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CS4815 Week04 Lab Exercise

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CS4815 Week04 Lab Exercise
Lab Objective: We will continue our introduction to OpenGL again this week by
• observing how selection of an object on the screen can be achieved
• observing the deficiencies of single buffering when drawing moving objects
• by creating and using a menu to modify program behaviour
• though not immediately related to graphics programming we will also see in action the
make utility that is a life-saver when developing any type of software
Here’s a quick summary of the tasks:
❶ Copy the source code for this week’s lab from the class directory ~cs4815/labs/week04
❷ Learn how to do a beginner’s make that will compile programs easily for you
❸ Examine the source code and behaviour of the glut7 and glut8 programs; do likewise
with the program spin
❹ Modify the behaviour of spin so that it responds to menu events
❺ Hand in your completed work (by the standard deadline) using handin
In Detail
❶ Following the procedure of previous weeks create a directory for this week’s lab in
~/cs4815/labs/week04. Now copy over from the class directory the three source files
glut7.cc, glut8.cc and spin.cc, and the makefile Makefile. These are to be found
in ~cs4815/labs/week04.
❷ This week we will introduce you to makefiles, one of the foundation stones of developing
even simple programs.
While our programs here are necessarily short and self-contained I hope that you can
imagine that this is not how mid-sized – never mind large – systems are developed. Code is
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distributed across many source files and include files (.h files) and when code is changed in
one or several maintaining a current executable is not always so easy. Enter make.
The main thing that the make program seeks to assist in is managing the compilation
process when your code is spread out across several files. In the Makefile you tell it about
your files, your programs and their dependencies. That is, you tell it what files are used in
the compilation of what programs and how your programs should be compiled (made) and
what to do if you update the source code of one of them. In a properly written makefile when
a change is made only those programs affected by the change should be updated / remade.
There are lots of tutorials available on the web that describe the make program so we
are going to limit ourselves here to how to run make. Because of the way the Makefile is
written you can either tell it to compile one of the three programs (glut7, glut8 or spin) or
you can tell it to compile them all. The command for telling make to just update the spin
executable would be make spin. In the terminology of make, we call spin the target. Often
you want to have the system “do everything and remake everything that is not up to date”.
You can do this with the command make all or, even shorter, just make, since the default
target is all.
❸ The programs this week are glut7, glut8 and spin. These are to be made form their
respective source files glut7.cc, glut8.cc and spin.cc. The first two are to be had from
the extremely useful site http://www.lighthouse3d.com/tutorials and these will be the
basis for the spin program.
You should not be put off by the complexity of the source code of the two glut programs
this week. By running the programs and watching its behaviour you can learn a lot from
the source code you are presented with.
In order to run the programs you will need to compile them first. Follow the make
instructions you were given earlier for compiling them with:
make glut7 glut8 spin
Note carefully the compile commands that make generates. Thus make is not a compiler but
is, more usefully, a program to manage compilation.
There is very little difference between the glut7 and glut8 programs. In the former a
menu is created that changes the display colour of the triangle; in the latter a second level
of menus (a sub-menu) is introduced to do exactly the same thing. Your task for this week
is to implement a menu system like this for the spin program (see below). Note in both
cases how a menu is
1. created
2. associated with a mouse button
3. tied to an event handler
All of this goes on in the createGLUTMenus() function. The function glutAddMenuEntry(“name”,
VALUE) adds the entries to the menu saying that the menu entry should appear as “name” and
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the corresponding return value should be VALUE, which will be processed in the processMenuEvents()
function.
Note how simple the structure of the code is in all three cases. While you may not
understand every line down at the detail level, since they are so similar to the programs you
have been seeing over the past weeks you should certainly be able to follow the general flow
of control of the program.
You will spend the remainder of the lab modifying the spin program.
❹ You should now modify the program so that its behaviour can be changed from a menu.
The menu should be associated with the right button and it should have three top-level
options:
1. direction
2. speed
3. quit
The direction option should, in turn, have three options:
1. reverse
2. clockwise
3. anti-clockwise
This will control the direction of rotation of the square.
The speed option should have at least a way of increasing or slowing the speed of rotation
but you might want to provide something a bit more flexible such as adding (subtracting)
different increments and having a twice (half) the speed option.
What would be nice also would be adding a fourth menu option that controls the colour
that the square is painted in.

You should now hand in your completed program for marking and evaluation. The
command for handing in this week’s assignment, is:
handin -m cs4815 -p w04
That’ll do pig, that’ll do.
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