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).
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.
Important: After finshing with the demo
put the cursor in the small box and hit "q". This will kill the demo.
- 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!!
Cutting and Zooming
A common question in image processing is:
How do I zoom in on an area?
Click here for a demo.
Important: After finishing the demo, put the
cursor in the box and hit "q". This will kill the 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
- 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.
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