CSE 457 Introduction to Computer Graphics
Spring Quarter 2006
Trace Sample

Project 3 : Trace

Assigned : Thursday, April 27th, 2006
Due : Thursday, May 11th, 2006 @ 11:59pm
Artifact Due : Monday, May 15th @ 11:59pm

Help sessions (Sieg 327):
#1: Tuesday May 2nd, 2:00pm
#2: Wednesday May 3rd, 3:30pm

Project TA : David Jones


Quick Links

  1. Just the sample raytracer, no code
  2. Sample Scenes (download)
  3. Skeleton Code
  4. Project Roadmap
  5. Triangle Intersection Handout
  6. .ray File Format
  7. How to use the Debugging Display
  8. Tool Kit Resources(FLTK, OpenGL)
  9. Help Session Slides

Project Description

Trace is a program that constructs recursively ray-traced images of fairly simple scenes. It is similar in functionality to the POV-Ray raytracer. You might try browsing around the POV-Ray web site for artifact and extra credit inspiration. And POV-Ray can be downloaded for free, so if you want a taste of what a really powerful raytracer can do, then go check it out!

Getting Started

To get started, checkout the skeleton code from your CVS repository.   In the "scenes" subdirectory of your project folder, you'll find some sample scene files (all the files with the .ray extension). These are text files that describe some geometry and the material that should be applied to them. Refer to this page for a brief explanation.  

The Trace project is a very large collection of files and object-oriented C++ code. Fortunately, you only need to work directly with a subset of it. However, you will probably want to spend a bit of time getting familiar with the layout and class hierarchy at first, so that when you code you know what classes and methods are available for your use.

The starting point for where ray tracing begins, and where you will be needing to add a lot of functionality, is in the RayTracer.cpp file. This is a good file to start studying and exploring what methods get called and what they do. In addition, the raytracer features a debugging window that allows you to see individual rays bouncing around the scene. This window provides a lot of visual feedback that can be enormously useful when debugging your application. Look at this web page for a detailed explanation of how to use the debugging window.

Running the Sample

Before you begin coding, you should run the sample solution; it is included in the skeleton distribution as "sample_ray.exe". It has all of the requirements implemented, along with some extra features.

Creating Your Own Scenes

As you get into the project, you'll probably want to use some scenes of your own invention. There is a help page available about the file format. This file also describes the specifications of all the primitives you are required to implement.  To create realistic refractive objects, you'll need their Indices of Refraction.

Required Extensions

After running the sample solution, you should build the skeleton code and see how it compares. You will probably notice that there is a significant difference in the quality of images rendered with the two versions. This suggests what parts of the raytracer have been written and what parts are left undone.

If you compare the outputs of the skeleton and solution, you will see that most of the basic geometry-handling code is done. The skeleton code is able to cast rays into an image and draw color on the screen, resulting in some flat-shaded polygonal shapes.  The skeleton code is doing ray-casting and nothing more.  Furthermore, the triangle and sphere primitives will not appear. While all the code to cast a ray exists, not all of the object intersections code is there.  You need to implement sphere and triangle intersections and expand ray-casting into ray-tracing by adding support for reflected and refracted rays. You also must implement the Phong specular-reflection model and include support for color-filtered shadows cast through transparent objects. 

Specifically, each group must implement recursive ray tracing as described in class. This entails making the extensions to the program listed below. Your ray tracer should recursively trace rays to account for these. Recursion should proceed to a maximum depth as set by the user.

Extension Foley, et al. Alan Watt 
(Errata -- not needed for handout)
Triangle Intersection - - -
Sphere Intersection 15.10 pp.18-19 pp.91
Phong specular-reflection model 16.1 and 16.2.5 6.2-6.3 -
Contribution from:
  multiple light sources,
  distance attenuation,
  and these additional types of lights:
  • Directional lights
  • Point lights
16.1 (particularly in 16.1.5 and 16.1.6) - -
Shadow Attenuation
  • Objects blocking light sources
  • Color-filtering through transparent objects
16.12 and 16.4 9 -
Reflection 16.12 1.4 4.2.1
Refraction 16.12 1.4 WARNING!  4.2.3 - 4.2.7

NOTE: You may assume that objects are not nested inside other objects. If a refracted ray enters a solid object, it will pass completely through the object and back outside before refracting into another object. Improving your refraction code to handle more general cases such as a refractive sphere contained inside another refractive sphere is an extra credit option as described below. In addition, you may assume that the camera itself is not placed inside an object. The initial rays that are sent out through the projection plane will always be moving through air.

