Project 1

Introduction: Green Screen

Almost certainly, you're familiar with green screening, where a plain green background is replaced with some other image. The most common example is weather maps on the news - you'll never see a weather person wearing green clothes, or they would disappear.

In this project, we're going to do some green screening, combining a background from one picture with a foreground from another.

For example, consider a picture of Bill the Goat (on the left below). He deserves to be someplace nice. So, for every pixel that is green, we'll replace it with the corresponding pixel from the image in the middle below, giving us the image on the right below.

To do this, we're going to use a horribly space-inefficient file format called "plain PPM." Here is a bunch of ppm images for download. Untar the images with the command

tar xzf ppms.tgz
If you open one up in a text editor, you'll see something that looks like this:
1280 960
0 255 0 125 255 5 0 255 0...

Every PPM file must be in this format. To view a PPM image on your Linux machine, use the command:

eog thepicture.ppm
In our green screened images, every pixel in the background is 0 255 0.

Tips for success


80% of your grade will be based on functionality (does your program work as specified), mostly based on the test cases that you see when you submit, but perhaps also extra test cases according to the specifications of the project. The code you submit must work if you expect to earn a passing grade.

20% of your grade will be based on coding style, which includes:

All of this is factored into your maximum score based on which part you have completed. So for example, if you complete up to part 2 perfectly (i.e., 85/100) working and also get 15/20 on style, then your final grade would be 85 * (0.80 + 0.15) = 80.75.


The course policy and the instructions it references, most pertinently COMPSCIDEPTINST 1531.1C, spell out what kinds of assistance are permissible for programming projects. The instructor can give explicit permission for things that would otherwise not be allowed.

For this project, you have explicit permission

  1. to get help from other current SI204 instructors as well as your own and from current SI204 MGSP leaders (any assistance must be documented though), and
  2. to use general purpose C++ resources online (though such use must be documented). Resources that might specifically address this project are not allowed.
With the instructions referenced in the course policy and the explicit permissions above, what you get is summarized as:
Warning: Do not even vaguely talk about the contents of the project with other people!!

If you have questions about any of these rules, email your instructor and ask about it. We know you want to do what's right and we are here to help you.

What to Turn In

Online: use the online submit system. Each part below has instructions on the command to run. You must turn in ALL parts up to the last part you completed. If you finish Part 3, you must submit your code for all parts before it too.

Hardcopy: you must print all your code with codeprint. You must also print and turn in the project coversheet to your instructor. The coversheet and code must be stapled, and turned in to your instructor by the following day after online code submission.

Part 1: Grayscale an image (25/100)

To get used to the PPM format, let's not worry about greenscreening yet and just make an image grayscale (black and white).

Write a program in a file called gray.cpp:

Instruction: To find the gray value of a pixel, take the r, g, and b values, find the max of those three values, and set all three values to that maximum number. So, the gray version of (5, 100, 16) would be (100, 100, 100). The gray version of (250, 80, 90) is (250, 250, 250).

As an example, running this program on our picture of flowers gives us the picture on the right.

Instruction: If the input file to be read in does not exist, the program should print an error message and gracefully exit.

An example run of the program is shown here:

~$ ./gray
Input file: flowers.ppm
Output file: grayflowers.ppm
Image saved to grayflowers.ppm

~$ ./gray
Input file: nonexisting.ppm
Output file: output.ppm
Error: Input file not found

Submit this program with the command

~/bin/submit -c=SI204 -p=proj01 gray.cpp

Deadline: 2359, Feb 2

Part 2: Combine images (85/100)

In this part, we'll do the part described in the introduction.

Your program should be in a file called green.cpp.

Your program should prompt the user

If the foreground and background images are not the same size, your program should output an error message, rather than trying to create the new file.

The pictures in the tarball are for your own testing, but your program should work for any two images that are the same size as each other.

If one of the two files to be read in don't exist, the program should print an error message and gracefully exit.

An example run of the program is shown below:

~$ ./green
Foreground file: bill.ppm
Background file: flowers.ppm
Output file: billflowers.ppm
Image saved to billflowers.ppm

~$ ./green
Foreground file: bill.ppm
Background file: nonexisting.ppm
Output file: billflowers.ppm
Error: Input file not found

Note: you may notice a thin green line around your foreground image. This is from the blurred edge of not-fully-green pixels. Don't worry about this.

Submit this program with the command:

~/bin/submit -c=SI204 -p=proj01 gray.cpp green.cpp
(don't forget the project coversheet!)

Part 3: Offset foreground image (100/100)

The problem with our program so far is that it only accepts pictures that are the same size. What if we want Bill somewhere other than in the middle of the picture?

Two of the pictures in the tarball you downloaded are cropped (billCropped.ppm and muleCropped.ppm), so we should be able to put them anywhere in a picture that is at least as big as they are.

Write a program in a file called shift.cpp that prompts the user for: The x shift is how many pixels over horizontally from the left side the foreground image should be placed. The y shift is how many pixels down vertically from the top of the background picture the foreground image should be placed.

For example, the three pictures below have an (x-shift, y-shift) of (0, 0), (900, 0), and (0,200), respectively.

If the given x-shift or y-shift is so large that the foreground image would extend past the background image, an error message should be given, and no output image created. In addition, if one of the two images input does not exist, the program should print an error message and gracefully exit. An example run of the program is shown below:

~$ ./shift
Foreground file: billCropped.ppm
Background file: flowers.ppm
Enter offset (x,y): (900, 0)
Output file: bf_900_0.ppm
Image saved to bf_900_0.ppm

~$ ./shift
Foreground file: billCropped.ppm
Background file: nonexisting.ppm
Enter offset (x,y): (900, 0)
Output file: bf_900_0.ppm
Error: Input file not found

~$ ./shift
Foreground file: billCropped.ppm
Background file: flowers.ppm
Enter offset (x,y): (1200, 0)
Output file: bf_1200_0.ppm
Error: The foreground image goes past the background image
Submit with the command
~/bin/submit -c=SI204 -p=proj01 gray.cpp green.cpp shift.cpp
(don't forget the project coversheet!)

Deadline: 2359, Feb 11

Test Vectors

Test Vectors

Check your output files with the following test vectors. Download them to your project directory.

Checking your output with the test vectors

You can check if your output file match the corresponding test file.
  1. Download script to the project directory.
  2. Rename it to
    ~/$ mv
  3. Change the permission of the script to be executable:
    ~/$ chmod 777
  4. Use the script to see if your output file is the same as a test data.
    ~/$ ./ grayflowers.ppm test_grayflowers.ppm
    ~/$ ./ muleflowers.ppm test_muleflowers.ppm
    ~/$ ./ mf_300_100.ppm test_mf_300_100.ppm
    Here, grayflowers.ppm, muleflowers.ppm, and mf_300_100.ppm refer to your output files.

Making your own pictures (optional, just for fun)

To turn a picture in some other format into a plain ppm on your VM, first install the needed software by running:

sudo apt-get install imagemagick
When it asks for your password, this is your VM password. The lab machines already have this software installed.

Once you have this software, if you have a picture somePic.jpg (or any other image format), you can convert it to a PPM with:

convert -compress none somePic.jpg somePic.ppm
If you want it to be a particular size (like 640x480), you would run:
convert -compress none -resize 640x480\! somePic.jpg somePic.ppm

Cutting out a foreground picture and putting it in front of a green screen requires Photoshop or its open-source equivalent GIMP, and the process is googleable, but longer than is appropriate for this page.