CS 551: Introduction to Computer Graphics

Goal: Implement Sutherland-Hodgeman clipping for arbitrary planes.

Assigned: Thursday, February 23, 2000

Due: Tuesday, March 7, 2000

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 Xforms; here is the XForms documentation. The XForms home page (http://bragg.phys.uwm.edu/xforms) is still down; http://world.std.com/~xforms seems to be a working mirror site.
• We also continue to use OpenGL (http://www.opengl.org) or Mesa (http://www.mesa3d.org). Brandon Shepard points out that http://heron.cc.ukans.edu/ebt-bin/nph-dweb/dynaweb/SGI_Developer/OpenGL_PG/@Generic__BookView appears to be a complete online copy of the red OpenGL book (we also have it under "insight" on some SGIs).

Synopsis: Your program will load a polygonal object and use OpenGL to render it. You will write code to clip the model to two arbitrary clipping planes before rendering using the Sutherland-Hodgman algorithm. You will need to design a form in fdesign that contains:

• An OpenGL canvas in which to display the object (name this canvas myCanvas).
• An XForms "file selector" object to specify the .poly file to be viewed.
• Controls for setting the orientation and position of the clipping planes.
• A button to toggle between wireframe mode (GL_LINE_LOOP) and filled mode (GL_POLYGON).
• A "Quit" button.

As usual, the single best assignment (measured by ease of use and "extra" features) will earn an 11 (one point extra credit).

Turning in the assignment: Before class starts on Thursday, March 7, you should:

• Verify that your code compiles and runs correctly on sgi-1.unixlab.virginia.edu. We will use this machine 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.
• Verify that the line in your Makefile beginning "SUBMIT =" includes all relevant files. This includes all source code (including header files!), fdesign form files (.fd suffix), your README, and the Makefile itself. You should not include object (.o) files, libraries, or the final executable.
• Type "make submit" in the directory containing your code. You will get mail in return from the CS551 account with a list of the files that were successfully received.
• If you submit the assignment multiple times be aware that we will only see the last attempt.

Specifics: We are providing the usual Makefile, a C file named gfx.c, which contains code to draw into an Xforms glCanvas, and a header file gfx.h, which contains prototypes for the functions in gfx.c. The gfx.c file will set up lighting so that front-facing polygons will be drawn one color and back-facing polygons another. As usual, you will modify the functions in gfx.c, adding code to draw the scene and handle callbacks. You will also create a form in fdesign. Saving that form (as a .fd file) will also create .c and .h files that contain the Xforms library code for creating the form, which you must include in your program via the Makefile. Some special things to notice about the code:

• Your code will need to create a global variable pointing to your form. This pointer is returned from the create_form_* call; the Xforms documentation has details. To make the code in gfx.c work correctly you must name this variable myForm. Your form must include a glCanvas object, which you must name myCanvas.
• Gfx.c contains a function called canvas_expose() which will redraw the canvas when it is created or covered and exposed by another window. After the create_form_* call, you must attach this function to your canvas with the line:
  fl_add_canvas_handler(myForm->myCanvas,Expose,canvas_expose,0);
• Here are some models in the .poly format. The horse has inconsistent polygon orientation and may look strange even before clipping--debug on simpler models.

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