MATLAB VIDEO AND IMAGE PROCESSING BLOCKSET 3 Guide de l'utilisateur télécharger pdf (Page 158)

7 Analysis and Enhancement

Feature Extraction

Feature extraction techniques return information about the structure of an

image. For example, you can use them to find edges, locations, and attributes

of objects.

This section includes the following topics:

• “Finding Edges in Im ages” on page 7-2 — Use the Edge Detection block to

find object boundaries

• “Finding Lines in Images” on page 7-9 — Use the Hough Transform, Find

Local Maxima, and Hough Lines blocks to find the longest line in an image

• “Measuring an Angle Between Lines” on page 7-17— U se the Hough

Transform, Hough Lines, and Draw Shapes block to draw tw o lines on an

image and measure the angle between them

Finding Edges in Images

YoucanusetheEdgeDetectionblocktofindtheedgesofobjectsinanimage.

This block finds the pixel locations where the magnitude of the gradient of

intensity is larger than a threshold value. These locations typically occur at

the boundaries of objects. In this section, you use the Edge De tection block to

find the edges of rice grains in an intensity image:

1 Define an intensity image in the MATLAB workspace. To read in an

intensity image from a PNG file, at the M ATLAB command prom pt, type

I= imread('rice.png');

I is a 256-by-256 matrix of 8-bit unsigned integers.

2 To view the image this matrix represents, at the MATLAB command

prompt, type

imshow(I)

7-2

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MATLAB VIDEO AND IMAGE PROCESSING BLOCKSET 3 Guide de l'utilisateur Page 158