Realtime Ray Tracing and Interactive Global Illumination - Scientific ...

14 Chapter 2: An Introduction to Ray Tracing
clearly separated the shading process from the actual ray tracing, and allowed
to describe the shading process using a convenient, easy-to-use high-level
language. Shaders could be written independently of the application and
independent of other shaders. The effects of different shaders could be easily
combined in a plug-and-play manner.
2.2.4.1 The Shader Concept
Originally, ray tracers usually supported only one kind of “shader” that was
attached to all objects at the same time. This function for computing the
“color” of a ray typically consisted of a physically motivated lighting/material
model that was the same for all primitives, and which could be parameterized
by different parameters like material properties or textures.
Today however ray tracers usually follow a much more general concept
in which each respective primitive may have a different function for “shading”
the ray (i.e. for computing its color). This function can be completely
independent of all other surfaces, and does not have to follow any physical
properties at all. All that has to be done to implement this concept is to
require that each object has one shader, and that this shader alone is responsible
for computing the color of each ray hitting that object. Using this
concept allows for a “plug and play” mechanism in which different shaders
can cooperate in rendering an image, without any shader having to know
anything out the other ones.
For example, a scene might contain a sphere with a specular metal shader,
as well as another object with a shader simulating wood with procedural
methods. In order to shade a ray hitting the metal sphere, the metal shader
simply casts a ray into the reflection direction, and recursively calls the shader
of the respective hitpoint to compute that reflected ray’s color. This way,
the wooden object can reflect off the metal sphere without the metal having
to know anything about a “wood” shader at all.
This kind of abstraction can also be applied to light sources, the camera,
or the environment. Each of these concepts is described by a separate kind
of shader, resulting in camera, surface, light, environment, volume, and pixel
shaders (also see Figure 2.3).
Camera Shaders: Camera shaders are responsible for generating and casting
the primary rays. Typically ray tracers use a perspective pinhole camera
(see e.g. [Glassner89]), but other effects like e.g. fish-eye lenses or realistic
lens systems are easy to implement, too. More importantly, it is possible
to also compute advanced effects like motion blur and depth of field by using
camera shaders that simulate real cameras with finite lens aperture and