Maximize Local Contrast

Roy Johnson — Wed, 01 Jul 2009 18:36:27 +0200

FX-Foundry/Photo/Enhancement/Maximize Local Contrast... Like Stretch Contrast, but instead of using the minimum and maximum pixel values from the entire image, uses the minimum and maximum in the vicinity of a pixel to compute its new value. Takes a while to run, but really brings out textures. Result is a Grain Merge layer that adjusts values of the original image.

This filter operates on the visible image, not on a layer or selection.

Radius indicates how wide a pixel's influence is. If you choose too small a value, adjustment will be too localized and the image will look patchy. Larger values take a bit longer to run.

Threshold indicates how many neighboring gray values above and below a value can be treated as the same, in finding minimum and maximum values. Purely a speed trade-off. Using very low values will make it very slow. Too high values will cause visible "steps" in color values. The latest version does not include threshold, but the older version is still available.

"Feather Haloes" helps smooth out haloes around high-contrast areas.

"Keep Min and Max Layers" leaves the intermediate layers in the stack so you can look at them if you're interested.

Update (21 Jul 2009):

New algorithm that works right without any compromises for speed. No scaling, no threshold. It does take a bit of time on large images; 45 seconds for a 1500x2000 pixel image on a modern but not especially powerful PC. The full 8-megapixel image took about 3 minutes. Most of the time is spent on grow-selection, so don't be alarmed when the progress meter seems to hang up for a while. There are just two grows and two feathers to do.
If you want a speed compromise, I've added Scaledown factor, which will reduce the time spent in selection a lot, but will add some time for scaling the image. There's essentially no loss of quality. Recommended for especially large images.

Updates (13 Jul 2009):

SPEED! Mapping of Min and Max is done on scaled-down layers. Now even large images can be done in reasonable time.
Feather haloes feature for more reasonable results on high-contrast images and images with big flat areas
Layer mask keeps narrow-contrast areas under control

Updates (9 Jul 2009):

Fixed the bug that left strange artifacts in the image.
Changed radius to work as a % of image size (maximum dimension)
Increased feathering and added limits to min and max layers to control haloes around small, high-contrast features
Got rid of sample images. Will find better examples.

gimp-ace2

shlomif — Tue, 01 Jan 2008 14:20:16 +0100

A port of gimp-ace to GIMP 2.2, based on gimp-ace 0.6.3.

Reading from the original description there: The basic "Stretch Contrast" operation takes in the whole image at once, and if there is a white pixel anywhere in the image and a black pixel anywhere in the image, it figures the contrast is already as good as can be. But Adaptive Contrast Enhancement works to increase the contrast locally, and brings out details that most wide-sweeping contrast-enhancements pass over.

Runs also on GIMP 2.4, but the GUI is not 100% adopted.