Phase Preserving Dynamic Range Compression is a new filter we have developed. It provides a useful alternative to histogram equalisation.
Images with a high dynamic range, such as aeromagnetic images, are difficult to present in a manner that facilitates interpretation. Histogram equalisation is often used to display images of this sort. However, a problem with histogram equalisation is that the contrast amplification of a feature depends on how commonly its data value occurs, rather than on the amplitude of the feature itself. Thus, features of equivalent magnitude can have very different contrast adjustments applied to them.
We have developed a method that compresses the dynamic range of an image while preserving and enhancing local features. The method is based on forming a 2D equivalent of the analytic signal via quadrature pairs of oriented log-Gabor filters to extract local phase and amplitude. The dynamic range of the image can then be reduced by applying a global tone mapping function to the amplitude values, for example taking the logarithm, and then reconstructing the image using the original phase values. An important attribute of this approach is that the phase information is preserved, this is important for the human visual system in interpreting the image. The result is an image that retains its essential features within a greatly reduced dynamic range. An additional advantage of the method is that the range of spatial frequencies that are used to reconstruct the image can be chosen to maximise the enhancement of local features.
The output is typically easier to interpret than a histogram equalised image. For aeromagnetic images this produces images that are a considerable improvement over the tilt derivative that is often used to compress the dynamic range. Tilt derivative images typically equalise anomaly amplitudes to an excessive degree and do not readily allow spatial frequencies of interest to be selected.
The phase preserving dynamic range compression filter is available within the CET Grid Analysis Extension
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Interactive Image Blending
Our linear image blending tool allows images processed at different scales to be interactively viewed. The movie below shows the operation of our prototype interface. Movements of the cursor up and down within the image vary the scale of analysis that is presented to the viewer.
The aeromagnetic data is the property of Fugro Airborne Surveys Pty Ltd.