[ Thanks to Michael Scott for this
link. ]
“Photographers use Star Filters sometimes to really make the
highlights stand out, but if you do not have one you can use GIMP
to reproduce the effect.“1. Uncovering the Light sources with Levels
* Open up an image with plenty of individual light sources.
* Duplicate the Original layer:
* On your Light Sources layer go, Colors -> Levels
* Slide the middle to the right, and slide the left to the
middle/left, as shown:”