contrastEnhancementPP110120

contrastEnhancementPP110120 - GIS 4037C and GIS 5033C...

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GIS 4037C and GIS 5033C: Digital Image Analysis Lab 3– Contrast Enhancement
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Goals: Learn to work with contrast stretching techniques discussed in lecture Learn different ways that contrast stretches can be applied to data.
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Introduction Imagine assumes you want to preserve the spectral qualities of your data Several ways that you can operate on the image without actually distorting the data. – Adjust Look Up Table ( LUT ). This is the way that the computer assigns brightness to each value in the data set. Ex. Could preserve raw data as 1-100, but display it as 155-255. – Perform contrast stretch on the image and display it to the screen but not save the results.
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Introduction – Permanently alter the image file by saving the results. Brightness values 1 to 100 would in our example be changed permanently to 155 to 255. Rarely want to do this – usually preserve original reflectance values of the dataset.
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Start by going into the contrast folder, getting the image called HOW.IMG and copying it to storage medium. Then open the image in a viewer. The HOW.img file is for this tutorial. We recommend running through this once then redoing it with the required lab files.
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Habitual Steps 1) Did you set your session preferences? 2) What is your skip factor for the histogram?
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Go into the viewer dropdown menu called Raster and select Band Combinations .
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Make sure Band 4 ( NIR ) is assigned to the red gun , Band 3 ( Red ) is assigned to the green gun and Band 2 ( Green ) is assigned to the blue gun . This creates a false color composite that simulates a Color Infrared (CIR) photograph. Click Apply and C lose the band combinations window. Color Guns
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Now look at the image info window. You are currently examining Layer_1, which is the blue band that is not being used in the composite. You are examining the raw data that is stored on the computer, rather than the image that is being displayed on the computer. It is often useful to write down the minimum and maximum values of the bands that you are working with. (this image is not the image for the lab) You are looking at the ImageInfo window at the General tab.
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Histogram tab, and look at the histograms for the four bands. Notice that the two peaked distribution only appears in the infrared band. This band is particularly good for discriminating water versus land. You can close the image
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contrastEnhancementPP110120 - GIS 4037C and GIS 5033C...

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