CS 248: Introduction to Computer Graphics

Assignment 4

Handed out:	Tuesday, November 17, 1998
Due:	5 pm, Tuesday, December 1, 1998

Note: In order to give us time to grade your assignment, the last day to turn in this assignment late will be Friday, December 4, 1998, at 5 pm. Please contact Prof. Hanrahan if you find that you will not be able to make this deadline.

Overview

Many graphics systems such as OpenGL have a fixed illumination model that describes how objects will appear in given lighting conditions. In the case of OpenGL, this illumination model is expressed as a formula. You have some ability to modify how this formula behaves by changing parameters of this formula, such as the material properties of the object being displayed and the color and position of the lights in the scene.

A more general approach to illumination is to allow a user-defined function to be called whenever the color at a particular point on the surface needs to be computed. This function (a surface shader) has access to information about the scene such as the location of the point on the surface being illuminated, the location of the light sources, the properties of the surface, and the parameterization of the surface (i.e. texture coordinates). With this information, the function computes the color of the point on the surface using some model of how lighting in the environment interacts with the surface.

You can also think of calling a shading function (called a light shader) every time you need to know the color of a light in the scene. This allows you to implement lights whose intensity or color changes based on the outgoing light direction. It also allows you to write a light shader that simulates a mask in front of the light source as follows: every time you call the light shader, the ray from the point on the surface being illuminated to the light source is intersected with the mask. If the ray passes through an "open" part of the mask, the point on the surface is illuminated. Otherwise, it is left in darkness.

What you need to do

First, start by reading the online guide we wrote to getting started on this assignment, which is linked off of the class Web pages. Then, you need to implement the following seven shaders:

Surface Shaders (50 points total)

• Plastic, 10 points. Add a specular lighting component to the diffuse shader provided to you, and use this new shader to model a smooth, red, plastic surface, illuminated by a single white light. Hint: a plastic surface has a highlight color that is related to the color of the light, not the color of the surface.
• 3D Checkerboard, 10 points. Write a procedural shader that implements a 3D checkerboard texture. Use the object-space coordinates of each vertex to obtain 3D texture coordinates, which you should then use to select one of two colors according to a 3D checkerboard pattern. The shader should reflect light according to a diffuse reflection model. Using this shader, model a diffuse, brown and grey checkerboarded surface in a scene with a single white light source.
• Wood, 15 points. Use a noise function like the one discussed in class or the one used in the online example to implement a 3D wood texture. Experiment with the parameters to the noise function and with different colors to get a good looking wood texture. As with the 3D checkerboard, derive the 3D texture coordinates from the object-space coordinates of each vertex. Use your wood shader to model a wooded object in a scene with a single white light source.
• Bump mapping, 15 points. Given an object that is parameterized by texture coordinates (u,v), write a shader that locally perturbs the surface normal to give the illusion of bumps. Use a specular shading model for the surface color. Demonstrate your shader using a scene with a white light and a surface color of gray.

• Spotlight, 14 points. Write a light shader that implements a spotlight as a point light source emitting a cone of light aimed according to the light direction. Vertices within the cone should be fully illuminated, while vertices outside the cone should not be illuminated at all. Configure your spotlight shader to have a cone that extends 30 degrees from its axis, and use it in a scene to cast light on an object shaded using one of the surface shaders described above. Hint: if you make clever use of the dot product, you can avoid the use of sin() and cos() on a per-vertex basis.
• Projected texture, 18 points. Write a slide projector shader that models a point light source with a color image in front of it. Any light ray that passes through the image has its color modulated (multiplied) by the corresponding image color. Any light ray that does not pass through the image does not illuminate the surface. Design your own image, or use one of the samples in the assignment image directory. As with the previous light shader, use your slide projector shader in a scene that contains an object shaded using one of the surface shaders described above.
• Disco ball, 18 points. Write a shader that produces a disco-ball effect by surrounding a light with a procedurally-defined mask or stencil consisting of many small holes (circles or other simple shapes). Only rays passing through the holes illuminate the surface. Again, use this shader in a scene that contains an object shaded using one of the surface shaders described above.

All of your shaders must be implemented in the file shader.c, and you must include one configuration file for each of your shaders. Please name your configuration files so that it's obvious which configuration file goes with which shader, and please comment your configuration files so we can figure out how the various shader parameters you placed in these files affect the rendered output.

You must also prepare an image for each of your shaders and put them on a web page. For each shader, save a PPM file from the viewer, then copy these files to your WWW directory and make a small web page with links to the images. Include a URL for this web page in your README file, and please verify that the links to images are actually correct. Please do not assume that we will go hunting down your web page. We will take off 5 points if the URL is not in your README file.

When grading your shaders, some of the criteria we will be looking at are:

• Do your shaders function like they're supposed to?
• Does the output of your shaders look reasonably nice?
• Can your shaders be manipulated interactively?

Please take these criteria into consideration when writing your shaders. Also, please make sure to check the Assignment 4 FAQ every so often. You are responsible for any information given there.

Extra Credit

For extra credit, you can implement additional shaders. The shaders should model real surfaces or real lighting effects. For example, you might pick a surface from your room, from outside your window, or from our page of sample real-world surfaces. You should clearly document your extra credit shaders in your README file, including a description of what each shader models and what choices you made while implementing each shader.

Two interesting, well-done shaders are worth a maximum of 10 extra points. The more interesting and convincing your shaders are, the more points will be awarded. Please do not hesitate to e-mail cs248@graphics if you have any doubts as to how interesting a particular shader is.

Submitting

• First, prepare images that you have rendered using your shaders, and put them on a web page, as described above.
• Second, run the /usr/class/cs248/assignments/assignment4/submit script, which will submit the file shader.c, which contains all of your shaders, the *.cfg files that define the parameters to your shaders and their default values, and your README file containing your web page URL, descriptions of your extra credit shaders (if any), plus any other items you would like to point out to your graders.