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Experimental Ray Tracer

  by Loren Blaney

These images were produced by a C program that can be downloaded below.

Click on an image for an enlarged view.

My first image. A red ball and two reflective spheres. The blue and white grid provides a reflected image.

Sierpinski Gasket on a Navajo Rug. Ten glass marbles and a steely. This uses only 16 colors. The Sierpinski gasket was actually made by mistake. I was trying for a much less interesting grid pattern. The gasket is surprisingly easy to make, the basic idea being to plot red where (X & Z) != 0.

Two glass spheres and a metal one. Reflections have been added to the glass. More than 256 colors were calculated for this image. The colors beyond 256 were were drawn using the closest available color. Color is a 3-dimensional vector (red, green, blue) and color calculations obey the laws of vector arithmetic, just as rays do. The reflective loss is modeled as a filter which is subtractive, rather than multiplying by a percentage, which is probably more correct.

Seventeen glass marbles (14 red, 3 blue) plus a light source up above. Multiple reflections and refractions.

Christmas Bulbs! Same as before but with metal instead of glass. This makes the reflected images easy to distinguish from the refracted images shown before.

Same as before but zoomed in on the center blue bulb. This shade of blue shows the reflected image more clearly than the red because it is not as sharp a filter.

Happiness is a Clean Floor. Two red and two blue marbles on a polished black floor with the moon above.

Inside a Christmas Bulb. Your eyeball is just inside the front surface of a large, reflective sphere with a radius of 2000. Also inside are three other balls colored red, green, and blue. These balls each have a radius of 75, and are located as shown below:

  |Your                                              1333                     |
  |Eye                                               i2                   Back|
  |i1                                   Red       Grn| Blue f             Wall|
  |4000                                 2000      1500 |    1000             0|
  |Front                               Center          1250                   |
  |Wall                                                                       |

As anyone who has really looked into optics knows, the formula for the inside of a Christmas bulb is:

 1/i1 + 1/i2 = 1/f

The focal length (f) is halfway between the center and the back wall, thus f = 1000. Your eyeball, at the front wall, (i1) is 4000 from the back wall. The point i2 is the point that is focused, and it is calculated as follows:

 1/4000 + 1/i2 = 1/1000
 1/i2 = 1/1000 - 1/4000
 i2 = 1/(1/1000 - 1/4000) = 1333

The blue ball is closest to this point, and it is reassuring to see that blue does indeed predominate the image.

The chaotic asteroid belts are a complete mystery. They might be due to rounding errors although double precision calculations are used throughout. I refuse to believe that even with 64 reflections, each magnifying the image, one can see the individual photons!

The green scratch is probably caused by a flaky ATI Wonder Card.

It'll do cylinders too. The blue and green cylinders intersect.


These images are the results of my ray tracing experiments. Perhaps you would like to take up where I left off. I've tried to make the code easy to understand by adding lots of comments.

The ray tracer is intended more for programmers who are curious about how ray tracing works, rather than for artists who are trying to make a nifty image, since the user interface is not exactly friendly.

The current version requires a super VGA display (640x480x256 colors), although it could easily be modified to use a standard VGA display (320x200x256 colors). You might need to change the definition of MODE in the program to work for your particular brand of super VGA card. I used an ATI Wonder Card.

A math coprocessor is nice but not essential. This program is much faster than other ray tracers that can take hours to produce similar images.

While the program is running, the arrow keys can be used to shift the image. Also the keys "." and "," can be used to zoom in and out in 10% steps, and the ">" and "<" keys zoom by a factor of 2. Each time one of these keys is pressed the ray tracing starts over from the beginning. "Esc" aborts the program.

Pressing the "Enter" key saves the current image to the file specified following "RAY" on the command line. If no file is specified, the default file name "RAYIMAGE.LBM" is used. LBM is the image format that I happen to be familiar with. It's the one that is universally used in the Amiga world, where it is known as IFF. I use Graphics WorkShop (GWS) to convert this format to GIF, which is more suitable for the PC world, and is considerably more compressed (the LBM has compression turned off for simplicity).

There are many books that discuss ray tracing; some seem to make it more difficult than it really is. I recommend the article in Byte Magazine, December 1990, page 263. If you want more than that, try "Interactive Computer Graphics" by Burger and Gillies. The key is to understand vectors. These are typically explained in books with titles such as "Calculus with Analytic Geometry". Don't let the calculus portion intimidate you -- just skip to the chapter on vectors.

If you improve this program, I'd sure like to see a copy. You can reach me at:
Download (192k, 23-Jan-92)

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Last updated: 13-Feb-2004