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Quick Tutorial for Using ImageJ

The purpose of this tutorial is to introduce a few basic image processing techniques and tools that will be necessary to complete the impact cratering activities.

Processing Images
It is often necessary to process, or “clean up” an image, and/or make certain features stand out more than others. In ImageJ, these functions can be found under the Process menu and include Smooth and Sharpen among others. The Smooth function blurs (softens) an image. It can be used to reduce “noise” in an image. An image with noise may appear speckled with random black, white, gray, or colored dots. The Sharpen function increases contrast and accentuates detail in an image. However, this function can produce, or contribute to, unwanted “noise” in the image. The Enhance Contrast function can also be useful in bringing out detail.

Sometimes one particular area of an image may need to be processed instead of the entire image. Selecting a particular area within an image can be done using one of a variety of selection tools. These tools can be found on the tool bar on the ImageJ applet window (Figure 1).

Figure 1. ImageJ selection tools include rectangular, circular, polygon, and free-hand.

TRY IT!!
Open the Mercury_47 file. Use the various selection tools and image processing functions described above to enhance different areas of this image of the planet Mercury. This is an opportunity to experiment with the different image processing functions and tools.

IMAGE PROCESSING TOOLS

In most situations it is not just enough to “clean up” an image. It is also important to know something about the features seen in the image. Craters, boulders, lava flows, mountains, and channels are geologic features seen on many planetary bodies. Image processing tools can be used to determine the different properties of these objects such as length, width, depth, diameter, etc. Tools important to the impact cratering activities are the Straight Line Selection tool, useful for measuring distances between two points, and the Angle Tool. (Using the Angle Tool requires selecting three points in an image.) Another tool that may be useful is the Magnifying Glass which is used to zoom in and out of particular areas of an image. All three of these tools can be accessed from the ImageJ applet window (Figure 2).

Figure 2. Location of ImageJ Straight Line Selection tool, Angle Tool, and Magnifying Glass.

TRY IT!!
Open the Mercury_47 file. Before taking any measurements it is important to set the scale of the image. If you know the scale, measurements in different units, such as kilometers, meters, etc., can be made. The scale for the Mercury_47 image is 0.14 kilometers per pixel. To set the scale, select Analyze -> Set Scale. Enter the measured distance as 1 pixel, and the known distance as 0.14 km. Enter “kilometer” in the "Unit of Length" field.

Using the Straight Line Selection Tool draw a line between the largest crater on the left edge of the image and any crater near the top of the image. Select ANALYZE -> MEASURE. A “Results” box will appear. Notice there are several different types of measurements reported in this box, including angle, area, and length. (In this instance, the length is the only measurement that means anything.) The number that appears under “Length” is the distance between the two craters. The units are in kilometers because that is what the scale was set in.

 

 

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