CS 551/645: Introduction to Computer Graphics

Goal: Implement clipping of triangles

Assigned: Wednesday, September 29, 1999

Due: Wednesday, October 6, 1999

Relevant reading/files/software:

• Foley and van Dam, section 3.11 to 3.14
• Check the class web page (http://www.cs.virginia.edu/~cs551) and this assignment frequently. We will add any updates in red.
• No resources on the Internet may be consulted.  "Paper research" is fine; if you can find an algorithm in the library, you can implement it (and yes, you can use Virgo to search). But cut-and-paste coding is not allowed.

Synopsis: You will design a clipping stage for the rendering pipeline we are building. Your clipper will take as input the three vertices of a triangle, clip the triangle, and draw the clipped polygon (if any) using libtri.a.

Specifics: We are providing a header file cliptri.h defining function prototypes for the library; you will have to fill in the source code for those functions. You can reuse the framebuffer library from Assignment 3 (frame.h, and frame.c) for debugging purposes. We also provide the usual Makefile, and a canonical version of tri.h and libtri.a (for solaris, and for irix, both of which were last modified on Friday at 5:25PM) to use if you wish. A few things to note:

• You will have to implement two functions: setviewport() and drawcliptri(). The first sets the rectangular viewport within the framebuffer to which triangles will be clipped. The second takes the coordinates of a triangle, clips the triangle against the specified viewport, and draws the resulting polygon with drawtriangle(). These functions should return the proper values, specified in the header file.
• You can visually debug your algorithm by setting the viewport to be smaller than the framebuffer, drawing triangles that overlap the edge of the viewport, and making sure that only the correct pixels are turned on.
• The setviewport() command takes integer pixel values; these represent the ranges of pixels that should be affected by triangles. For example:

• Note that your code need not (and should not) reference the framebuffer directly at all, but should simply make calls to drawtriangle(), which does write the framebuffer.

Test Files: You may wish to reuse the test files from Assignment 3.

Grading: The usual 10-point scale. An assignment that does everything correctly will receive 8 points. Fast algorithms (that still produce correct output!) can earn up to 9 points. Points will be deducted for assignments that don't compile "out of the box", or that crash on some input, or that produce incorrect output (e.g., turning on pixels that should be unaffected, or failing to turn on pixels that should be affected). The two fastest (and entirely correct) assignments get 10s.

Turning in the assignment: Before class starts on Wednesday, October 6, you should:

• Write a README file describing what you've done, any particular troubles you had, and the kind of optimizations you've made.
• Verify that the line in your Makefile beginning "SUBMIT =" includes all relevant files: your README, source code (including header files!) for your library and demo, and the Makefile itself. Do not include object files (.o suffix), your final library, or any supporting libraries (.a suffix).
• 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.

Advice: Get it working (shouldn't be too hard), then make it fast.