The previous tool, Brightness Scaling, operates on an image by examining the range of pixels in it and assigning new pixel values based on where each lies in the range between the values selected for black and white. Histogram Shaping, while looking superficially similar, works in an entirely different way. While both tools reassign each pixel value in the original image to a new value, Histogram Shaping does it by trying to force the histogram of the image, as a whole, to a new shape. While we used the Statistics Tool to examine the effects of the Brightness Scaling Tool, we will use the Histogram Display to show the effects of Histogram Shaping.
Step 1: Load the Image. For this tutorial, we will again use the image of the Eagle Nebula, "ml6.fts" from the Image Enhancement tutorial directory. Load it, and click the Default B/W Stretch button for this display mode.
Step 2: Examine the Image Histogram. Create a histogram of the image by clicking the Measure I Histogram... menu item. Examine it and notice how the spectrum peaks at 47 ADUs and falls off in an exponential fashion until it hits 10 pixels at a pixel value of around 1000 ADUs. It has a narrow peak at about 1500 and then a broader one at 3000. Most of the image structure is buried in that initial peak, and it's only about 600 ADUs wide. What we will do is to change the shape of this histogram in various ways to bring out the structure of the nebula.
Step 3: Invoke the Histogram Shaping Tool. Click on the Enhance \ Histogram Shaping... menu item to invoke the Histogram Shaping Tool. This tool window contains several tabs and a Preview display, just as with the Brightness Scaling Tool. Preview allows you to experiment with different histogram shapes until you find one that suits the image and features you are trying to bring out.
The Transfer tab contains a Scaling Type box with buttons showing ten types of histogram shaping available. Some of the different histogram shapes have a parameter which is set with the slider underneath the Scaling Type box, while others take no parameters.
Step 4: Preview an Exponential Histogram Shape. The default histogram shape is the Exponential Histogram. This option has proven to be effective for many faint deep-sky objects. Examine the Preview display and see how it works on the image of the Eagle Nebula. Slide the Peak Skew slider around and notice how the pre-calculated histogram shape changes. This is intended to help you visualize how you are shaping the image histogram. The red curve shows the histogram of the original image, while the blue curve shows the histogram that will result from the application of this tool, and the Preview display shows how the image will look.
Step 5: Create a New Image with an Exponential Histogram Shape. Set the Sigma Spread to a value of 42 and click Apply; a new image will be created with the Exponential histogram shape.
Step 6: Compare the Histograms of the Two Images. Now examine the histogram of the new image. Click on the still-open Histogram window and click the Refresh button. This histogram is radically different from that of the original image. First, it now stretches from down around zero to up around 65535, the maximum value represented in 16 bits. Second, it is evenly spread across the whole range. Because of the exponential shape, the range of values containing the nebula now stretches from about 10,000 to roughly 50,000, a full two-thirds of the histogram. As a result the nebula is very prominent in the image.
Step 7: Try Other Histogram Shapes. Close all the images, which will close all the tools, and reopen the "ml6.fts" image. Try the different histogram shapes on it. Also try other images in the collection on the CD-ROM. See how planetary images benefit from different histogram shapes than those used for deep-sky objects.
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