Applying Contrast to an Image

You may print these instructions before starting this activity.

If you have not downloaded a copy of NIH Image (MAC) (version 1.62 or later) then do that now. If you are computing on a PC, you will need Scion Image from the Scion Corp. Install the software onto your hard drive and open the program. You must be running Windows 95/98 for the PC version to operate. It was ported from the Mac version, and as such, not all features work in the same manner.

This activity will demonstrate some of the basic techniques in using the NIH Image Software. Open the NIH program and let's begin.

You will learn how to adjust the contrast and brightness of an image. Most of the images we will be working with, will be in .tif format. This means that most browsers will not open the image unless you are running Netscape 4.0. If you get an error message when asked to download a .tif file, then download the compressed version and use Aladdin UnStuffit Software to decompress the file.

There is a reason for working in .tif and not gif/jpg. One is that NIH prefers this format and the second is that a .gif/.jpg image is compressed to allow fast download times via the internet. The .gif and .jpg compression processes strip the images of much their scientific value. However this workshop deals more with techniques in using the NIH software than in the "scientific value" itself. Those techniques are for an advanced workshop.

Download this Florida image (image.tif.sit) and open it using NIH Image. When an image is first opened, the map bar (see Basic NIH Tools "Map" box in the NIH Tools) is in the neutral position in the image. In the Florida image place the mouse cross hair on the center dot (handle) and drag it to the lower right corner and observe the change in contrast and brightness of the image. Notice the difference in the image and how different pixels are changed in appearance only. The digital numbers do not change. By dragging the other dots, or sliding the brightness bar (B) or contrast bar (C) to the left or right, these parameters can be changed on an individual basis.

Play around with the brightness and contrast controls to see how they change the appearance of the image, and how different features are highlighted. The four images listed below can be downloaded and used for practice.

• Gulf of Mexico #1 (visible) ( tif version -gulf1.tif.sit )
• Gulf of Mexico #2 (visible) ( tif version - gulf2.tif.sit )
• Gulf of Mexico #3 (infrared) ( tif version -gulf3.tif.sit )
• Gulf of Mexico #4 (infrared) ( tif version - gulf4.tif.sit )

If you do not wish to download each tif image one-at-a-time, then download all four images in the compressed file folder (gulftif.sit). You will need Aladdin UnStuffit Software to decompress the folder before the images can be used.

As a reminder, a visible image is similar to a black and white photograph. An infrared image shows temperature. You will learn more about the difference in a later activity.

Hurricane Opal Project Activity