Image Processing Fundamentals

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IMAGE PROGRESSING FUNDAMENTALS

Technically Speaking Part 2: Math, Math, Math

by James Antonakos

Start ı Blob Analysis ı Edge DetectionıBehind the Mask ı A Helping Hand from Fourier ı Please Sir, May I Have Some More? ı Sources and PDF

BLOB ANALYSIS

A group of pixels organized into a structure is commonly called a blob. For example, the pixels making up each cell in an image of red blood cells can be thought of as blobs.

What do you want to know about a blob? There is a short list of questions.

1. Where are the edges?
2. Where is the center?
3. How many pixels does it contain?
4. What is the average pixel intensity?
5. What is the blobıs orientation (angle)?

Figure 1ıTo find the center of this blob of 26 pixels, use a technique based in calculus.

Figure 1 shows a sample blob located between rows four through 11 and columns six through 13. By borrowing a technique from calculus, you can easily determine the center of the blobıs mass. The technique involves counting the number of pixels in each row and column and multiplying the pixel counts by the respective row or column numbers, as indicated in Tables 1 and 2. The sum of all the row products is divided by the total number of pixels in the blob to find the center row.

Row	Pixels per row	Product
4	0	0
5	3	15
6	5	30
7	6	42
8	6	48
9	4	36
10	2	20
11	0	0
Total		191
Table 1ıHere you can see the row calculations for the blob of pixels. The center row equals 191 divided by 26, which equals 7.3.

Column	Pixels per column	Product
6	0	0
7	3	21
8	5	40
9	6	54
10	6	60
11	4	44
12	2	24
13	0	0
Total		243
Table 2ıAfter completing the row calculations, you may compute the column total for the blob of pixels. The center column equals 243 divided by 26, which equals 9.3.

The same method is used to find the center column. Figure 2 indicates where the center of the blob is located. You may think, "Hey James, thatıs not the center," but I encourage you to make up your own example and try the method. Youıll see that it really works.

Figure 2ıThe center of the blob is located at row 7.3 and column 9.3.

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