Week05 Lab Exercise

CS4815 Week05 Lab Exercise
Lab Objective: We will complete our introduction to OpenGL over the next two weeks by
• looking at the use of output primitives (GL_LINE_STRIP), etc.
• looking at a chart drawing program for plotting some silly data, and
• modifying its behaviour to react to menu and keyboard events
Here’s a quick summary of the tasks:
❶ Copy the source code for this week’s lab from the class directory ~cs4815/labs/week05
❷ Take a look at a function for plotting circles efficiently
❸ Use the provided makefile to compile two source files in to one executable
❹ Run the program and be prepared to be un-Dazzled
❺ Modify the program so that it switches between chart-drawing modes
❻ Although there is no handin due this week what you do in the lab will be crucial for
next week’s handin.
In Detail
❶ Following the procedure of previous weeks create a directory for this week’s lab in
~/cs4815/labs/week05. Now copy over from the class directory the two source files chart.cc
and circlePlot.cc. These are to be found in ~cs4815/labs/week05.
❷ One of the two files you have been provided with this week is called circlePlot.cc and
it contains a function, circleMidpoint(), that takes a centre-point and a radius, and plots
the corresponding circle. This is a very interesting method for plotting a circle as it takes
huge advantage of the symmetry in plotting circles in order to speed up the rendering. This
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is achieved in circlePlotPoints() where 8 different pixels can be coloured just from one
known point. Note also the function setPixel() which “colours” a pixel.
❸ The source code you are being given this week is C++ , although only barely: a very
simple class that defines a point in the xy-plane makes it C++ as opposed to C. We will use
.cc to denote C++ files and the compiler we will use for compiling C++ programs is g++ .
If you look at the Makefile you will see where we specify CXX=g++ .
What is also new about this week’s set-up is that the executable comes in two parts: the
main bulk of it is in a file called chart.cc and this is the one you should do all of your work
on.
In terms of getting the program to be runnable you will need to compile
together the two source files to create an executable, chart. A way to do
this is with the command:
g++ chart.cc circlePlot.cc -l GL -l GLU -l glut -o chart
This takes all of the source code in both files, syntax checks them, generates machine code for them and searches in the glut library for “anything
that wasn’t supplied” in the two source files. From the point of program
development the purist would say that this generates unnecessary work:
since we will never change circlePlot.cc why should we repeatedly have
to syntax check it and generate machine code? The solution here is to syntax check it and generate the machine code (called compilation) once and
save it in object form; this is achieved with the -c flag to the compiler and
results in a .o file being generated (.obj in the Devil’s tongue). As we
change chart.cc we will need to compile it each time. Then we link the
two object files together to create the executable. This is summarised in
these three steps:
g++ -c circlePlot.cc
g++ -c chart.cc
g++ chart.o circlePlot.o -l GL -l GLU -l glut -o chart
The first command is done once only, the other two are done any time you
make a change to chart.cc.
While all of the above works very well we can now begin to see the power of a properly
set up makefile. If we modify, say, one of the .cc files we need to recompile it and link
the .o version to recreate the executable. Alternatively we just type make and the makefile
manages the process for us.
❹ Run the program and note how the bar chart is not fully displayed in the window. Will
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resizing 1
the window fix this problem? Whether it does or not, this is not a proper solution.
Can you fix this? Hint: don’t do anything until you have read and edited/rerun the program
as per the next item.
❺ You will see from the code in chart.cc that there are 2+ chart-drawing routines. There
are functions barChart(), pieChart() and and empty (stub) lineChart(). Comment out
the call to barChart() and replace it with the call to pieChart(). Remake and rerun. You
should have the same data presented without labels this time. Try resizing the window.
Why is the circle becoming elliptical? Can you fix this? Hint: the glViewport(); function
does the job. See glut07.cc of last week for its use.
The second and final task for today is to set up a menu system so that the type of
chart can be selected from a menu; the data should be redrawn in the selected format on
exiting the menu. Refer to last week’s sample code for how to tackle this. As an add-on,
add a keyboard handling callback so that hitting ’c’ or ’b’ will toggle between the two styles
of chart, also. You should add a quit() function that can be used by the menu, or the
keyboard, ’q’.
1Clicking on the bottom right corner and dragging.
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