LOCAL CONTRAST ENHANCEMENT(DEFOGGING)

Wednesday, January 04, 2006

It is a technique I have started using lately, though I use it sparingly and only if I find a need for it. It is a technique to improve contrast of the image without harming the image like the contrast adjustment command does. Interestingly, it is done by applying unsharp mask on the image. The primary uses of local contrast enhancement, which is often called defog, are -

1. Improve the image that looks dull because of any reason such as foggy weather, image taken through a glass or just a bad lens
2. Improve the contrast of the image to make the image look more pleasant and colorful.

The process is simple. Apply unsharp mask(Filter >> Sharpen >> Unsharp Mask) on the image with low amount and threshold, and high radius. The recommended values are 20, 50 and 0. Below is an example of how defogging works to remove haze. This image was taken in a foggy morning and hence looked dull.



I applied unsharp mask with the values 30, 50 and 0 on this image.



Below is the resulting image, where you can see the improved contrast. The haze is also minimized.



As always, unsharp mask has to be used with caution and applying too much of it on the image is undesirable and can result in ugly output. I usually use local contrast only if it is necessary. The Local Contrast enhancement is usually applied at the beginning of the processing. It is best done in LAB colors to prevent any color distortions.

Further reading:
1. Luminous Landscape: Local Contrast Enhancement
2. Cambridge in Color: Local Contrast Enhancement