C76 Wavelet Spatial Filtering

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One of the most exciting image processing technologies to come along in recent years is wavelet processing. AIP4Wiri s Wavelet Spatial Filter provides unique capabilities for image enhancement not matched by other processing techniques. To get the most out of this tutorial it is recommended that you read Chapter 18, so you will have a better idea of what is happening, as well as a better understanding of the terminology.

For this tutorial, be sure that Auto Lo/HiStretch is selected as the default display mode in the Image Display Control window.

Step 1: Load an image. Load the image "IC5070HaStack.fts" from the Image Enhancement tutorial subdirectory. This image is a Hydrogen-alpha filtered image of the Pelican Nebula. (The pelican namesake is recognizable if you use the Transform\Translate-Rotate-Scale... tool to perform a quick 90 degree counterclockwise rotation of the image. Try it and see.)

Step 2: Invoke the Wavelet Spatial Filter tool. Click on the Enhance I Wavelet Spatial Filter... menu item, and the Wavelet Spatial Filter tool will open. This tool is divided into four panels with the following controls:

• Maximum Wavelet Scale. Sets the limit of the largest pixel scale that will be processed.

• Wavelet Profile sliders. Set the amount of enhancement for each pixel scale.

• Add Constant checkbox. Allows you to add a bias to the image, to help keep the processed values from going negative.

• No Dark Rings around Stars checkbox. Does just what it says; checking this box prevents the dark rings that can occur during aggressive filtering operations.

• Select/Un-Select All. These buttons can be used to control whether the entire range of pixel scales is processed or not.

• Settings. Allows you to save and recall your favorite filter settings.

• Apply Filter button. Starts the wavelet filtering process.

To make this tutorial easier, we have saved a filter using the Save button and will recall it to quickly set up the tool.

Step 3: Load a predefined filter. Click the Recall button to load the filter that has already been set up. This tool allows you to configure a filter and save the settings in a file so that you can recall it later. In this way you can create a library of your favorite filters and share them with other users. The filter is called "Sharp-en.wlt" and is located in the Enhancement tutorial subdirectory where you found the current image file.

Step 4: Run the Wavelet Spatial Filter. Now click the Apply Filter button. A status bar will appear to let you know the progress. Wavelet filtering demands an enormous amount of computation, so filtering a really large image can take several minutes. The status bar provides a Cancel button if you need to stop the process before completion.

You will see that when stretched to the same range as the original image, the resulting image has much greater contrast. This tool tends to enhance brighter features and abrupt edges and is wonderful for enhancing nebulae. It works particularly well on images taken through a Hydrogen-alpha filter.

Try an experiment, and compare the result with the Unsharp Mask and De-convolution tools.

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