CIS681 Lab 3

Winter 2K11
due ~~Monday, February 7~~ Wednesday, February 9

FINALIZED


directional light,tilt=0	directional light, tilt=30	point light	spot light

snell = 0.95	snell = 1.05


1 Reflective	2 Reflective	2 Reflective; 1 Refractive

Objective

To modify lab2 to:

incorporate recursive ray tracing with shadows,
extend the types of light sources that are handled
process the head tilt parameter

Assignment

Use the scene file specification found in the Lab #2 specification with the addition lightsource commands found below.
Modify your ray tracer to incorporate refraction, reflection, and shadows. Use at least 5 levels of recursion.
- Shadows: For each visible point, loop through the light sources. For each one in front of the visible point, form a ray from the point to the light source and see if it intersects any object. If it does, and the object is not transparent, ignore the effect of that light source on the point. If the object is transparent, attenuate the intensity of the light source.
- Sphere parameters: Use 3 of the last 5 parameters of the sphere for this lab.
```
sphere <r> <g> <b> <texture-name_s> <kd> <ks> <n_i> <kr> <gl> <kt> <n1> <tr>
```
  - kr is the coefficient of reflection
  - kt is the coefficient of refraction
  - n1 is the Snell coefficient
  Side note: It's probably more common to use one coefficient for both specular illumination and reflection of the environment (i.e. kr = ks) because both are reflections off the surface of the object. Using different coefficients is more flexible.
- Reflection: Do reflection when kr > 0.0. Recursively call for shading in the reflection ray direction. Add the color from the reflection to the surface color scaled by the reflectioncoefficient, kr.
```
Color = Color + kr*shade(reflection_ray)
```
- Refraction: Do refraction when kt > 0.0. Recursively call for shading in the refraction ray direction. Use an index of refraction, n1, of 1.0 for air and choose another value for all other material. Dr. Shen suggest 1.5. In my samples, I found that 1.05 worked well for the input files I was using (for example input files, refer to the links associated with the images above). Negate the normal for exiting intersections of transparent objects. Linearly blend the color of the surface of the object with refracted light using the transparency coefficient, kt. If both reflection and refraction are computed, add in the reflection color and then do the linear blend of transparency.
- Here is some high-level pseudo-code (leaving out some details like recursion depth control):
```
Color = ambient, diffuse and specular illumination w/ shadows
if (reflection)
    Color = Color + kr*shade(reflection_ray)
if (refraction)
    Color = (1-kt)*Color + kt*shade(refraction_ray)
```

Add the ability to handle the other types of light sources:

Directional light sources
```
dlight <dx> <dy> <dz> <intensity> 
```

Spotlight

slight <x> <y> <z> <dx> <dy> <dz> <hood-angle> <falloff-power_i> <intensity> <size>

Include the processing of the head tilt parameters. A positive head tilt angle should rotate the head clockwise and, therefore, rotate the image counter-clockwise. Compute the camera coordinate system, <u,v,w>, as in Lab #2, then redefine u and v using the sine and cosine of the tilt parameter times the u, v vectors.

Read in a scene description file just like Lab #2. The next 3 parameters of the sphere command are the parametes for reflectivity and transparency including the Snell coefficient: ks, kt, and n1

Call your program lab3. As before, your program should take one argument, the name of the scene description file. The grader should be able to do the following (if the output file specified in output.ppm) on a departmental machine under UNIX:

make
lab3 input.txt
convert output.ppm output.jpg
display output.jpg

Include with your submission sample input.txt files that demonstrate to the grader effective use of:

shadows, refraction and reflection
point, directional and spot lights
head tilt (use two input files: one with tilt = 0 and one with tilt set to something)

You can use as few as two input files or as many as you'd like. It is your responsibility to create input files that demonstrate to the grader that your lab satisfies the lab requirements.

Notes

In this and future labs, you are not required to exactly match my images. You are required to generate reasonable images (pretty pictures) based on the input file.

Extra Credit

Submission of input files may receive extra credit if they are particularly interesting or complex scenes and/or effects. (up to 10%). Start being creative!
Handle near and far clipping distance parameters. You can either consider them as distance along w or distance along pixel rays. To recieve extra credit, consider:
- visibility starting inside of a refractive object. (5%)
- treating an opaque sphere as a solid object (not a shell)

Last updated 2/1/11

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