Autodesk LIGHTSCAPE, Руководство пользователя

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Introduction 1
4
Lightscape
Ray Tracing
One of the first global illumination algorithms to be
developed is known as
ray tracing . In ray tracing, it is
recognized that while there may be billions of
photons traveling about the room, the photons you
primarily care about are the ones that enter the eye.
The algorithm works by tracing rays
backward, from
each pixel on the screen into the 3D model. In this
way, it computes only the information needed to
construct the image. To create an image using ray
tracing, do the following procedure for each pixel on
the computer screen:
1.
Trace a ray back from the eye position, through
the pixel on the monitor, until it intersects with a
surface.
2.
The model provides the reflectivity of the sur-
face, but not the amount of light reaching that sur-
face. To determine the total illumination, trace a ray
from the point of intersection to each light source in
the environment
(shadow ray) . If the ray to a light
source is not blocked by another object, use the light
contribution from that source to calculate the color
of the surface.
3. The intersected surface may be shiny or trans-
parent. The algorithm must determine either what is
seen in or through the surface being processed. Re-
peat steps 1 and 2 in the reflected (and, in the case of
transparency, transmitted) direction until another
surface is encountered. The color at the subsequent
intersection point is calculated and factored into the
original point.
4. If the second surface is yet again a reflective or
transparent surface, repeat the ray tracing process
until a maximum number of iterations is reached or
until no more surfaces are intersected.
Ray tracing is a very versatile algorithm because of
the large range of lighting effects it can model. It can
accurately account for the global illumination char-
acteristics of direct illumination, shadows, specular
reflections (for example, mirrors), and refraction
through transparent materials. The main disadvan-
tage of ray tracing is that the process can be slow and
computationally expensive for environments of even
moderate complexity.
Another significant disadvantage of ray tracing is
that it does not account for one very important char-
acteristic of global illumination—diffuse
interreflections.
Traditional ray tracing techniques accurately
account for only the light arriving directly from the
light sources themselves. But, as shown in the room
example, light does not only arrive at a surface from
the light sources (direct lighting), it also arrives from
other surfaces (indirect lighting). If you ray trace an
image of the table (as shown in the example), the
area under the table appears black because it receives
no direct light from the light source. You know from
experience, however, that this area would not really
be completely dark because of the light it would
receive from the surrounding walls and floor.
Ray tracing