Digital Image Processing

Introduction to the Matlab Image Processing Toolbox

Basics

• A digital image is equivalent to a matrix.
• Displaying an image essentially involves placing a pixel of the correct color or brightness, depending on the value of the matrix element, at the correct location, corresponding to the position of the element in the matrix.
• The matrix-to-display mapping is as shown in this figure:

• Since the MATLAB Image Processing Toolbox is a complete color image processing system, for this Digital Image Processing class, we have to ensure that we use its monochromatic capabilities only (rather a shame).

Types of Images in Matlab

1. RGB or True-Color Images: There are three matrices associated with a true-color image of size M x N - the Red matrix, the Green Matrix and the Blue matrix, each of size M x N. Typically, each of these matrices contain values between 1 and 256; 1 indicating the least intensity and 256 the highest for each of the colors. The three matrices are combined into a single M x N x 3 multidimensional matrix with 3 planes - the R plane, the G plane and the B plane.
2. Indexed Images: Although indexed images are used for displaying color images, we shall use them for displaying gray-level images. There are two matrices associated with an indexed image of size M x N to be diplayed using G gray levels:
• The image matrix which contains elements corresponding to the pixel intensities. The elements take on values from between 1 and infinity.
• The colormap matrix is G x 3 matrix, with each row containing the Red, Blue and Green components for each level of intensity. For a monochromatic gray-level image, this matrix starts with [0 0 0] for black and ends with [1 1 1] for white. Each row inbetween contains equal numbers between 0 and 1 for various shades of gray.
An image matrix element with a value of, say 5, will be displayed with the graylevel corresponding to the 5th row of the colormap matrix.
3. Intensity Images: These are the actual graylevel images. There is only one matrix, the image matrix, but the elements of the matrix must have values between zero and 1 only.
4. Binary Images: Again, only one image matrix with elements being 1 or 0.

Importing and Exporting Image Files

• To import an  image into MATLAB:

>> [x,map] = imread('filename.fmt', 'fmt');
where fmt refers to an image file format such as jpeg, pcx, etc.
Then, matrix [x] will contain the intensity information and matrix [map] the colormap information.
• To export an image matrix [x] and a colormap matrix [map] in a specified format, use:

>> imwrite(x, map, 'filename.fmt');
• Occasionally, data may be in other formats; I will provide commands and m-files to read them. 

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Conversions Between Different Image Types

 


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Displaying Different Types of Images

1. Indexed Images: If [x] is the image matrix and [map] is the colormap matrix,

>> imshow(x, map);
2. Intensity Images: If [x] is the image matrix and G is the no. of gray levels,

>> imshow(x, G);
3. Binary Images: If [x] is the image matrix,

>> imshow(x, 2);

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Pre-Lab Assignment

1. Download a true-color (RGB) image off the web. Import this image into Matlab and display it. Save the image to a file; open with any other imaging software and display it.
>> imread, >> imshow and >> imwrite commands.
2. Convert this image into a gray-level indexed image and display it. Save the image to a file; open with any other imaging software and display it.
>> rgb2gray, >> imshow and >> imwrite commands.
3. Convert the indexed image into an intensity image and display it. Save the image to a file; open with any other imaging software and display it.
>> ind2gray, >> imshow and >> imwrite commands.
4. Convert the intensity image into a binary image and display it. Save the image to a file; open with any other imaging software and display it.
>> im2bw, >> imshow and >> imwrite commands.
5. Generate a 256x256 matrix of numbers; convert this to an intensity image and display it. Save the image to a file; open with any other imaging software and display it.
>> mat2gray, >> imshow and >> imwrite commands.
6. Generate a column profile and a row profile of the intensity image you have downloaded earlier.
7. Convert the indexed image you have downloaded earlier into an intensity matrix. Save this matrix in Matlab's native format (.mat file). Read this file and display image.
>> mat2gray, >> imshow and >> save and >> load commands.

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