NOTE: Alan Watt's book is full of errors, be sure to check out this errata before blindly trusting any of the equations.

NOTE: Some of the provided scenes need features that are not required in order to render correctly. dragon.ray, shell.ray, sier.ray, and trimesh2.ray all require material interpolation support (which is extra credit) in order to appear as they are rendered by the sample solution. And, box.ray, texture_map.ray, and texture_box.ray use texture mapping (also extra credit). So, if you are testing with these scenes, do not worry if your results are different (and other scenes appear fine).

For this project, you are not required to implement any bells or whistles. At this time, we hope that you are already in the habit of thinking about extra features when you start the project. Even the simple bells and whistles can make significant changes in your ray traced scenes.


Artifact Requirements

Each team is required to submit one artifact, at most two, jointly for voting. The scene traced cannot be one of the provided .ray files but must at least be modified in some way (or a completely new scene). With each artifact submitted for voting a text file describing the artifact must also be submitted. This can be as simple as two sentences describing the placement of the objects and lights to get the desired effect or a detailed description of the bells and whistles used to create the scene. The text file will be linked with your artifact.

If you implement any bells or whistles you need to provide examples of these features in effect. You should present your extra credit features at grading time either by rendering scenes that demonstrate the features during the grading session or by showing images you rendered in advance. You might need to pre-render images if they take a while to compute (longer than 30 seconds). These pre-rendered examples, if needed, must be included in your turnin directory on the project due date. The scenes you use for demonstrating features can be different from what you end up submitting as an artifact.


Bells and Whistles

In many cases, the material in the Foley, et al. book goes into more depth on a particular subject than the Alan Watt text. In addition, Glassner's book on ray tracing is a very comprehensive exposition of a whole bunch of ways ray tracing can be expanded or optimized (and it's really well written). If you're planning on implementing any of these bells and whistles, you are encouraged to read the relevant sections in these books as well.

Remember that you'll need to establish to our satisfaction that you've implemented the extension! You should have test cases that clearly demonstrate the effect of the code you've added to the ray tracer. Sometimes different extensions can interact, making it hard to tell how each contributed to the final image, so it's also helpful (that is, necessary!) to add controls to selectively enable and disable your extensions.

Here are some examples of effects you can get with ray tracing. Currently none of these were created from past students' ray tracers. Soon I will expand this page to include examples of effects students have achieved with their programs.

[whistle] Implement an adaptive termination criterion for tracing rays, based on ray contribution.  Control the adaptation  threshold with a slider.

[whistle] Implement antialiasing by supersampling and averaging down.

[whistle] Modify your antialiasing to implement the first stage of distribution ray tracing by jittering the sub-pixel samples.  The noise introduced by jittering should be evident when casting 1 ray per pixel.

[bell] Implement spot lights (described in the Angel text).

[bell] Improve your refraction code to allow rays to refract correctly through objects that are contained inside other objects. You must put together a .ray file to demonstrate this effect.

[bell]Add code for interpolating the material properties of a triangle's vertices. This is necessary for viewing some of the polymesh scenes such as sier.ray.

[bell] Add a menu option that lets you specify a background image to replace the environment's ambient color during the rendering.  That is, any ray that goes off into infinity behind the scene should return a color from the loaded image, instead of just black.  The background should appear as the backplane of the rendered image with suitable reflections and refractions to it. This is also called environment mapping. Click here for some examples.

[bell]Find a good way to accelerate shadow attenuation.  Do you need to check against every object when casting the shadow ray?  This one is hard to demonstrate directly, so be prepared to explain in detail how you pulled it off.

[bell] Deal with overlapping objects intelligently.  While the skeleton code handles materials with arbitrary indices of refraction, it assumes that objects don't intersect one another. It breaks down when objects intersect or are wholly contained inside other objects. Add support to the refraction code for detecting this and handling it in a more realistic fashion.  Note, however, that in the real world, objects can't coexist in the same place at the same time. You will have to make assumptions as to how to choose the index of refraction in the overlapping space.  Make those assumptions clear when demonstrating the results.

[bell+whistle] Implement antialiasing by adaptive supersampling, as described in Foley, et al., 15.10.4.  For full credit, you must show some sort of visualization of the sampling pattern that results.  For example, you could create another image where each pixel is given an intensity proportional to the number of rays used to calculate the color of the corresponding pixel in the ray traced image.  Implementing this bell/whistle is a big win -- nice antialiasing at low cost.

