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LAB 10: IMAGE PROCESSING, continued (Due Thursday, May 13th) In this exercise, you'll continue to explore the basics of image processing on ERDAS Imagine software. I encourage you to experiment, but be sure to not save any changes on the images.
I. Thematic Images. First, we'll have a look at some image files that have already been processed, or created based on an original digital image.
1. Viewer should already be open, but if not, ask for assistance.
2. Pull down FILE menu to OPEN/RASTER. Click on "Lanier.img" file
Click OK to load image.
This image should look somewhat like a color infrared photo.3. Open a Second Window by pulling down VIEW menu to SPLIT/VERTICAL.
4. Then view them side by side by pulling down VIEW/TILE VIEWERS.
You can make both windows larger by clicking/dragging on sides. You can rearrange the two viewers on your screen by clicking on the top bar of each and dragging to a new position.
5. While cursor is on Second Viewer window, load a second file by clicking on the OPEN icon on the Tool Bar (or by pulling down the FILE menu, as usual). This icon sends you directly to the raster file dialog box.
6. Highlight "inslope.img" file. Click OK to load. This is obviously an image based on the original lanier.img file, but looks considerably different. The file name suggests that this image may have some slope information on it, but it's certainly not obvious from what you can see now.
Question 1. What more do you need to know to help you obtain useful data from the two images on your screen? (Don't be chicken and look ahead -- just think a moment.)
7. Since you've come to the grand conclusion that you need a KEY to the colors on the second image, Pull down the RASTER menu to ATTRIBUTE EDITOR.
The various numbers in boxes don't look very useful here, but take some time to explore this dialog box. You can see the range of colors on the left-hand side. What do they correspond to besides various columns of numbers?
Question 2. After you have found the KEY, answer the following two questions:
2a. How steep (in degrees) is the relief of most of the land in the Lanier Lake area?
2b. How steep is the steepest slope shown on this image?
8. Try another thematic view of the Lanier Lake area. DO NOT CLOSE the Attributes Box. OPEN the raster file menu again, and highlight a second "in.....img" file (all of the Lanier Lake image files begin with "in" except the primary file). If you make sure that the CLEAR DISPLAY button is toggled, each time you load another image into Viewer No. 2, the new image will replace the former image in the viewer. This time, open "inlandc.img" . Then check out the values for the colors on this image in the Attributes box.
Question 3. Comparing the KEY for the "inlandc.img", answer the next three questions.
3a. What kind of a thematic image is this? (What does it depict?)
3b. What is the dominant land cover in the Lanier Lake region?
3c. What is that object whose area you measured in the last lab? (The whitish double-blob just northwest of the highway in the upper-right corner of the image...)
9. Check out the other "in......img" files in the Open Raster dialog box. Each depicts different aspects of the Lanier Lake area. (Answer Question 4, next page)
Question 4. Which two of these "in....img" files are different from all the others in the types of information they depict? In what way are they different?
II. Image Enhancement. First, CLOSE Viewer 2 by double-clicking on the upper-left corner or pulling down FILE/CLOSE. In this section of the lab, you're going to experiment with manipulating brightness and contrast of the image. Then you'll see how you can change the appearance of the image by manipulating the values of each of the color guns, Red, Green, and Blue.
1. First, create a Magnifier so that you can view the image at full size and magnified at the same time. Pull down VIEW/CREATE MAGNIFIER. Alternately, while the cursor is in the Viewer window, Click the RIGHT button of the mouse to bring up the VIEW MENU, then highlight Create Magnifier. A Viewer No. 2 box will appear in the upper-left corner of the viewer. Beneath the box is a white grid box, you can just see the edge of it.
2. Click your cursor arrow on this white box. The Viewer No. 2 will disappear. Then Click and Drag on the X at the center of the box. Drag it over to an area on the image that has both water and land, and let go. You can change the size of the box by dragging on the sides or corners. The smaller the box, the greater the magnification (I know you knew that!).
3. But you can't see what's happening in Viewer No. 2. So pull down VIEWER/TILE VIEWERS. The second Viewer will appear on the right of Viewer No. 1. Resize the two viewers until you're happy with what you can see.
Question 5. From your lecture notes and text, what is the computer doing to create the magnified image on the right? How are the pixels adjusted/changed? Draw a sketch to illustrate your answer.
