GEOG272-Lab3 (1)

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University of Maryland *

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272

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Geography

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Feb 20, 2024

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docx

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Uploaded by Josephmass

Lab 3 Contrast Stretching and Histogram Equalization Lab Data : GEOG 272 Lab 2 Data.zip . Make sure to download, unzip and save all the files into your own folder. Overview We will be working with image histograms and several contrast stretching methods. Histograms before and after stretching will be visualized and compared. Please complete the tasks and answer the questions written in RED. The easiest way to view histogram information for your data is through the Image Viewer Window toolbar: Enhance/Interactive Stretching. The input histogram shows the raw data values for your image, while the output histogram of the stretched image is given on the right side. The change between these two histograms represents the contrast enhancements that have automatically been performed to the image by ENVI. The range of the x-axis consists of brightness values from 0 to 255 (corresponding to 8-bits; refresher: 0 is black and 255 is bright white). The y- axis starts at 0 and increases upwards, showing the total number of pixels that are being placed into each x-axis range from 0 to 255. [1] By either taking screenshots or drawing on a sheet of paper, recreate the input histograms for the NIR and red bands and briefly interpret the general characteristics of each band's histogram based on your knowledge of the electromagnetic spectrum. Label the value with the highest frequency represented. Now experiment with different types of contrast stretches. Look under the Histogram_Source and select Image. This will allow you to view your contrast stretches as applied to the entire image. This option also allows you to use the entire image as the source for your histogram. As you work through this lab, see what happens when you alter the histogram sources for each of the bands you are viewing. Use the Defaults drop down menu to select different stretching methods

and answer the following questions. You can also view the images to compare before- and after-stretching. [2] What happens to the histograms when using histogram equalization? How does it affect the image? [3] What happens to the histograms when using the linear 0-255 contrast stretching? How does it affect the image? [4] What happens to the histograms when using a Gaussian contrast stretching? How does it affect the image? [5] What happens to the histograms when using the square root contrast stretch? How does it affect the image? Note: square root contrast stretching simply assign the square root of a pixel’s value as the new value of the pixel. Next, select the Piecewise Linear under Stretch Type. This will allow you to interactively change the stretch by grabbing the corners of the line and changing the slope of the line for each band. Each slope line that crosses the histogram illustrates the transformation of image data values into brightness values and is a graph of the lookup table. The line shows how the input file value of x is changed to produce an output brightness value of y. Experiment with the various different methods of contrast stretching until you are satisfied with the visual display of your image. [6] Based on the lecture and the lab exercise, discuss the advantages and disadvantages of image contrast enhancement. [7] Based on the lecture and the lab exercise, discuss the strategy of selecting an appropriate contrast stretching method. That is, how to choose a best stretching method for a specific remote sensing image?

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