[bell+whistle] Implement more versatile lighting controls, such as the Warn model described in Foley 16.1.5. This allows you to do things like control the shape of the projected light.

[bell] [bell]

Add texture mapping support to the program. To get full credit for this, you must add texture mapping support for all the built-in primitives (sphere, box, cylinder, cone) except trimeshes. The square object is already done for you. The most basic kind of texture mapping is to apply the map to the diffuse color of a surface. But many other parameters can be mapped. Reflected color can be mapped to create the sense of a surrounding environment. Transparency can be mapped to create holes in objects. Additional (variable) extra credit will be given for such additional mappings.  The basis for this bell is built into the skeleton, and the parser already handles the types of mapping mentioned above.  Additional credit will be awarded for quality implementation of texture mapping on general trimeshes.

[bell] [bell] Implement bump mapping (Watt 8.4; Foley, et al. 16.3.3). Check this out!

[bell] [bell] Implement solid textures or some other form of procedural texture mapping, as described in Foley, et al., 20.1.2 and 20.8.3. Solid textures are a way to easily generate a semi-random texture like wood grain or marble. Click here for a tutorial on making realistic looking marble using Ken Perlin's noise function.

[bell] [bell] Add some new types of geometry to the ray tracer. Consider implementing torii or general quadrics. Many other objects are possible here.

[bell] [bell] Add support for height-fields. Click here for a discussion on what they are and how they can be generated.  

[bell] [bell]

Extend the ray-tracer to create Single Image Random Dot Stereograms (SIRDS). Click here to read a paper on how to make them. Or, create 3D images like this one, for viewing with red-blue glasses.

[bell] [bell] for first, [bell+whistle] for each additional
Implement distribution ray tracing to produce one or more or the following effects: depth of field, soft shadows, motion blur, or glossy reflection (See Watt 10.6, Glassner, chapter 5, or Foley, et al., 16.12.4).

[bell] [bell] [bell] Add some higher-level geometry to the ray tracer, such as surfaces of revolution, extrusions, metaballs, swept surfaces, or blend surfaces.  You may have implemented one or more of these as a polygonal object in the modeler project.  For the Raytracer, be sure you are actually raytracing the surface as a mathematical construct, not just creating a polygonal representation of the object and tracing that.  Yes, this requires lots of complicated math, but the final results are definitely worth it (see Transparent Metaballs).  Here is a really good tutorial on raytracing metaballs. For an additional bell, add texture mapping to your higher-level geometry. The texture mapping must look good in order to get credit for it!

[bell] [bell] [bell] Implement ray-intersection optimization by either significantly extending the BSP Tree implemented in the skeleton or by implementing a different optimization method, such as hierarchical bounding volumes (See Glassner 6.4 and 6.5, Foley, et al., 15.10.2).

[bell] [bell] [bell] Implement 3D fractals and extend the .ray file format to provide support for these objects. Note that you are not allowed to "fake" this by just drawing a plain old 2D fractal image, such as the usual Mandelbrot Set. Similarly, you are not allowed to cheat by making a .ray file that arranges objects in a fractal pattern, like the sier.ray test file. You must raytrace an actual 3D fractal, and your extension to the .ray file format must allow you to control the resulting object in some interesting way, such as choosing different fractal algorithms or modifying the base pattern used to produce the fractal.

Here are two really good examples of raytraced fractals that were produced by students during a previous quarter: Example 1, Example 2
And here are a couple more interesting fractal objects: Example 3, Example 4

[bell] [bell] [bell] [bell] Implement 4D quaternion fractals and extend the .ray file format to provide support for these objects. These types of fractals are generated by using a generalization of complex numbers called quaternions. What makes the fractal really interesting is that it is actually a 4D object. This is a problem because we can only perceive three spatial dimensions, not four. In order to render a 3D image on the computer screen, one must "slice" the 4D object with a three dimensional hyperplane. Then the points plotted on the screen are all the points that are in the intersection of the hyperplane and the fractal. Your extension to the .ray file format must allow you to control the resulting object in some interesting way, such as choosing different generating equations, changing the slicing plane, or modifying the surface attributes of the fractal.

Here are a few examples, which were created using the POV-Ray raytracer (yes, POV-Ray has quaternion fractals built in!): Example 1, Example 2, Example 3, Example 4. And, this is an excellent example from a previous quarter.

To get started, visit this web page to brush up on your quaternion math. Then go to this site to learn about the theory behind these fractals. By now you should have a high level overview of what's going on, so you are ready to take the plunge and dive into this hardcore web page, which contains a practical discussion of how quaternion fractals are implemented in POV-Ray.