IIA. Adjust Image Contrast. (Contrast Stretching, p.419-424, Avery and Berlin). With this tool, you can enhance the image by changing its brightness and contrast.
1. Pull down RASTER/BRIGHTNESS/CONTRAST. This brings up a dialog box with two slide bars for changing brightness and contrast.
2. You can try out the different combinations by adjusting the slides on the bars, then clicking on APPLY. Observe the values change in the boxes to the left. You can also change the box values incrementally by clicking on the arrows.
Question 6. What settings would help bring out details of the urban vs. rural areas? (Give numbers, and indicate results -- lighter, darker, etc.)
6a. Look at the airport in the lower-central part of the image, south of the city and northwest of the highway -- what settings make the runways clearest?
6b. What settings make the rural areas clearest? What about boundaries between water and land?
3. When you're finished playing with this tool, return to original contrast and brightness by clicking on RESET. Then CLOSE contrast tool.
IIB. Piecewise Linear Stretches. This tool allows you to manipulate the range of contrast and brightness of each of the color guns individually.
1. Pull down the VIEWER menu, then go to RASTER/PIECEWISE CONTRAST. A more complex dialog box appears that still has the slide bars, but also includes Range Specifications and Select Color. As one slider is moved, the other is automatically adjusted so that there is no gap between values in the Range Specifications Table. Thus there is always a certain percentage of pixels in each range, even if most can be put into Low, Middle, or High Range. With Select Color, you can specify which color gun you want to manipulate.
2. With your cursor over the image, click the right mouse button and bring up the QUICK VIEW menu. Then highlight INQUIRE CURSOR. You can now see the white crosshair on both images. (Chances are the Contrast Tool Box is hiding one of your images, but you can move it as you like.)
3. Move the crosshairs over the water in the image, and see how the values in the File Pixel box and LUT (Look Up Table) box change for the Blue Color Gun. Look at the magnification in Viewer No. 2 and note the subtle range of blue and blue-green colors in the water.
4. In the Piecewise Contrast Box, Click on the BLUE color gun, then change the Range Specifications for LOW to:
From: 35
To: 55
Note how the Middle and High ranges adjust to almost all the pixels going to LOW.
5. Click APPLY.
Question 7. Any comments on changes you see? I didn't see much change in the water values, but the urban features seemed to have a bit more contrast.
6. Click RESET, then CLOSE.
IIC. Histogram Stretching. This tool allows you to see the histograms for each individual color gun and manipulate the stretch of each or all three together.
1. Pull down the RASTER menu, and go to HISTOGRAM CONTRAST. This brings up the Histogram Tool, with three histograms displayed, for RED, GREEN, and BLUE. I suggest you experiment with manipulating each of these histograms. Note if you put your cursor in a histogram box, you see readouts of the values of the coordinates of your cursor to the left of the box. The gray shading in the background represents input values of the digital image. The X axis of each graph represents the range of pixel values (0 to 255) for the represented color gun; the Y axis represents the frequencies (number of pixels) and the range of values for the Look Up Table. The histogram of each color represents the Output Values -- it changes as you change the values of the Look Up Table.
2. To change the stretch of each histogram, place your cursor on the straight line on the graph. This line represents the graph of the Look Up Table. If you click and drag this line, you can see the histogram change.
3. Try different configurations for each color, then click APPLY ALL. If you make a change to just one histogram, click on the Lightning Bolt icon to the left to apply the change.
Question 8a. What happens when you stretch out the RED color histogram (that is, make the distribution spread out to the right)? What does the Lanier Lake image look like?
8b. What happens when you bring all of the histograms up to match the gray shading in the background?
4. To go back to the original settings, you have to UNDO the settings. CLICK THE RIGHT mouse button in one of the histograms. This will bring up a dialog box, then highlight UNDO ALL EDITS. Do this for each of the histogram boxes.
CLOSE ERDAS IMAGINE.
1. Pull down the FILE menu for Viewer No. 1 and highlight CLOSE OTHER VIEWERS.
2. Then choose CLOSE.
3. Then pull down the SESSION menu at the top of the window and highlight QUIT. Choose NO when the dialog box asks you if you want to print your log.
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