Project Handouts

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Project 2: Ray Tracing

Assigned:  Wednesday, January 30, 2002
Due:  Tuesday, February 12, 2002
Artifacts Due:  Friday, February 15, 2002

Project Description

You will build a program called Ray that will generate ray-traced images of complex scenes. The ray tracer should trace rays recursively using the Whitted's illumination model.

Getting Started

To install the starting point source code, unzip the files (get them from the projects page). The skeleton code can load scenes and save images. It generates extremely simple "ray-traced" images. The pixels are shaded only by the diffuse terms of the material at the ray intersections.

The skeleton code can run in both text mode and graphics mode. Text mode is considerably faster. Running without no arguments will execute the program in the graphics mode. For usage see 'ray --help'.

Required Functionality

We'll describe these requirements in more detail afterwards:
  1. The skeleton code has no triangle-ray intersection implementation. Fill in the triangle-intersection code so that your ray tracer can triangle meshes. 
  2. Implement the Whitted illumination model, which includes Phong shading (emissive, ambient, diffuse, and specular terms) as well as reflection and refraction terms. You only need to handle directional and point light sources, i.e. no area lights, but you should be able to handle multiple lights.
  3. Implement Phong interpolation of normals.
  4. Implement anti-aliasing. Regular super-sampling is acceptable, more advanced anti-aliasing will be considered as an extension.
  5. Implement data structures that speed up the intersection computations in large scenes. There will be a contest at the end of the project to determine the team with the fastest ray tracer.

Notes on Whitted's illumination model

The first three terms in Whitted's model will require you to trace rays towards each light, and the last two will require you to recursively trace reflected and refracted rays. (Notice that the number of reflected and refracted rays that will be calculated is limited by the "depth" setting in the ray tracer. This means that to see reflections and refraction, you must set the depth to be greater than zero!)

When tracing rays toward lights, you should look for intersections with objects, thereby rendering shadows. If you intersect a semi-transparent object, you should attenuate the light, thereby rendering partial (color-filtered) shadows, but you may ignore refraction of the light source.

The skeleton code doesn't implement Phong interpolation of normals. You need add code for this (only for meshes with per-vertex normals.)

Anti-aliasing

Once you've implemented the shading model and can generate images, you will notice that the images you generated are filled with "jaggies". You should implement an anti-aliasing technique to smooth these rough edges. In particular, you are required to perform super-sampling and averaging down. You should provide a slider and an option to control the number of samples per pixel (1, 4, 9 or 16 samples). You need only implement a box filter for the averaging down step. More sophisticated anti-aliasing methods are left as bells and whistles below.

Acclerated ray-surface intersection

The goal of this portion of the assignment is to speed up the ray-surface intersection module in your ray tracer. In particular, we want you to improve the running time of the program when ray tracing complex scenes containing large numbers of objects (they are usually triangles). There are two basic approaches to do this:
  1. Specialize and optimize the ray-object intersection test to run as fast as possible.
  2. Add data structures that speed the intersection query when there are many objects.

Most of your effort should be spent on approach 2, i.e. reducing the number of ray-object intersection tests. You are free to experiment with any of the acceleration schemes described in Chapter 6, ''A Survey of Ray Tracing Acceleration Techniques,'' of Glassner's book. Of course, you are also free to invent new acceleration methods.

Make sure that you design your acceleration module so that it is able to handle the current set of geometric primitives - that is, triangles spheres, squares, boxes, and cones.

The sample scenes include several simple scenes and three complex test scenes: trimesh1, trimesh2, and trimesh3. You will notice that trimesh1 has per-vertex normals and materials, and trimesh2 has per-vertex materials but not normals. Per-vertex normals and materials imply interpolation of these quantities at the current ray-triangle intersection point (using barycentric coordinates).

Acceleration grading criteria

The test scenes each contain up to thousands of triangles. A portion of your grade for this assignment will be based on the speed of your ray tracer running on these scenes. The faster you can render a picture, the higher your grade.

For grading on the rendering speed, the scenes will be traced at the specific size with one ray traced per pixel, and the rays should be traced with 5 levels of recursion, i.e. each ray should bounce 5 times. If during these bounces you strike surfaces with a zero specular reflectance and zero refraction, stop there. At each bounce, rays should be traced to all light sources, including shadow testing. The command for testing rendering speed looks like: 

ray -t -w 400 -d 5 in.ray out.bmp
[For fairness, don't include other stop criteria (except for the one mentioned above) for -t option.]

You are welcome to precompute scene-specific (but not viewpoint-specific) acceleration data structures and make other time-memory tradeoffs, but your precomputation time and memory use should be reasonable. Don't try to customize your ray tracer for the test scenes; we will also use other scenes during grading. If you have any questions about what constitutes a fair acceleration technique, ask us. Coding your inner loops in machine language is unfair. Using multiple processors is unfair. Compiling with optimization enabled is fair. In general, don't go overboard tuning aspects of your system that aren't related to tracing rays.

Bells and Whistles

This assignment is large, and, furthermore, the optimization element is completely open-ended, so you can profitably work on that until the project is due. Therefore we don't neccessarily expect a bunch of bells and whistles. We can't stop you, though. Here are some interesting extensions to this project.

Approved Bells and Whistles

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

[Whistle]  
Implement stochastic (jittered) supersampling. See Glassner, Chapter 5, Section 4.1 - 4.2 and the first 4 pages of Section 7.

[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.

[bell] Deal with overlapping objects intelligently.  In class, we discussed how to handle refraction for non-overlapping objects in air.  This approach 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] Implement spot lights.

[bell+whistle] Implement antialiasing by adaptive supersampling, as described in Glassner, Chapter 1, Section 4.5 and Figure 19 or 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]  Add some new types of geometry to the ray tracer. Consider implementing torii or general quadrics. Many other objects are possible here.

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

[Bell] [Bell]
Implement texture mapping.

[Bell] [Bell]
Implement bump mapping.

[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.

[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.  An introduction, and a paper discussing a ray-trace project that included caustics.

References