How are images represented in a computer?
Image Objects
CS111 Computer Programming
Department of Computer Science
Wellesley College
Image Objects
Digital Images
Digital Images
Images are often represented in a computer as a 2-dimensional
(2D) array of pixels.
Images are often represented in a computer as a 2-dimensional
(2D) array of pixels.
20-2
38 pixels high
44 pixels wide
50 x 50
Image Objects
20-3
Image Objects
20-4
1
Representing Pixel Colors
Image Formats
Instances of the Java Color class have red/green/blue
(RGB) components ,each in the range 0 to 255.*
Images can be encoded in different ways (e.g., JPEG, GIF, BMP, PNG).
One way to represent images is to encode each pixel as an integer.
An RGB triple can also be represented as a negative
integer that falls in the range -16777216 to -1. *
You can play with these representations in the
ColorPad applet.
Color
R
G
B
int
-1
0
0 -1 -1
0
0 -1 -1
0
0
0
0
0
0 -1
-1
0
0 -1 -1
0
0 -1 -1
0
0
0
0
0
0 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
black
0
0
0
-16777216 or 0†
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
white
255
255
255
-1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0
0
0
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0
0
0
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
red
255
0
0
-65536
green
0
255
0
-16711936
blue
0
0
255
-16776961
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
yellow
255
255
0
-256
magenta
255
0
255
-65281
cyan
0
255
255
-16711681
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
bagel brown
200
100
50
-3644366
-1
0
0 -1 -1
0
0 -1 -1
0
0
0
0
0
0 -1
-1
0
0 -1 -1
0
0 -1 -1
0
0
0
0
0
0 -1
* Take CS240 to understand these ranges!
† O is a Color synonym for -16777216.
Image Objects
16 x 16
20-5
Mirror Image
Image Objects
20-6
Checkerboard as 2D array
How would you mirror (i.e., flipHorizontally) an image for which
each pixel was represented as an integer?
-1
0
0 -1 -1
0
0 -1 -1
0
0
0
0
0
0 -1
0
0
0 -1
-1
0
0 -1 -1
0
0 -1 -1
0
0
0
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
-1
0
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
0
0
0
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0
0
0
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1 -1 -1
0
0 -1 -1 -1
-1
0
0 -1 -1
0
0 -1 -1
0
0
0
0
0
0 -1
-1
0
0 -1 -1
0
0 -1 -1
0
0
0
0
0
0 -1
0
0
?
-65536
-65536
-256
-256
-65536
-65536
-256
-65536
-65536
-256
-256
-65536
-65536
-256
-256
-256
-256
-256
-65536
-65536
-256
-256
-65536
-65536
-256
-256
-65536
-65536
-256
-256
-65536
-65536
-65536
-65536
-256
-256
-65536
-65536
-256
-256
-65536
-65536
-256
-256
-65536
-65536
-256
-256
-256
-256
-65536
-65536
-256
-256
-65536
-65536
-256
-256
-65536
-65536
-256
-256
-65536
-65536
8x8
0 -1 -1 -1
0 -1 -1 -1
Image Objects
20-7
Image Objects
20-8
2
Mona Lisa as 2D array
Creating a Class
We want to define the data and operations relevant to an image
object. What would such a class look like?
Pixels starting from the upper left corner:
-10389929
-10522028
-10720177
-11049399
…
-10324649
-10522028
-10785970
-11049912
…
-10259372
-10456751
-10720436
-10921927
…
-10259633
-10391732
-10589881
-10853310
…
.
.
.
.
.
.
.
.
.
.
.
.
520 rows x 334 columns
Image Objects
20-9
Some Helpful Methods
We ve provided a few methods in the class PicOps that may be
useful when designing and testing your image class.
/* Returns a String representing the path to the
* location of the source/class files.
*/
public static String getFolder();
/* In a new frame with the given name,
* displays the corresponding RGB image
* specified by the 2D array of ints.
*/
public static void display (String name, int[][] pix);
/* In a new frame, displays the corresponding RGB image
* specified by the 2D array of ints.
*/
public static void display (int[][] pix);
/* In a new frame, displays the corresponding RGB image
* specified by the given image file name. The specified
* image file can be of type JPEG, GIF, PNG, BMP, or a
* TXT file containing a 2D integer array representation
* of an image.
*/
public static void display (String imageFileName);
Image Objects 20-11
Image Objects 20-10
Some Helpful Methods (2)
We ve provided a few methods in the class PicOps that may be
useful when designing and testing your image class.
/* Given the name of a TXT file containing a 2D integer
* array representation of an RGB image, returns a new
* 2D integer array that corresponds to the RGB image
* represented in the TXT file.
*/
public static int[][] readInTextFile(String textFileName)
throws FileNotFoundException;
/* Outputs the given 2D integer array to a TXT file with
* the given name, so that the TXT file contains a 2D
* integer array representation of an RGB image.
*/
public static void outputTextFile(String textFileName, int[][] pix)
throws FileNotFoundException;
/* Given an image file of type JPEG, GIF, PNG, or BMP,
* outputs a TXT file containing a 2D integer array
* representation of the RGB image.
*/
public static void toTextFile (String imageFileName);
/* Outputs a JPEG file corresponding to the RGB image represented in
* the given TXT file as a 2D integer array.
*/
Image Objects 20-12
public static void textFileToJPEG(String textFileName);
3