ES 555 Image Processing I
James S. Aber
The first exercise is based on a digital kite aerial photograph (KAP) from Quivira National Wildlife Refuge in south-central Kansas. This refuge is a salt-marsh environment that attracts huge flocks of waterfowl and shorebirds during their spring and autumn migrations--see Quivira NWR.
The goal of this exercise is to learn basic methods for enhancing the visual appearance of aerial photographs. Transfer the following file via FTP into your personal computer work space: quivira1.jpg
||Quivira National Wildlife Refuge, high-oblique view toward the northwest in mid morning. This image was acquired with the Canon PowerShot 230 (Digital Elph) on 4 May 2003. Raw image without enhancements. Click on small image to see larger version.|
Display the image with Adobe Photoshop (or equivalent) software. On most monitors, the image will be reduced in order to display fully. Look at the information bar (top line) to see the reduction factor, given in percentage. Note: monitor resolution should be at least 1024 columns; resolution of 1400 columns or higher is best.
Next click the "Image" button (upper left) and select "Image Size." The width value is the number of columns in the image; height shows the number of rows.
- 1. What reduction percentage appears for the initial display? What does this mean?
The first operation is to rotate the image to make the horizon approximately level. Click the "Image" button (upper left), and select "Rotate Canvas: Arbitrary." Click the "°CCW" button, insert 10 as the angle, and click OK. The horizon should now appear nearly level.
- 2. How many rows and columns does this image have? How many megapixels is this? Hint: one megapixel is 1024 x 1024 pixels.
- 3. What is the length to width ratio? Hint: reduce to simple fraction, such 3 to 2.
||Quivira image rotated 10° counterclockwise. Note the addition of blank margins around the image, which increase its overall size. Click on small image to see larger version.|
Tip: During this procedure, you may wish to save a temporary copy of the image at each stage. Use the "Save As" command under File menu, and give a suitable name, such as temp1, etc.
Next remove the blank margins using the crop tool. Find the tool bar, and click on the button that looks like two crossed carpenter's squares. Note: specific location of this tool varies on different versions of Adobe Photoshop. Move the mouse pointer to the image. Click and hold the mouse button near the upper left corner of the image. Drag the crop box to the lower right corner and release the mouse button. The area to be cropped is now highlighted. You may adjust the size, shape, and position of the box as needed. Try to include the horizon at top and maximize the ground area. When you are satisfied, click the "Image" button (upper left) and select "Crop."
||Cropped image. Note the shape has changed--wider relative to its height. Click on small image to see larger version.|
Next you will improve the brightness and contrast of the image. Under the "Image" menu, select "Adjust: Levels." A small box will appear with a histogram showing brightness levels in the image. There are three "Input Levels" that can be adjusted. The default values are 0, 1.00, and 255. Change the 0 to 40, change the 255 to 240, and leave 1.00 as is. Notice what happens to the display image. Click OK to implement the change in brightness.
- 4. Now check the size of your cropped image. How many columns and rows does it have?
- 5. What is the length to width ratio? How does this compare with the original image?
||Brightened image. Note the increase in color contrast compared with previous example. Click on small image to see larger version.|
The next enhancement step is to sharpen the image. Find the "Filter" menu, and select "Sharpen: Unsharp Mask." This is one of several options for sharpening an image. The Unsharp Mask box has three input values; enter the following.
The effect of sharpening is subtle. To compare before and after versions, use the "Step Backward" and "Step Forward" functions under "Edit" menu. Now try different input values for unsharp mask. You also can try other types of sharpening (under Filter menu) to compare results.
- Amount = 30%.
- Radius = 3.0 pixels.
- Threshold = 0 levels.
As a final task, you can annotate the image with a title, date, and your name using the text function (T on tool bar). Experiment with different font sizes, placement, colors, etc. Save your final version with the name QNWR.jpg
- 6. What type of sharpening do you like best? Describe how the sharpened image differs from the previous (unsharpened) image.
Note: The caption should not overlap the horizon, which is an important visual element in the scene.
||Final image of Quivira National Wildlife Refuge. This image was created using rotate, crop, brightness and sharpness operations, plus annotation. Click on small image to see larger version.|
- Written answers (1-6).
- QNWR.jpg image file.
Return to SFAP schedule.
ES 555 © J.S. Aber (2014).