main file.
Requirements
Complete the ray tracer so that you end up with an image that has
shadows and mirror reflections. Similar to this:
Implementation Suggestions
• MAKE SURE YOU LOOK THROUGH ALL THE FILES AND
UNDERSTAND THEM
• There is a TODO in every function where you need to write code.
“grep TODO *” <--- type that in the terminal
• It helps to have a function like bool inShadow(…)
• You will probably have to write a function called Trace() !!!
• The first thing your gonna have to work on is writing the intersection
functions and the eyeray generating function.
• Work on this every day.
Resources
•
There are resources on the webpage. PLEASE download them, there are helpful
pictures, and information on eye ray generation.