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Assignment 1 - Greyscale Image Converter


Due: September 11



To gain experience with storing data in Java programs including using classes, multi-dimensional arrays, and program parameters.



For this program, you will write a program that converts images from full color into greyscale.


The PPM Image Format

Most image file formats write the image data in binary, and utilize compression in order to keep the file sizes small. Unfortunately, this makes reading them in quite difficult. In contrast, the PPM image format is textual and quite simple and easy to work with.

A PPM image file contains the following:

  1. A line containing "P3". This is a marker that identifies the file as being a PPM image.
  2. A line contains two integers, separated by a space. These are the width and the height of the image. You can assume that the width and height will both be 500.
  3. A line with one integer, the maximum color value for the image. This should be the value 255.
  4. The rest of the file is composed of $3 \times W \times H$ values which give the color data of the image. There is one set of three values for each pixel - each is an integer between 0 and 255. The first of these is the red component, the second is the green, and the third is the blue. Together these three values represent the color value for one pixel. The color values will be separated by whitespace.


Greyscale Conversion

To convert an image to greyscale, you can simply average the red, green and blue values for a given pixel, then write that value over the red, green and blue values.

For example, if a pixel has a red value of 250, a green value of 100, and a blue value of 10, it will be a bright orange color. If we wish to convert the color to greyscale, we average 250, 100 and 10 which gives us 120. If we set the red, green and blue values all to 120, we get a grey color roughly as bright as the orange.

If we apply this process to each of the pixels in the image, it will be greyscale.


Sample Images

Below are some sample images you can use to test your program:

Input Image PNG Version Expected Output PNG Version
eagle.ppmeagle.png eagle-grey.ppmeagle-grey.png
farmer.ppmfarmer.png farmer-grey.ppmfarmer-grey.png
snow.ppmsnow.png snow-grey.ppmsnow-grey.png


Program Details

  1. Your program should take the input file name as a program parameter (i.e. do not read it in with a Scanner).
  2. You will need to load a PPM image file into a 2D array. If the file is not found, print an error message and exit.
  3. You should create a class to represent a single pixel. It should include the red, green and blue color components as member data.
  4. You should store the image as a 2D array of these pixel objects.
  5. You will then need to write code to apply the greyscale conversion to the 2D array of color objects.
  6. You will then need to write the data back to a file named "greyscale.ppm". You must make sure that you follow the PPM image format exactly when writing the file so it can be viewed correctly!



In order to test your program's output, you will need to view the images. Image viewers on Linux and Mac usually support PPM images natively. So if you are using one of these systems, you should be able to just open the image file like usual.

On Windows, this is usually not the case. You can use the image viewer IrfanView which is free and supports many image formats, including PPM. With this installed, you should be able to see your generated PPM images and make sure they look right.


Extra Credit

For extra credit, you can perform any other kind of image manipulation effect you wish. Examples might include flipping the image, lightening, darkening, applying Instagram-like filters etc. If you complete an extra credit effect, you should write the modified image to a file called "extra.ppm". You also must briefly describe the effect in your email submission.


General Requirements



When you are done, submit your program by emailing your code to ifinlay@umw.edu.

Copyright © 2019 Ian Finlayson | Licensed under a Creative Commons Attribution 4.0 International License.