Assignment 4

Assignment 4: Procedural Modeling

Due: 11/22/99 at 11:59PM

Announcements

Wednesday, 12/08: Check out the submissions.
Sunday, 11/21: Due to the network down time yesterday, most people were unable to access their files. To make up for this, the due date has been changed to 11/22/99 at 11:59PM.
Thursday, 11/18: As some people pointed out during precept yesterday, it would make more sense to have the xi values instead of the lambda values in the ShellArgs structure (xi is the name of that squiggly greek letter). For example, if you increase Nt, you would like to have a shell with the same overall shape, but smoother. If you keep Lr fixed, what happens is that the overall shape changes, because now the radius grows faster. The solution is to keep Xr constant, and compute Lr for the new value of Nt (the same is valid for Lz and Lc). Also, as many people noticed, we also need to specify the initial scale factor c0 for the generating curve.
These changes have been made to the ShellArgs structure, and the files in /u/cs426/4 were fixed accordingly. You should copy the new files to your directory, and make sure your code works with this new format. Make sure you don't overwrite your shell.c.
Sunday, 11/14: If you are developing on a PC, running Microsoft Visual C++, make sure the Tcl/Tk libraries are installed on the machine you are using (look at "C:\Program Files\Tcl"). If they are not, please download the Tcl/Tk self-extracting installer (tcl822.exe), and run it. During the installation process, make sure you make a full install. In particular, check the box "Header and Library Files."
Saturday, 11/13: The SGI Tcl libraries were recompiled for the o32 file format. You should get the updated Makefile.sgi at /u/cs426/4, and change the LD_LIBRARY_PATH environment variable so it contains the /u/cs426/lib directory:
```
% setenv LD_LIBRARY_PATH /usr/lib:/lib:/u/cs426/lib
```

Overview

In this assignment you will implement a procedural modeling program that generates seashells. The method we will use is described in the following paper:

Modeling seashells

The basic idea is to construct a 3D polygonal mesh by sweeping a curve along a helico-spiral. With this relatively simple model, several different types of seashells can be generated by varying a few input parameters.

What You Have to Do

You should implement a program that generates seashells with the method described in the paper "Modeling Seashells." The inputs to the program will be:

A set of parameters tmax, r0, Xr, z0, and Xz describing the shape of the seashell's helico-spiral axis (as described in Section 2.1 of the paper).
A sequence of 2D points (u, v) describing the shape of the seashell's cross-section (the "generating curve" described in Section 2.2 of the paper).
A set of parameters c0 and Xc describing the growth of the scale factor applied to the generating curve as it is swept along the helico-spiral axis (as described in Section 2.3 of the paper).

The output of the program will be:

A .ray file containing a triangle mesh representing a sea shell.

Your program should first construct a helico-spiral using the procedural method described in Section 2.1 of the paper. It should represent the generating curve as a Catmull-Rom spline generating curve in the uv plane. It should construct a 3D triangle mesh by sweeping the generating curve along the helico-spiral, aligned with the helico-spiral's Frenet Frame, as described in Sections 2.3 and 2.4 of the paper, and output the resulting mesh as a .ray file. Other features can be included as described below.

The assignment is worth 15 points. The following is a list of features that you may implement. The number in parentheses corresponds to how many points it is worth. Options in bold are mandatory.

