This lab we will be writing programs that manipulate images. We will be using a format called PGM (.pgm), that is very simple. It represents an image in a text file in which each pixel is represented by a number from 0 and 255. 0 is black, and 255 is white. The first line of the file is P2, the second line is the width-space-height in pixels, the third is 255, and from then on we get pixels values. So, for example, here is the file smallest.pgm: :

P2 4 5 255 0 255 0 0 0 0 255 0 0 0 0 255 0 80 160 240 240 160 80 0

Note: this image has 5 rows and 4 columns. So we see that the width is listed first, and the height is listed second!

So we will be reading, storing and writing files in these formats. I want you to use your own pictures for this stuff. Hopefully you already know how to download and save image files. What you need to know, however, is how to convert them to pgm files. Fortunately, the lab machines (and youf VMs) have a suite of tools called "Imagemagick" installed. One of these, convert, can do this conversion for you. Suppose foo.jpg is a file of yours (could be .png, .gif, whatever). To convert it to pgm do the following:

convert -compress none foo.jpg fpp.pgm

You can convert back to .jpg with the following:

convert bar.pgm bar.jpg

You can also resize with Imagemagick. For example, to stretch/compress the an image tmp.jpg into a 300 wide by 500 pixel high image, I can do:

convert -resize 300x500! tmp.jpg tmpNEW.jpg

Finally, if you want to get the pixel dimensions of an image, the identify command will do it for you:

$ identify cat.jpg 
cat.jpg JPEG 1024x768 1024x768+0+0 8-bit Grayscale Gray 256c 73.2KB 0.000u 0:00.000

... from which we see it is 1024x768. With these simple commands in hand, you should be able to prepare your own images for this lab. To view an image, use the eog program. For example:

eog foo.pgm &

I really prefer you find some images of your own. If you want, however, here are two for you to play with: dog.pgm / dog.jpg and cat.pgm / cat.jpg (and you can test further with ianfh.pgm and cd.pgm).

Part 1: Posterize

Write a program part1.cpp that reads in a file in .pgm format, and produces a "posterized" version of that file. Note: even though this particular part doesn't require it, I insist that you read the file completely and store it in a 2D array, and only then create the posterized version. Here's an example:

run of program	cat.pgm	postercat.pgm
~/$ ./part1 filename: cat.pgm output filename: postercat.pgm

The rule for posterization I used was this: if a pixel in the original has value > 128, in the new image that pixel has value 255. Otherwise, it has value 0.

Submit: ~/bin/submit-external -c=SI204 -p=lab09 part1.cpp

Important: I recommend you test your program on smallest.pgm as shown below.

smallest.pgm	run program	poster.pgm
~/$ cat smallest.pgm P2 4 5 255 0 255 0 0 0 0 255 0 0 0 0 255 0 80 160 240 240 160 80 0	~/$ ./part1 filename: smallest.pgm output filename: poster.pgm	~/$ cat poster.pgm P2 4 5 255 0 255 0 0 0 0 255 0 0 0 0 255 0 0 255 255 255 255 0 0

Part 2: reflections

Write a program part2.cpp that reads in a file in .pgm format, and produces a version of that file that is the image with its reflection along the bottom of the original image. Here's an example:

run of program	dog.pgm	mirrordog.pgm
~/$ ./part2 filename: dog.pgm output filename: mirrordog.pgm

Submit: ~/bin/submit-external -c=SI204 -p=lab09 part1.cpp part2.cpp

Part3: merging (you can try striping too)

Write a program part3.cpp that reads in two files in .pgm format, both of the same size, and produces a new file of that size that merges the two files. My rule for merging is that the value of pixel i,j in the merged file is the average of the values of the first file's pixel i,j and the second file's pixel i,j value (note: truncate any fractional part, don't round!). Here's an example:

run of program	cat.pgm	dog.pgm	catdog.pgm
~/$ ./part3 file 1: cat.pgm file 2: dog.pgm output filename: catdog.pgm

Submit: ~/bin/submit-external -c=SI204 -p=lab09 part1.cpp part2.cpp part3.cpp

Another intersting thing to try with two pictures of the same size (not to turn in!) is to write a program that reads in two files in .pgm format, both of the same size, and produces a new file of that size consisting of vertical strips alternatingly from one then the other of the input files. The user will input the number of vertical strips to use. Here's an example:

run of program	cat.pgm	dog.pgm	catdog.pgm
~/$ ./part3b file 1: cat.pgm file 2: dog.pgm Number of strips: 25 output filename: stripcatdog.pgm

Part 4: rotate

Write a program part4.cpp that reads in a file in .pgm format, and produces a version of that file with the image rotated 90 degrees counter-clockwise. Here's an example:

run of program	cat.pgm	rotatecat.pgm
~/$ ./part1 filename: cat.pgm output filename: rotatecat.pgm

Submit: ~/bin/submit-external -c=SI204 -p=lab09 part1.cpp part2.cpp part3.cpp part4.cpp

~/$ ./part5
filename: cat.pgm
blur size: 100
output filename: blurdog.pgm