I have to do something like this. So, I have to apply glare efects etc’ in composer. The problem is, that the light source behind the glass and I can’t use ID passes or Cryptomatte for light source, because it is impossible in Blender. So… Any ideas how to make light effects for such thing?
Add glare globally. It’s how the real world works. Using mat/obj IDs to control it is something we can do to add more control over it; either for artistic reasons (isolating lamps for tiny area to cause streaks), to fight things that aren’t real (cheesing light falloff/inverse square law), or to help fight our own optimizations (i.e. clamping to gain control over fireflies or shadows not real because we cheat with caustics).
Look at the silly default settings in 2.80; a fixed clamping that only kinda works with neutral exposure. That tells me they want us to light our scene for neutral exposure rather than expose our scene for varying real lighting values. Turn it off first; if fireflies are problematic, turn it on with the intent of tweaking it to match lighting.
For glare, I always use mix=1 and add them to the image later. That allows me to tweak the effect with image area filters if I want.
For streak glow, you can try a few superbright pixels in the middle of the “lamps”, maybe as a separate pass. So basically, create your own mask.
What does it mean you add them “later”?
Color mix, add, original image and glare node.