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ENVI

Background Signature Selection

Background Signature Selection

Use the Background Signature Selection panel to define the background clutter that can interfere with searching for target signatures in the hyperspectral image. This is an optional task in most cases. Background signatures are required as input into the BandMax or Advanced Signature Optimization band selection methods, or if you intend to use the TCIMF or OSP target detection methods.

To define background signatures, unselect the Skip background signature selection option. You can select background signatures by defining regions of interest (ROIs) on the input or atmospherically corrected image, or you can let THOR automatically derive background signatures using various unsupervised methods.

See the following sections:

Selecting Backgrounds Using ROIs


To select background signatures by manually defining ROIs, click the Regions of Interest tab. The atmospherically corrected (or input) image loads into a display group, and the ROI Tool appears. The Available ROIs list on the Background Signature Selection panel and the list of ROIs in the ROI Tool are synchronized so that when one list changes, the other list updates too.

Use the ROI Tool to define ROIs for each desired background, and draw the ROIs. Name the ROIs so that you will recognize them later in the workflow. As you create ROIs, they will appear in the Available ROIs list in the Background Signature Selection panel.

Click Plot Signature(s) to plot the mean signatures for the selected ROIs. Click ROI Options to create new ROIs, delete ROIs, or save the ROIs to a file. You can also perform these functions through the ROI Tool. If you accidentally close the image or the ROI Tool, click Reload Image and ROIs.

To designate ROIs as backgrounds, select the desired ROIs in the Available ROIs list, then click the arrow button to move them to the Background ROIs list. You can remove ROIs from the Background ROIs list by clicking Remove ROIs or by right-clicking the desired ROIs and selecting the appropriate option. This will remove them from the Background ROIs list, but it will not delete them.

Automatically Deriving Background Signatures


To automatically derive background signatures, select the Auto-Derive tab. Use the Processing Method drop-down list to select whether signatures should be auto-derived using SMACC or K-Means classification.

These processes should ideally run on a reduced-resolution version of the input image to save processing time. Set the Spatial scale factor to the desired value for spatial reduction. For example, a value of 0.25 means that every fourth pixel will be used. Set the Number of background signatures to the number of background signatures you wish to generate. For SMACC, you may also change the default value for the SAM threshold (in units of radians). SMACC generates a set of signatures, which THOR then uses in a Spectral Angle Mapper (SAM) classification so you can see the distribution of the backgrounds within the image. Smaller SAM threshold values mean that image pixels must be closer to the background signatures to be classified as that background type.

Click Process when you are ready to generate the background signatures. When processing is finished, the reduced-resolution versions of the input image and background classification image will be displayed, along with a plot window showing the derived background signatures. In addition, the Select Background Signatures list in the Background Signature Selection panel will be populated with the derived signatures. After examining the images and signatures, select the signatures you want to designate as background signatures (if any) from the Select Background Signatures list.

Confirming Your Selections


When you click Next, THOR will display the selected background signatures (both from ROIs and auto-derived) and ask you to confirm before proceeding. Click Yes to confirm or No to go back and change your selections.



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