CS 445/645: Introduction to Computer Graphics

Goal: Implement Sutherland-Hodgeman clipping for arbitrary planes.

URL: http://www.cs.virginia.edu/~gfx/Courses/2000/intro.fall01/Exercises/assignment3.htm

Assigned: Thursday, October 4th, 2001

Due: Sunday, October 21, 2001 at Midnight

Test Cases Used For Grading:

• sphere.p1
• sphere.p2
• sphere.p3
• tpot.p1
• tpot.p2

Relevant reading/files/software:

• It is important that you check back with this page frequently to see any bug reports or updates. We will add any new information in red.
• We continue to use OpenGL (http://www.opengl.org). The OpenGL Red Book is available here:
    http://fly.cc.fer.hr/~unreal/theredbook/

Synopsis: Your program will read a command-line specified parameter file, load a polygonal object, clip the polygonal object with two clipping planes, and use OpenGL to render it.  You will write code to clip the model to the two arbitrary clipping planes using the Sutherland-Hodgman algorithm. Your OpenGL program will also support mouse navigation through the model. Grad students, note that your program must also support interactive repositioning of the clipping planes.

Collaboration/Web Resources:  Unlike previous assignments for this course, you cannot use resources from the web to do this assignment.  Code segments for the tasks in the assignment are probably floating around the web and it is inappropriate to use any of them.  As per the earlier assignments, you can still talk to your classmates about your work, but you cannot look at each other's code or work side-by-side to generate your code.

Specifics: We are providing a Makefile and a C++ skeleton file named assign3.cpp. The assign3.cpp file will set up lighting so that front-facing polygons will be drawn one color and back-facing polygons another. You will modify the functions in assign3.cpp, adding code to draw the scene and handle callbacks. Some special things to notice about the code:

• Your program must accept one command line argument that will be a character string of no more than 256 characters. The program must execute from the command line like this:

  prompt> assign3 parameter.file

• The four lines in your parameter file must look like this:

  modelname.poly
  A B C D
  E F G H
  X
  

   

  • Where the .poly file is defined according to these rules. We will be using these .poly files to test your code.

  • A B C D are the four doubles that define the parameters of the first implicit plane equation: Ax + By + Cz + D = 0. Clip away all geometry found in the negative half space formed by this plane

  • E F G H are the four doubles that define the parameters of the second implicit plane equation: Ex + Fy + Gz + H = 0. Clip away all geometry found in the negative half space formed by this plane

  • X is the integer that specifies rendering globals. A value of '1' indicates that the clipping planes should be drawn. A value of '0' will turn off the rendering of the clipping planes.

• The user must be able to navigate through the world using the mouse.
  • Left button + drag = rotating the camera around the polygonal model. The camera should remain a constant distance from the model, but the translation of the mouse should correspond to longitudinal and latitudinal rotations of the camera about the model. Picture the way you use a trackball.  (Don't worry about the poor implementation of interactive rotation of the world. Try rotating 180 degrees about x and then rotate about y to see the problem to which I am refering.)
  • Right button + drag = zooming in and out of the model. Forward  movements of the mouse move the camera closer to the model's center and Downward  movements move the camera away from the model.

• Use the Sutherland-Hodgeman algorithm to clip all polygons that penetrate, or fall within, the negative half spaces of either (or both) of the clipping planes.  If a polygon is partially clipped, compute the new polygon that is within the positive half space.

• (Grad students only) Use the center mouse button to interactively select one of the clipping planes. Once selected, use the left and right mouse buttons to allow the user the freedom to reposition the selected clipping plane. Translation is all that I require, but rotation would be nice.

Advice: OpenGL has direct support for clipping to arbitrary planes. Don't even think about it. :-)

Turning in the assignment:

• I have had Small Hall SGI accounts created for everyone in the class.  Use the same login/password as you use for Blue.unix.
• Verify that your code compiles and runs correctly on one of the SGI machines in Small Hall (sgi-1.unixlab.virginia.edu for example). We will use these machines for compiling all assignments.
• Write a README file with your login, your name, what you did, and anything you'd like us to notice or know about your demo.
• Deposit the README and all files required to compile and run you code in the following directory:
       /courses/cs/445_brogan/Students/<username>/Exercise3