Tuesday, February 23, 2010

Homomorphic Range Compression

After watching the video presentation from Dr. John C. Russ, many readers wrote in to ask specific questions about the procedures that he describes. Today we'll deal with Homomorphic Range Compression.

Some folks were trying to duplicate Dr. Russ' work in Photoshop and getting frustrated. They were using Curves and Levels adjustments, but couldn't seem to get the same results.  These histogram-based contrast adjustments work differently. You'll need Fovea Pro, a reasonably priced set of plug-ins from Reindeer Graphics, to duplicate Dr. Russ' presentation examples.

According to Dr. Russ, Homomorphic Range Compression "... performed using the Fourier transform, uses the magnitude of the power spectrum itself to design a custom filter for each image. The result reduces the overall contrast range of the image but increaes the contrast for those particular frequencies (and orientations) that are most important in the image."

Below is an image of a vehicle, shot in an underground parking garage, with a darkened interior. Homomorphic range compression can be utilised to bring out details from within the vehicle.

Here's how. Once loaded, select an area of interest using the Marquee Tool and click on Filters>IP•Adjust>Homomorphic Compression. There's no user input, no sliders, nothing to do beyond selecting the filter. Just sit back and wait for the filter to work it's magic.

And here's the result:

There's a lot more detail visible.


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