Digital Image Processing

All of the pictures which you are viewing on the computor are 8-bit digital images, which means that they have 256 colors (2^8). The image below illustrates this principle. Digital images are tables of numbers, which in this case range from 0 to 255. Note that the "bright" squares (called pixels) have high number values (ie. 200 to 255), while the "dark" pixels, have low number values (ie. 50-100). This image is an extreme closeup of a satellite image of West Hancock. The dark region is a stream valley, which has low reflectance, while the bright area is a gravel pit, which has high reflectance. The satellite sensor records the reflectance in its field of view, and then scales the signal to an 8-bit number (0 to 255).

Color Images

The image shown above is relatively easy to interpret, bright regions have high reflectance, and dark areas have low reflectance. The interpretation becomes more difficult when we combine different bands of data to produce false-color composites. A color image is produced by loading a band of data into the red, green, and blue planes of the computor monitor.

Click here for a hands on demonstration of producing color images.

A small white box should appear after you clicked on the word "here".
Place the cursor in the white box and hit the "e" key.
A large color edit panel will pop up. Find the part of the color edit panel that looks like this:
Place the cursor arrow on the end of one of the color arrows (red, green, or blue). Hold down the left mouse button and move the arrow. When you release the arrow, the color of the small box will change.
By changing the red, green and blue values, you can make any color. Try to produce the following colors:
Red: r=255, g=0, b=0
Grey: r=128, g=128, b=128
Yellow: r=255, g=255, b=0
Purple: r=255, g=0, b=255
Cyan: r=0, g=255, b=255
Or any combination you like!!
Important: After finshing with the demo put the cursor in the small box and hit "q". This will kill the demo.

Cutting and Zooming

A common question in image processing is:
How do I zoom in on an area?
Click here for a demo.
A small satellite image of Houghton will pop up on your screen.
Put the cursor in the image, hold down the left mouse button, and drag the mouse to define a box.
After selecting a box, release the center button, and hit the "c" key (for cut). That piece will be cut out of the image. Use the "<" key to shrink the image and the ">" key to enlarge it.
If you want to return the image to its normal size, hit the "n" key.
To paste the image back together, hit the "u" key.
Important: After finishing the demo, put the cursor in the box and hit "q". This will kill the demo.

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