In the filters section of gimp, the “generic” section is probably one you would skip. But in the generics is a treasure that can add an old painting style or an intense neon effect to a scene. If you don’t use it correctly though, it can completely screw up your image. From what I know, when you put a positive number in, it changes some values to be more intense and bright and vice-versa for the negative numbers. However, a mixture of numbers can lead to something like a neon oil painting effect or something amazing.
One positve + one negative = effect.
Positive + Positive or nothing = Bright neon.
Negative by itself or + another negative = darker
Other features in the matrix section are the divisor and the color channels buttons. Changing the divisor from what I know will change the opacity / color saturation. Changing the color channel buttons will exclude specific color channels which can lead to changing maybe yellow streaks into blue streaks by excluding the green channel.
Before:
After:
