CS140 -- Lab 2

Makefile and executables

There is a makefile for this lab in the file /home/plank/cs140/Labs/Lab2. Copy it to your directory, and then when you want to compile your programs, type "make" or "make exectuable-name". It is a nice way to automate the often tedious task of compilation.

As always, there are executables for each part of the lab in the lab directory. Make sure you test them out and make sure that your answer matches them exactly. If you have simple questions about what your program should do, try checking the executable first rather than asking the TA. Often, the answer is there.

PGM Files

This lab lets you create and manipulate PGM files. PGM stands for portable gray-map, and is file format for non-color digital pictures. PFG files are nice because they are ASCII files that you can view with an editor like vi or emacs, and that you can also view with certain software.

Unfortunately, most web browsers do not have support for PGM files. However, most every environment has a program for dealing with them. In standard Linux/xwindows, the display program (or xv if it is available) lets you view PGM files. On Macs, the GraphicConverter program allows you to view them, and you can set up your browser to run it whenever it sees a PGM file. I'm not sure about Windows support. However, on the cetus machines, there is a program called convert, which allows you to convert a PGM file to a JPG file, which you can then view easily with a web browser. Do the following to test it out: 

UNIX> cp /home/plank/cs140/Labs/Lab2/Nigel.pgm .
UNIX> convert Nigel.pgm Nigel.jpg
You may now view Nigel.jpg with a web browser. Test it out. It should look like:

Here are a bunch of sample PGM files that you can use with this lab. They contains some of my most important role models. Unfortunately, you shouldn't try to create your own PGM files with the above software, as most PGM converters insert comments, which we ignore here.

A picture is simply a two-dimensional array of pixels. In a PGM file, a pixel is a gray value between 0 and 255. Zero is black, and 255 is white. Everything in between is a different shade of gray.

PGM files have a specific format. (Actually, they are more robust than this -- I've removed comments to make them easier for you). A PGM file is composed of words. The first word is ``P2''. Then the next two words define the number of pixels in the picture -- first is the number of columns in the two-dimensional array, and then the number of rows. The next word is the number 255. So, if you look at the beginning of Red.pgm, you'll see: 

UNIX> head Red.pgm
P2
235 183
255
 67  74  76  80  76  77  70  67  75  82 102 119 129 133 130 127 128 
130 134 131 130 129 130 133 132 125 116 109 101 102 105 105 113 119 
128 133 138 137 137 137 141 143 143 144 140 143 146 144 140 138 128 
122 118 107 106 104 106 107 115 124 127 121 114 108 103  98  97 108 
122 138 140 140 139 129 123 125 127 141 145 147 147 146 147 147 147 
149 150 150 150 155 168 185 195 198 202 201 201 200 200 199 201 199 
200 201 207 205 206 203 204 205 206 206 205 206 207 207 207 207 207
This means that Red.pgm is a 183 * 235 array of pixels. After the 255 come all the pixels. First, the pixels in the top row, then the pixels in the next row, etc. Note that the ASCII formatting of the file doesn't mean anything -- there could be one pixel per line and the file would still be a legal PGM file. In Red.pgm above, the first 183 pixels are those in the top row, then the next 183 are in the second row, and so on. There will be a total of 235*183 = 43005 pixels. After the last pixel, the file ends.

Before you go any further, create a PGM file of your own -- make it 10x10 and give the pixels any value you want. Take a look at it. Something like: 

P2
10 10
255
0 10 20 30 40 50 60 70 80 90
10 20 30 40 50 60 70 80 90 100
20 30 40 50 60 70 80 90 100 110
30 40 50 60 70 80 90 100 110 120
40 50 60 70 80 90 100 110 120 130
50 60 70 80 90 100 110 120 130 140
60 70 80 90 100 110 120 130 140 150
70 80 90 100 110 120 130 140 150 160
80 90 100 110 120 130 140 150 160 170
90 100 110 120 130 140 150 160 170 180

This should look like:

Cool, no?

Program #1: Pgminfo

Now, your first PGM program should take a PGM file on standard input, and report the number of rows, the number of columns, the total number of pixels, and the average value of all the pixels, padded to three decimal places. Your program should work on all valid PGM files, and should print out an error (on stderr) on any invalid PGM file. Examples of invalid PGM files are:

Here is an example of pgminfo running on some of the PGM files. Note, I have a few bad PGM files in Bad-1.pgm, etc. You need to make sure that your output matches mine exactly. And I mean exactly, meaning the same punctuation, same number of spaces, and same capitalization. 

UNIX> pgminfo < Red.pgm
# Rows:         183
# Columns:      235
# Pixels:     43005
Avg Pixel:  120.142
UNIX> pgminfo < Pike.pgm
# Rows:         235
# Columns:      197
# Pixels:     46295
Avg Pixel:   99.932
UNIX> pgminfo < Bad-2.pgm
Bad PGM file -- No column specification
UNIX> pgminfo < Bad-5.pgm
Bad PGM file -- pixel 99 is not a number between 0 and 255
UNIX> pgminfo < Bad-6.pgm
Bad PGM file -- Extra stuff after the pixels
UNIX>
Note, when I print a pixel number, it is zero-indexed. So the first pixel is pixel zero.

Note also, you don't need to call malloc() with this program, and the only procedure that you need to use to read in information is scanf().

Program #2: Bigwhite

This program takes two numbers as its command line arguments -- the number of rows and the number of columns. It then writes a PGM file on standard output which contains that number of rows and columns, all of white pixels. Again, you should error check to make sure that the proper number of command line arguments are given, that they are integers and in the proper range. On an error, print the error statement to stderr. As an example, try: 

UNIX> bigwhite 20 10 > a.pgm
This will create a PGM file a.pgm, which has 20 rows and 10 columns of white pixels.

Your output in this and the next two programs should match mine exactly when there is an error. Otherwise, the PGM files that it emits should be equivalent to mine (they should make the same picture), but do not have to have the same output exactly.

Program #3: Neg

Neg takes a PGM file on standard input, and prints a PGM file on standard output that is the negative of the input file. If the PGM file is not valid (same parameters as pgminfo), print an error to standard error.

For example, here is the negative of Red.pgm:

Program #4: Hflip

Hflip reads a PGM file on standard input, and prints a PGM file on standard output. The output file should be the horizontal reflection of the input file -- in other words, left is right and right is left.

You'll have to use malloc() for this program. Read in the first four words, and then malloc() a row's worth of pixels. Then, for each row, read in the row and write it out backwards. Test it out. Here's the hflip of Red.pgm: