Question

1. Type in the command

>> help imdemos

This will give you a list of sample TIFF images which come with the Image Processing Toolbox.

Make a list of ten sample images, and for each image

(a) determine its type (binary, greyscale, true colour or indexed colour),

(b) determine its size (in pixels)

(c) give a brief description of the picture (what it looks like; what it seems to be a picture of)

2. Pick a greyscale image, say cameraman.tif or some other. Using the imwrite function, write it to files of type JPEG, PNG and BMP. What are the sizes of those files?
3. Repeat the above question with (a) a binary image, (b) an indexed colour image, (c) a true colour image.
4. Take some indexed images, convert it to gray image and display the result, convert it to integer data type and display the result. compare the two images in terms of apperarance and pixel values and describe your understanding.

Answer 1

Introduction:

>>help imdemos

There are a series of photos that come as part of the image processing toolkit. To get the list of images and credits, you can type :

help imdemos

or

doc imdemos

(a)Determine its type:

(1) Binary:

Type:

Logical (Boolean) array of 0’s and 1’s

0 is Black and 1 is White

Greyscale (Intensity) :

Type:

Values have ranges as follows:

single or double arrays [0,1]

uint8 [0,255]

unit16 [0,65 535]

int16 [-32 768,32 767]

(3) true colour (RGB):

Type:

Values have the following range:

Single or double arrays [0,1]

Uint8 [0,255]

Uint16 [0,65 535]

Indexed (Pseudocolor) :

Type:

Values in image are indices into a colormap

Range of values where p is the length of the colormap :

single or double arrays [1,p]

logical, uint8, or unit16 [0,p-1]

(b) Determine its size(Pixels) : The image size is: 206*345*3

(c) Description of the picture:

Truecolor :

Until now you have worked with grayscale images and you have seen that there are different image types. Most of them work similarly. For each pixel there's a single value, from a range of values appropriate to a data type, stored in a two dimensional matrix. To represent color we have to add another dimension. For instance, if you read image "autumn.tif" into I2 and view the details of I2 using:

I2 = imread(‘autumn.tif’)

Whos I2

The image size is: 206*345*3

The extra dimension is the color component. For each pixel, we have a representation of the intensity of red, green, and blue. In autumn.tif, 255 represents the highest intensity, and 0 represents the lowest intensity. For instance, to make a pure red, red would be 255, green would be 0, and blue would be 0.

At pixel (93,180) in autumn.tif,

the red intensity is represented by I2(93,180,1)

the green intensity is represented by I2(93,180,2)

the blue intensity is represented by I2(93,180,3)

The following diagram represents the three color components at pixel values. Often, they are represented as channels as is shown in the diagram below. Usually, there is a red channel, a green channel and a blue channel.

If you only wanted to view the red channel, which of the following statements would you write?

Imshow( I2(1, :, :) );

Imshow( I2(:, :, 1) );

Imshow( I2(:, 1, :) );

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