(2) Construct a helico-spiral according to parameters tmax, r0, Xr, z0, and Xz as described in Section 2.1 of "Modeling Seashells."
(1) Sample the helico-spiral at Nt points.
(2) Construct a generating curve by interpolating a sequence of points (u,v,w) with a uniform piecewise-cubic Catmull-Rom spline scaled by a factor c0 * Xc^t as described in Section 2.3 of "Modeling Seashells."
(1) Sample the generating curve at Nc points.
(1) Compute a Frenet frame for each of the Nt sample points along the helico-spiral, with e1 tangent to the helico-spiral, e2 pointing in the principal normal direction, and e3 perpendicular to both e1 and e2, as described in Section 2.3 of "Modeling Seashells."
(1) Transform each of the Nc sample points on the generating curve into the coordinate system of each of the Nt Frenet frames, aligning the w axis with a vector tangent to the helico-spiral (e1) and aligning the u axis with the principal normal vector (e2) as described in Section 2.3 of "Modeling Seashells."
(1) Construct a 3D triangle mesh by connecting all of the Nt*Nc transformed sample points with their immediate neighbors as described in Section 2.4 of "Modeling Seashells."
(1) Generate a normal for each mesh vertex either by transforming a vector normal to the Catmull-Rom generating curve or by averaging the normals of adjacent faces in the mesh.
(1) Generate texture coordinates for each mesh vertex and apply an interesting texture to the surface of a sea shell.
(1) Output the resulting 3D triangle mesh as a .ray file.
(1) Add a periodic displacement to the transformed sample points in a direction normal to the generating curve as described in Section 2.5 (see Figures 7, 8, and 9) of "Modeling Seashells."
(1) Add a periodic displacement to the scale factor applied to the generating curve as described in Section 2.5 (see Figure 10) of "Modeling Seashells."
(1) Add a periodic perturbation to the mesh vertex normals to simulate bump mapping (only for sufficiently large Nc and Nt).
(1) Support alignment of the generating curve with a different coordinate system, such as aligning the v and u axes respectively parallel and perpendicular to the shell axis z, as suggested in Section 2.3 of "Modeling Seashells."
(1) Generate a ray traced image of an interesting sea shell (not the same shell used for your art contest submission).
(2) Add a crusty appearance to the shell surface by subdividing and perturbing the mesh with a fractal random process in which each triangle is split into four and the newly generated points are displaced randomly in a direction normal to the mesh surface.
(2) Subdivide and smooth the triangle mesh by repeatedly applying a smoothing subdivision rule.
(2) Reparameterize the generating curve to partition the generating curve into more equally spaced points as described in Section 2.4 of "Modeling Seashells."
(3) Generate pigmentation patterns using the "activator-substrate" model described in Section 3.1 of "Modeling Seashells."
(3) Generate pigmentation patterns using the "activator-inhibitor" model described in Section 3.2 of "Modeling Seashells."
(?) Impress us with something we hadn't considered...

By implementing all the required features, you get 12 points. There are many ways to get more points:

implementing optional features (as listed above),
submitting images for the art contest (1), and
winning the art contest (1).

It is possible to get more than 15 points. However, after 15 points, each point is divided by 2, and after 17 points, each point is divided by 4.

Getting Started

You should use the code available at /u/cs426/4/, on the MECA machines, as a starting point for your assignment. We provide you with several files:

main.c: Implements a parser for the Tcl script that describes a shell.
algebra.[ch]: Implements basic linear algebra functions for manipulating vectors and matrices.
shell.[ch]: Implements the functions to construct, write, and draw a shell.

The only file you will have to touch is shell.c. There you will find the three functions that you have to implement:

ShellConstruct(ShellArgs *args): constructs a data structure (of your choosing) describing the shell triangle mesh with the given arguments.
ShellWrite(const char *fileName): writes the most recently constructed triangle mesh to a file in .ray format.
ShellDraw(void): draws the most recently constructed triangle mesh using OpenGL commands.

After you copy the provided files to your directory, the first thing to do is compile the program.

If you are developing on a PC, running Microsoft Visual C++, first make sure the Tcl/Tk libraries are installed on the machine you are using (look at "C:\Program Files\Tcl"). If they are not, please download the Tcl/Tk self-extracting installer (tcl822.exe), and run it. During the installation process, make sure you make a full install. In particular, check the box "Header and Library Files." Then, from the MS-DOS prompt, type:
```
% copy Makefile.pc Makefile 
% "c:\Program Files\Microsoft Visual Studio\VC98\Bin\vcvars32"
% nmake
```
If you are developing on an SGI machine, first type:
```
% setenv LD_LIBRARY_PATH /usr/lib:/lib:/u/cs426/lib
```
It's a good idea to add the previous line to your ~/.cshrc file, so you don't have to type it again. Then, type:
```
% cp Makefile.sgi Makefile 
% make depend
% make
```

An executable called seashell or seashell.exe will be compiled and linked. To run a simple test, type:

% seashell < shell1.tcl

The shell1.tcl file contains a sample script (for the shell in Figure 1 of "Modeling Seashells"). Note that the file is a Tcl script that is interpreted by the parser created for you in main.c. You don't have to worry about how the parser works at all.

What to Submit

You should submit:

the complete source code with a Makefile that works on the MECA machines,
any interesting .tcl scripts, and files you created (do not submit .ray the files - we'll generate them using your program)
the images for the art contest, and
a writeup.

The writeup should be a HTML document called assignment4.html which may include other documents or pictures. It should be brief, describing what you have implemented, what works and what doesn't, how you created your .tcl files, how you created the art contest images, and any relevant instructions on how to compile and run your program.

Make sure the source code compiles in the MECA workstations. If it doesn't, you will have to attend to a grading session with a TA, and your grade will suffer.

Always remember the late policy and the collaboration policy.

Last update: 11/10/99 10:17 AM