Retinex

Disclaimer:

All the information here provided was published on my former website Photmysteries.com with written permission from Dr. Glenn Woodell, Responsible NASA Official and Dr. Daniel Jobson, and fully applies to all rules. No confidential or classified information is published here. Images and quotes from NASA are properly credited. Images without credits are my own.

Please take a look and support NASA Langley Research Center. Those guys are awesome people and do a great job to make our world a better place.

RETINEX

One of the most versatile and interesting image processing algorithms for scientific, military and industrial use is the Retinex.

It was developed by NASA (Langley Research Center) to enhance low contrast images to a never seen before level. It's a wonderful tool for use in astronomy, medical and so many other areas.

For example, many of the deep space astronomy imagery from Hubble, Spitzer was processed with Retinex to enhance the usual very faint contrast found in this kind of images. The same applies to Solar System images of comets to the amazing HiRise's orbital images from Mars surface.

Also, many Jupiter and Saturn atmospheres images were Retinex processed to make complicated low contrast weather pattern visible.

It's not my goal to enter into deep details about the Retinex guts. It's a complex algorithm and all beyond the scope of this page. If you're really interested in learning how it works or how to implement it, just do some research on the internet. The algorithm is fully documented. The idea here is to show what are the uses for Retinex.

Now, let's see some cool examples of what Retinex is capable of.

Foggy image from Lago Maggiore, Italy

Retinex processed image

The image above shows a dramatic contrast gain over the original image, but to be honest, nothing that a Photoshop autolevels can't do very closely.

Below, the same original image processed with PS and autolevels. With some curves adjustments, I'm sure it can match the Retinex example.

Same image processed using Photoshop Autolevel

Pushing Retinex harder, things start to change a lot, especially at the very low contrast parts of the image, for example, the smoke trails at the hill's houses. It has a very distinct look and some features are very enhanced.

More aggresive processing

At this time, it's fair to say that Retinex is not aimed to render a realistic looking image. Its function is to make things visible !

Below, some images I made from some glaciers in Patagonia. The image was shot from something like 12km of altitude.

Foggy Glacier

Retinex Processed

On the processed image, all the rivers, vegetation and terrain features are easily visible.

Let's continue...

Pio XI Glaciar in Patagonia

Again, a dramatic change !

Well, I think that's becoming repetitive =)

Now some surveillance examples. The photo below was shot in Italy just after the sunset. Compare it with the Retinex processed image !

Low light image

Retinex enhanced

Some very cool turbidity examples below.

High turbidity image
Credit: NASA 

Credit: NASA

X-Ray image
Credit: NASA

Same image Retinex Enhanced

Credit: NASA

If you like to do some experiments with Retinex, there are some cool and free options:

- imageJ
- The Gimp
- RawTherapee
- Fred Weinhaus Retinex script for ImageMagik