Star Reduction in GIMP

I found a really slick trick to reduce stars in an image using GIMP. It’s really simple and works amazingly well.

Many of my images are absolutely loaded with stars, especially most nebulae which live in the galactic plane. After stretching, sharpening, and other processes, these stars can be quite distracting from the appeal of the target object. My last project, the Wizard nebula, is an example of a very rich star field that hid the details within the nebula. Here is an example after my normal processing steps:

You can see that all of the stars are greatly exaggerated from sharpening and stretching the image.

Now comes the trick. Duplicate you final image, add a layer mask, and apply Filters → Generic → Erode. Then, adjust the layer mask using the Colors → Curves tool so that it reduces the stars but does not really effect the rest of the image. The erode tool reduces the quick luminosity changes from stars, but doesn’t much affect the target or background. Using a mask can limit the effect to saturated stars and not the target.

Here’s a closeup of the effect:

The erode filter looks at a 3×3 kernel, and the luminosity of the center pixel is changed to be the same as the brightest of the other pixels. Since stars are high contrast, a neighboring pixel can be much darker than a pixel within the star. Thus, the bright star is reduced.

After some playing around, I found that this process is great to do just after the initial stretch. At this point, the stars tend to be much brighter than the target, so they can be easily isolated using a mask.