Lesson 3. Correction and Binarization
of Color Images
Brief
description: During this lesson you will learn how to correct
color images by histogram, binarize a color image and correct the results
of binarization.
For detailed information on these subjects see the following sections
of the Quick Start guide:
Correction by histogram,
Separating color and greyscale images to monochrome layers, Filtering
Monochrome Images.
Using correction of a color image by histogram you can tune the image
brightness, hue and contrast. You can apply automatic correction by histogram
or tune the parameters manually.
Choose Open from the File
menu. Open the utrecht.cws file, created during
the previous lesson from the Lesson_2-3 folder (or
the Utrecht.tif file from the same folder if you skipped
the previous exercise).
- Choose Equalize from the Image menu.
During this exercise we will use the manual correction
by histogram. To correct an image manually you need to specify three values:
lightest and darkest levels and image gamma (that defines the position
of the middle brightness value relative to the current threshold values)
- for each channel or for Master channel.
In the Equalize dialog choose Master
from the Channel list:
- Lightest level is set using the White point eyedropper
 ,
using which you can specify the colours to turn white. (click
one of the darkest pixel in the white part of the image).
All pixels included in the histogram area from the right slider
to 256, will turn white.
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- Darkest level is set using the Black point eyedropper
 ,
using which you can specify the colours to turn black. (click
one of the lightest pixels in the black letters).
All pixels included in the histogram area from 0 to the left slider,
will turn black.
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- Move the Histogram middle marker (setting the image gamma) to
obtain the best result in the preview window.
Moving the middle slider to the left increases the gamma value;
the image becomes lighter.
Moving the middle slider to the right reduces the gamma value;
the image becomes darker.
- When you achieve satisfactory result in the Preview window,
press the Apply button.
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Binarizing a color image
Binarization allows you to extract pixels of specified
colors (or close to them) to a monochrome layer. For this, you
need to specify colors on the original image to transfer to another layer.
During this exercise we use binarization to obtain a
monochrome layer of roads, which are yellow on this image.
- Choose Binarization from the Image menu.
- Choose a binarization method - Range; for this,
open the Range tab in the Binarization
dialog.
- Chose range method - by HSV and channel
- Hue in the corresponding lists of the Range
tab.
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- Specify a layer and its color to place the new monochrome image
to.
- Using one of these eyedroppers
 specify
the colours on the screen; they are added to the Colors
list. The estimated binarization results are highlighted in the
preview window.
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INFORMATION:
the first eyedropper is for specifying a color for a specified pixel,
the second eyedropper is for specifying the averaged color of all pixels
adjacent to the one specified by you; the third eyedropper is for specifying
the averaged color of the polygon defined by you.
- You can delete incorrectly selected colors by pressing the Delete
button.
- Tune the results using the sliders located in Ranges.
- You can delete small speckles that appear using the Noise Reduction
slider - move it right, observing the changes in the preview window.
- When you achieve satisfactory results in the Preview window, press
Apply.
- You can save the binarization template for future use. To do this,
press the Template button, choose Save,
and in the Save template File dialog specify a name and
location, and then press the Save button.
Improving the monochrome layer obtained after binarization
To improve the monochrome layer obtained after binarization,
you can apply one or more of the WiseImage tools for enhancing monochrome
images.
Let’s apply the Hole Remover and Smoothing
filters to correct text areas.
For convenience, open the Image Manager
by choosing Images from the Tools menu, and
then turn off the display of the color image by pressing the  button.
Close Image Manager.
Hole Remover
- Choose Hole Remover from the Filters menu.
- Select the Auto estimating checkbox, and then press
the Measure Max Size button and specify the largest hole.
- Press Apply.
- Choose Smoothing from the Filters
menu.
- Using the sliders - Medianning & Threshold
- achieve the best result in the Preview window.
- Press OK.
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Before applying Smoothing
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After applying Smoothing
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- Choose Thinning from the Filters menu.
- Specify Number of Passes - 1.
- Press OK.
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