CS 101 - Intro to Computing, Spring 2012

Lab 8

Posterization

The code to posterize is often a modification of the black & white (grayScale) code. Once the "grayAmount" has been determined for a pixel, we use that value in if statement(s) to determine which of the posterized colors that pixel will be set. The following shows that code for posterizing into 2 colors: black and white.

• If the grayAmount is in the range from 0 to 127, the color of the pixel becomes black.
• If the gray Amount is in the range from 128 to 255, the color of the pixel becomes white.

// determine the grayAmount from the color at the pixel int grayAmount = (int) (red * 0.299 + green * 0.587 + blue * 0.114); // determine the final color value of the pixel if ( grayAmount < 128 ) { // set the pixel to black red = 0; green = 0; blue = 0; } else { // set the pixel to white red = 255; green = 255; blue = 255; }

To posterize to more colors we just need to use nested if statements (often called "else-if" clauses). The following posterizes to 4 colors (black, blue, green and white). In this one the ranges for the grayAmount are:

• 0 - 63 for black,
• 64 - 127 for blue,
• 128 - 191 for green, and
• 192 - 255 for white.

// determine the grayAmount from the color at the pixel int grayAmount = (int) (red * 0.299 + green * 0.587 + blue * 0.114); // determine the final color value of the pixel if ( grayAmount < 64 ) { // set the pixel to black red = 0; green = 0; blue = 0; } else if ( grayAmount < 128 ) { // set the pixel to blue red = 0; green = 0; blue = 255; } else if ( grayAmount < 192 ) { // set the pixel to green red = 0; green = 255; blue = 0; } else { // set the pixel to white red = 255; green = 255; blue = 255; }

Lab Assignment

For this lab assignment, you are to posterize a picture into 6 colors. We will use the "grayAmount" that was used when creating a black and white (grayscale) pricture to determine which of the six color values will be used.

Since our range for the "grayAmount" can vary from 0 to 255, we need to divide that range into 6 parts to distribute the six colors evenly throughout the picture.

We will use the following shades of blue for our 6 colors. The names come from the web page at: http://www.tayloredmktg.com/rgb/.

grayAmount Range	0 - 42	43 - 84	85 - 127	128 - 170	171 - 212	213 - 255
Color Value	Midnight Blue r = 25 g = 25 b = 112	Dark Slate Blue r = 72 g = 61 b = 139	Royal Blue r = 65 g = 105 b = 225	Deep Sky Blue r = 0 g = 191 b = 255	Pale Torquoise r = 175 g = 238 b = 238	Light Cyan r = 224 g = 255 b = 255

For this lab assignment, you are to write a java program that will complete the following:

1. Prompt the user for an picture and open that picture.

2. Call a method that will posterize the picture as described above. The picture opened in step 1 is to sent as a parameter to this method.

3. Display the posterized picture.

4. You must write your programs using good programming style which includes:
   • Good variable names
   • in-line commenting
   • header block commenting for the program and each method written
     Be sure to include the following with the header block comment for the program.
     • your name
     • day and time of your CS 101 lab section (i.e. Wednesday at 9:00)
     • A description of the project.
   • proper indentation of program statements
   • use of blank lines to separate blocks of code.

Submission of the Lab

The lab must be submitted electronically to the Assignment Link for Lab 8 inside of Blackboard. You will only need to submit the java source code file (the ".java" file). Please only submit source code file (the .java file, not the .class).