[bell] [bell] [bell] [bell] Implement a more realistic shading model. Credit will vary depending on the sophistication of the model. A simple model factors in the Fresnel term to compute the amount of light reflected and transmitted at a perfect dielectric (e.g., glass). A more complex model incorporates the notion of a microfacet distribution to broaden the specular highlight. Accounting for the color dependence in the Fresnel term permits a more metallic appearance. Even better, include anisotropic reflections for a plane with parallel grains or a sphere with grains that follow the lines of latitude or longitude. Sources: Watt, Chapter 7, Foley et al, Section 16.7; Glassner, Chapter 4, Section 4; Ward's SIGGRAPH '92 paper; Schlick's Eurographics Rendering Workshop '93 paper.

This all sounds kind of complex, and the physics behind it is. But the coding doesn't have to be. It can be worthwhile to look up one of these alternate models, since they do a much better job at surface shading.  Be sure to demo the results in a way that makes the value added clear.

Theoretically, you could also invent new shading models. For instance, you could implement a less realistic model! Could you implement a shading model that produces something that looks like cel animation? Variable extra credit will be given for these "alternate" shading models.  Links to ideas: Comic Book Rendering

Note that you must still implement the Phong model.

[bell] [bell] [bell] [bell] Implement CSG, constructive solid geometry. This extension allows you to create very interesting models. See page 108 of Glassner for some implementation suggestions. An excellent example of CSG was built by a grad student here in the grad graphics course.

[bell] [bell] [bell] [bell] Add a particle systems simulation and renderer (Foley 20.5, Watt 17.7, or see instructor for more pointers).


[bell] [bell] [bell] [bell]Implement caustics.  Caustics are variations in light intensity caused by refractive focusing--everything from simple magnifying-glass points to the shifting patterns on the bottom of a swimming pool.  A paper discussing some methods, and some images.  2 bells each for refractive and reflective caustics.

Here is a really good example of caustics that were produced by two students during a previous quarter: Example

There are innumerable ways to extend a ray tracer. Think about all the visual phenomena in the real world. The look and shape of cloth. The texture of hair. The look of frost on a window. Dappled sunlight seen through the leaves of a tree. Fire. Rain. The look of things underwater. Prisms. Do you have an idea of how to simulate this phenomenon? Better yet, how can you fake it but get something that looks just as good? You are encouraged to dream up other features you'd like to add to the base ray tracer. Obviously, any such extensions will receive variable extra credit depending on merit (that is, coolness!). Feel free to discuss ideas with the course staff before (and while) proceeding!


Disclaimer: please consult the course staff before spending any serious time on these. These are all quite difficult (I would say monstrous) and may qualify as impossible to finish in the given time. But they're cool.

Sub-Surface Scattering

The trace program assigns colors to pixels by simulating a ray of light that travels, hits a surface, and then leaves the surface at the same position.  This is good when it comes to modeling a material that is metallic or mirror-like, but fails for translucent materials, or materials where light is scattered beneath the surface (such as skin, milk, plants... ). Check this paper out to learn more.


Metropolis Light Transport

Not all rays are created equal.  Some light rays contribute more to the image than others, depending on what they reflect off of or pass through on the route to the eye.  Ideally, we'd like to trace the rays that have the largest effect on the image, and ignore the others.  The problem is:  how do you know which rays contribute most?  Metropolis light transport solves this problem by randomly searching for "good" rays.  Once those rays are found, they are mutated to produce others that are similar in the hope that they will also be good.  The approach uses statistical sampling techniques to make this work.  Here's some information on it, and a neat picture.



Photon Mapping

Photon mapping is a powerful variation of ray tracing that adds speed, accuracy and versatility.  It's a two-pass method:  in the first pass photon maps are created by emitting packets of energy (photons) from the light sources and storing these as they hit surfaces within the scene.  The scene is then rendered using a distribution ray tracing algorithm optimized by using the information in the photon maps.  It produces some amazing pictures.  Here's some information on it.

Also, if you want to implement photon mapping, we suggest you look at the SIGGRAPH 2001 course 38 notes.  The TA's can point you to a copy, if you are interested.


design by Ian Li, Spring '06 maintenance by Elizabeth Arrowsmith, esais@cs CSE 457 Introduction to Computer Graphics
Spring Quarter 2006
  Last modified: Thursday, 27-Apr-2006 11:19:54 PDT