Editing Ray casting (section)

== Realistic shaded pictures ==

Ray casting is a natural modeling tool for making shaded pictures. The grayscale ray-casting system developed by Scott Roth and Daniel Bass at GM Research Labs produced pictures on a Ramtek color raster display around 1979. To compose pictures, the system provided the user with the following controls:
<dl>
<dt>View</dt>
<dd>
    <ul>
        <li>Viewing direction and position</li>
        <li>Focal length: width-angle perspective to parallel</li>
        <li>Zoom factor</li>
    </ul>
</dd>
<dt>Illumination</dt>
<dd>
    <ul>
        <li>Number of light sources</li>
        <li>Locations and intensities of lights</li>
        <li>Optionally shadow</li>
        <li>Intensities of [[Shading#Ambient_lighting|ambient light]] and background</li>
    </ul>
</dd>
<dt>Surface reflectance</dt>
<dd>
    <ul>
        <li>% reflected [[diffuse reflection|diffusely]]</li>
        <li>% reflected [[specular reflection|specularly]]</li>
        <li>% [[transmittance|transmitted]]</li>
    </ul>
</dd>
</dl>

[[File:Two point light sources.jpg|thumb|left|Two point light sources produce shadows]]
This figure shows a table scene with shadows from two point light sources.

Shading algorithms that implement all of the realistic effects are computationally expensive, but relatively simple. For example, the following figure shows the additional rays that could be cast for a single light source.[[File:Follow up rays for effects.jpg|400x400px|center|Follow up rays for effects]]

For a single pixel in the image to be rendered, the algorithm casts a ray starting at the focal point and determines that it intersects a semi-transparent rectangle and a shiny circle. An additional ray must then be cast starting at that point in the direction symmetrically opposite the surface normal at the ray-surface intersection point in order to determine what is visible in the mirrored reflection. That ray intersects the triangle which is opaque. Finally, each ray-surface intersection point is tested to determine if it is in shadow. The “Shadow feeler” ray is cast from the ray-surface intersection point to the light source to determine if any other surface blocks that pathway.

Turner Whitted calls the secondary and additional rays “Recursive Ray Tracing”.<ref>{{Citation
  | last1 = Whitted| first1 = Turner
  | title = An Improved Illumination Model for Shaded Display
  | journal = Communications of the ACM
  | volume = 23
  | date = June 1980
  | pages = 343–349
  | issue = 6| doi = 10.1145/358876.358882
 | s2cid = 9524504
 | doi-access = free
  }}</ref> [A room of mirrors would be costly to render, so limiting the number of recursions is prudent.] Whitted modeled refraction for transparencies by generating a secondary ray from the visible surface point at an angle determined by the solid’s index of refraction. The secondary ray is then processed as a specular ray. For the refraction formula and pictorial examples, see Whitted’s paper.