CS 248: Introduction to Computer Graphics

Pat Hanrahan

Assignment 4

Handed out:	Thursday, February 26, 1998
Due:	5pm, Tuesday, March 10, 1998

Note: Late days cannot be used for this assignment since we need to grade it right after the due date in order to turn in final grades on time. If you need to take more time, we will need to turn in an incomplete for your final grade; please send e-mail to Prof. Hanrahan to arrange this.

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 (ie. texture coordinates). Using 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

Surface Shaders (16 points each, 64 points total):

• Bump mapping. 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 simple diffuse shading model for the surface color. Use one light and a surface color of gray.
• Wood. Use a noise function like the one discussed in class or the one from the online example to implement a 3D wood texture. Experiment with the parameters to the noise function and with colors to get a good looking wood texture. Derive the 3D texture coordinates from the object-space coordinates of each vertex (in info.h, these are called rawSurfacePosition), rather than from the texture coordinates. You'll have to scale these positions down to lie in a suitable range (between 0 and 1, for example). The size of the object's bounding box (in info.h, this is given by objXMin, objXMax, etc.) will be very helpful in doing this.
• Marble. Use a noise function like the one discussed in class or the one from the online example to implement a 3D marble texture. Experiment with the parameters to the noise function and with colors to get a good looking marble texture.
• Real surface. Pick a real surface (one from your room, outside your window, or perhaps from our page of sample real-world surfaces and write a shader to recreate it. Document the choices that you make while implementing it and submit it as a README file.

• Projected texture. Write a shader that shades a surface (using any surface shader, like the ones above) that is illuminated by a light with a stencil in front of it. Implement the stencil as a black-and-white image that you can place in front of the light source. Any light ray that passes through a black area on the image does not illuminate the surface, and any light ray that passes through a white area on the image illuminates the surface. Any light ray that misses the image entirely also does not illuminate the surface. Design your own stencil image, or use one of the samples from the assignments directory.
• Disco ball. Write a shader like the previous one that produces a disco-ball like effect as follows: Instead of using a planar stencil derived from an image as in the previous part, use a procedurally-defined mask consisting of small holes (circles or other simple shapes). The easiest way to think of the stencil is that it is spherical mask around the light source.

Extra Credit

Surface Shaders

• Ripply Water: write a bump-map shader that simulates windswept water and combine it with reflection mapping to generate good-looking water.
• Thin-film interference: if you look at a compact disc, or a puddle of gasoline, you'll notice a rainbow of colors, caused by thin-film interference of the light. Write a shader that simulates this.
• Moldy donut: use bump maps and an interesting surface reflection shader to make the torus model look like a moldy donout.

Submitting

• First, 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 the description of your real-world surface shader, plus any other items you would like to point out to your graders.
• You also need to prepare images that you have rendered using your shaders and put them on a web page. For each shader, you should save a PPM file from the viewer. Copy these files to your WWW directory and make a small web page with links to the images. There is a template web page in /usr/class/cs248/assignments/assignment4/248.html which you can use as a starting point. Please verify that all of the links on the web page are correct.