Hello
Vlj told me to try to make Lens dust effect.
The effect:
Basically when there is a bright light source, it lights dusts on the lens of the camera
The shader needs two input. The rendered texture + a dust texture
An example of dust texture. It should be on dark ground
I put blend mode so you can reproduce easily on gimp if you want (https://en.wikipedia.org/wiki/Blend_modes for math formula).
illuminated dust = darken only(Dust texture, Highly contrasted version of the rendered texture)
Final output = screen (illumated dust, rendered texture)