System Information
Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: GeForce GTX 970/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 436.30
Blender Version
Broken: version: 2.81 (sub 12), branch: master, commit date: 2019-09-27 22:44, hash: rB1c1a3198af9d
Short description of error
At first I thought that the effect doesn't care if the part of the stroke is actually visible or not, but that isn't the case: if you hide a GP object in another object it will disappear.
What I think is happening is that the effect holds out areas where there would be a stroke, but that check doesn't include whether or not that part of the stroke is actually being drawn (because it is obscured). I think the screenshots and the file might demonstrate it better.
All of the other effects seem to respect the geometry of the scene properly, so I think this is a bug and not a technical limitation.
Exact steps for others to reproduce the error
- Add a Grease Pencil object.
- Add some geometry that obscures it
- Add the Glow effect


