User`s guide

Image Enhancement

Image enhancement techniques improve images. You can use them to remove

noisefromimages,increasethesignal-to-noiseratio,makecertainfeatures

easier to see by modifying the colors or intensities, or sharpen the image.

This section includes the following topics:

• “Sharpening and Blurring an Image” on page 7-27 — Use the 2-D FIR

Filter block to improve the clarity of an im age

• “Removing Salt and Pepper No ise f rom Images” on page 7-35 — Use the

Median Filter block to eliminate noise from an intensity image

• “Removing Periodic N ois e from Video” on page 7-41 — Use the 2-D FIR

Filter block to eliminate noise in a video stream

• “Adjusting the Contrast in IntensityImages”onpage7-48—Usethe

Contrast Adjustment and Histogram Equalization blocks to modify the

contrast of intensity images

• “Adjusting the Contrast in Color Images” on page 7-53 — Use the

Histogram Equalization block to modify the contrast of a color imag e

Sharpening and Blurring an Image

To sharpen a color image, you need to make the l um a i ntensity transitions

more acute, while preserving the colorinformationoftheimage. Todothis,

you convert an R’G’B’ image into the Y’CbCr color space and apply a highpass

filter to the luma portion of the image only. Then, you transform the image

back to the R’G’B’ color space to view the results. To blur an image, you apply

a lowpass filter to the luma portion of the image. This example illustrates

these two processes. The prime notation indicates that the signals are gamma

corrected.

1 Define an R’G’B’ image in the MATLAB workspace. To read in an R’G’B’

imagefromaPNGfileandcastittothedouble-precisiondatatype,atthe

MATLAB command prompt, type

I= im2double(imread('peppers.png'));

7-27