Info About 'Use Subspace Background' Option in Target Detection Wizard

In step 6 of ENVI's Target Detection Wizard (Select Target Detection Methods) if you click on the Show Advanced Options button, you will see an option for 'Use Subspace Background.' If you select this option, a Background Threshold parameter will appear. The documentation describes this option in this way:

"If you selected the ACE, CEM, MF, TCIMF, MTMF, and/or MTTCIMF method(s), you can model the scene background by removing anomalous pixels before calculating background statistics. To model the subspace background, enable the Use Subspace Background check box. By better characterizing the subspace background, you achieve greater target-to-background separation and can potentially improve ACE, CEM, MF, TCIMF, MTMF, and/or MTTCIMF detection results, particularly in scenes that contain a lot of clutter or man-made objects.

If Use Subspace Background is enabled, specify the Background Threshold to use for calculating the subspace background statistics. This value indicates the fraction of the background in the anomalous image to use. The allowable range is 0.500 to 1.000 (the entire image)."

It may be useful to know that this process involves calculating an anomaly image using the RxD method (Chang, C. and Chiang, S., 2002). The areas detected as anomalous are then excluded from the calculation of the background statistics, to remove the influence of anomalous features from the characterization of the general background.

It may also be helpful to know that setting the threshold to 1.0 means the entire image will be used to calculate the background stats. Setting the threshold to 0.8 means that the 80% of pixels that have lowest anomaly values will be used to calculate the background stats.

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Chang, C. and Chiang, S. 2002. Anomaly detection and classification for hyperspectral imagery. IEEE Transactions on Geoscience and Remote Sensing, 40(6):1314-1325.