Week02 Lab Exercise

CS4815 Week02 Lab Exercise
Lab Objective: The object of this week’s lab is to take a first look at OpenGL, our graphics
library and to compile and run a program. Here’s a quick summary of the tasks:
❶ Create a series of hierarchical directories to manage our work throughout the semester
❷ Copy this week’s lab from the class directory ~cs4815/labs/week02
❸ Examine the gasket.c file there using a file browser or emacs or whatever editor you
like to use
❹ Compile and run the program and play around with the number of iterations that it
runs for
❺ Modify the colour that the program draws with
❻ Submit your lab for marking using handin
In Detail
❶ Note that there are a few alternatives given below for this first step so please read the
entire instructions before acting. Over the semester we will have labs, programming assignments, etc. and it is a good idea to keep these in an organised way. My suggestion is that
you create a subdirectory of your home directory called cs4815 that will be the top-level
point for all module-related material. The command to create a directory in Linux is mkdir,
so you could do
mkdir ~/cs4815
which makes a subdirectory located in your home directory. Next you should make a subdirectory called labs in this for each week’s work. This can be done with
mkdir ~/cs4815/labs
Finally, for this week’s lab, week02, you should create its own subdirectory with
mkdir ~/cs4815/labs/week02
An alternative to making each level of the hierarchy at a time is to tell mkdir to make
the “parent” subdirectory if it doesn’t exist. So the following command can take the place
of all of the previous ones.
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mkdir -p ~/cs4815/labs/week02
The -p is for “make parents if they don’t already exist.” Note that it is very like the
command before it, but you had to do a lot of previous work in order to achieve that.
Yet another alternative is to bring up a file manager and use the “Create ...” menu
option there.
❷ With the semester’s hierarchy set up, now copy to your ~/cs4815/labs/week02 directory
the program we will be examining today. This is called gasket.c and it can be found in
~cs4815/labs/week02 sub-directory. Change your working directory into this week’s lab
directory with
cd ~/cs4815/labs/week02
The command to perform the copy is
cp ~cs4815/labs/week02/gasket.c .
Note the dot at the end of the command that signifies copying it into the current working
directory and keeping the name the same.
❸ The program you are looking at is, by graphics programming standards, a pretty straightforward one. Don’t worry if it is confusing to you now, the object of the lab is to give you a
chance to see one from the inside.
The program generates a very famous graphic called the Sierpinski Gasket. Later you
can compile and run the program and see the resulting output but first we will briefly discuss
what the program is doing.
Given a triangle comprising three points in the xy-plane the construction goes as follows:
1. Pick an initial point at random inside the triangle select one of the three corners at
random
2. Find the point halfway between the initial point and the randomly selected vertex
3. Display this new point by putting some sort of marker, such as a small circle, at its
location
4. Replace the initial point with this new point
5. Return to Step 2
The first two iterations of the algorithm are shown below; in the program the loop repeats
5000 times, as shown in for loop of line 43, in the function display().
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It would be reasonable to think that this algorithm would just create a random smattering
of points inside the given triangle. We will compile the program now and see what happens
when we run it.
❹ In order to compile the program you will need to open a “terminal session” so that you
can issue commands. Follow one of the menus in the top-lefthand corner and you should be
able to open a terminal, if there isn’t one opened by default.
The command for compiling the program is
gcc gasket.c -l GL -l GLU -l glut -o gasket
Note that that’s not a “dash 1”, it’s “dash l” (short for library file).
Linux is always sensitive to character case so be careful to type it exactly as shown. The
program should compile without errors. After that you can run it with the command
./gasket
Don’t worry about the error reported to the screen (if any). Play around with the number
of iterations that the program runs for and see what effect it has.
❺ The final part of the lab exercise is to change the colour that it draws the points in. The
default drawing colour, red, is given by the program statement
glColor3f(1.0, 0.0, 0.0);
on line 14 in the function myinit(). Click here and follow the menu items to glColor for
documentation for an explanation of glColor3f(1.0, 0.0, 0.0);
You should now modify the colour that it draws in to your own “private” colour using
the last 6 digits of your student ID. If your student ID is 11051234 then for the RGB values
you should use 05 12 34 (and poor old “11” at the front gets ignored). Use these three
integers in the range of 0 to 99 and by dividing each by 100 you will get a value between 0
and 1 as required for each of the basic colour components.
Make this change to the function call glColor3f(1.0, 0.0, 0.0) and recompile. Please
leave the executable and .c file in your directory so that you can hand it in for inspection
using the handin program. And speaking of handin...
❻ Your lab work, like all CS4815 labs this semester, will be due no later than 09.00, Thursday
next. This will give everybody as close as is practicable to a week to work on each one. This
will be the continuing pattern throughout the semester, too1
: the lab sheet that you first
work on in Weekn will be due no later than 09.00, Thursday, Weekm, m = n + 1.
Below I will give you the command that you will need to run in order to submit your
work. In the early weeks you will get marks for simply handing something in and having it
compile. Your lab work will also be visually inspected by me later to make sure it does what
1with one or two exceptions
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it is supposed to do. Initially this won’t count for much but as the semester goes on, this is
where most of the marks will be allocated.
The command you need to run in order to make your submission:
handin -m cs4815 -p w02
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