Lab02: Drawing letters with Gnuplot

CISC181 Honors, Fall 2004

Background

In this exercise, you will use a program called gnuplot to draw letters of the alphabet. This exercise, together with some future labs, illustrates some of the basic ideas behind how "fonts" work in systems such as Microsoft Windows, Macintosh OS, and the X Windows system used on strauss.

This exercise will begin to teach you what you need to know about gnuplot.

In this exercise, you will first use a text editor (vi or emacs) to create a file that contains points that draw a letter of the alphabet. You will then use gnuplot to plot those points, and store the resulting graphic in an output file. The output file has the extension .png, which stands for "Portable Network Graphics". Files ending in .png are similar to files ending in .gif, .jpg, or .jpeg, except they use a different format for representing the pixels in the image, and for compressing those pixels.

This exercise also assumes you know how to put files on the web under your public_html directory, use the chmod command to make those files readable, and point your web browser at those files.

The suggested link for putting the files you create with this exercise is:

   http://copland.udel.edu/~userid/cisc181/lab02

where userid is your strauss userid. Hence, the directory name is:

   ~/public_html/cisc181/lab02

Why are we learning about gnuplot in a C++ programming course?

Gnuplot offers us an easy way to generate graphics from C++ programs. In future labs, we'll be generating data files using C++ programs, and then using Gnuplot to visualize that data.

Part 0: Copying files

Throughout this lab, you'l be using files from the gnuplot subdirectory of the labs directory. You might want to just copy all the files from that subdirectory into your current directory now. (First though, you should probably cd into your ~/cisc181 subdirectory.)

The following command will copy the entire gnuplot subdirectory, along with all its contents, into the current directory. You'll then have to cd into gnuplot to see all the files. The -r stands for recursive.

cp -r ~pconrad/public_html/cisc181h/04F/labs/lab02 .

Part 1: Creating a gnuplot data file: "A.dat"

A typical gnuplot data file consists of lines of text, where on each line there are two numbers, representing an (x,y) coordinate. Here is a gnuplot data file called "A.dat", followed by an explanation of its contents:

# A 0 0 2 8 4 0 1 4 3 4

Explanation:

The first line, the one starting with #, represents a comment.
When the gnuplot command "plot with lines" is used, gnuplot will draw lines that connect each point to the one following it. For example, the first three lines following the comment signify to draw a line from point (0,0), to point (2,8), then another line down to point (4,0). This forms the "tent" part of the capital letter A.
Blank lines represent places where gnuplot should "pick up the pen".
The final two lines signify to draw the cross brace of the A from position (1,4) over to (3,4).

Part 2: A file containing gnuplot commands: "A.gnuplot"

To plot this data file, you use a file that contains gnuplot commands. Here is an example file "A.gnuplot" that takes "A.dat" as input, and produces an output file "A.png". The actual graphic produced appears after the listing of A.gnuplot:

set xrange [0:8] set yrange [0:8] set output "A.png" set terminal png large color plot "A.dat" with lines

A.png graphic Here is an explanation of the gnuplot commands:

set xrange [0:8] signifies that the x-axis will run from 0 to 8. The set yrange command is similar.
set output "A.png" means that the output graphics will go to a file called "A.png". You could supply any other file here that you wish, though it should end with .png if you want it to work properly with your browser.
set terminal png large color means that we want .png output. (Note: There are other kinds of output that gnuplot can produce, but we won't need to know about those for this exercise.)
plot "A.dat" with lines means to get the data points from the file called "A.dat", and connect the dots with lines. There are other kinds of plots that gnuplot can do, but we won't need to know about those in this exercise.

Note: if you want to learn more about gnuplot commands, you can read about these by finding the gnuplot documentation on the web, or looking at the online help inside of the gnuplot program itself. The commands listed above are probably sufficient for this exercise, however.

Part 3: Actually plotting the graphic: creating the "A.png" file

To actually create the "A.png" file using "A.dat" and "A.gnuplot" (see above), type the Unix command:

gnuplot A.gnuplot

Then, to see the file, copy it into a directory on your webpage (i.e. somewhere under your ~/public_html directory tree) and do the "chmod" command that makes it readable. You should then be able to point your web browser to the file and see the output.

Sometimes setting the xrange and yrange commands in the gnuplot file to [-1,8] works better than [0,8]; when you go from -1, the xaxis and yaxis don't cover up part of your letter, and its sometimes easier to see what is going on. Experiment with changing the value from 0 to 1 to see what happens when you repeat the gnuplot command, and copy the resulting .png file to your webpage and refresh.

Part 4: Another example: CIS.dat, CIS.gnuplot, CIS.png

You will also see another set of three files. The CIS.dat file and CIS.gnuplot files can be used to produce a CIS.png file. However, note that if you look at the CIS.png file (by pointing a web browser to that file) the S in CIS is incomplete. As an exercise, see if you can modify the points in CIS.dat to make the S complete, and get the correct graphic to come up. That will help check your understanding of how gnuplot works before you go to the next step.

Part 5: Writing your own data file and gnuplot file

Create a data file called initials.dat that contains the points needed to print your first and last initial. Create each using block capital letters. Put your first initial in a box where the lower left corner is at position 0,0, and the upper right hand corner is at position 4,8. Put your second initial in a box where the lower left hand corner is at position 5,0, and the upper right is at position 9,8. That will leave one "point" of space between the two capital letters.

Also create a initials.gnuplot file that will take the input from initials.dat and produce an output file called initials.png. Adjust the xrange and yrange in your gnuplot file appropriately.

Grading and Submission

For this lab, you do not have to submit anything to WebCT. Instead, just make a directory under your web page called ~/public_html/cisc181/lab02, and copy your initials.dat, initials.gnuplot and initials.png files(from step 5) into that directory. Make the directory and all the files inside it readable. Your TA will look in that directory for the finished work. (You do not have to submit anything for any of the others steps in this lab.)

This lab is worth 50 points, and is due by 11:55pm on Monday September 27.

Final Thoughts

Writing a .dat file and a .gnuplot file by hand can be tedious. If we are going to plot a large graphic, we might want to write a program to do it instead. Next week in lab, we'll do just that.