On this page I detail my experiences with playing around with PBR and HDR in Flightgear 2020.4.0.
It looks absolutely amazing when using model-pbr.eff with ORM texture.
This happens sometimes in HDR. Only when using model-pbr.eff, model-combined.eff don't have the issue. It goes away if I sit and wait about 1 to 2 minutes.
Some aircraft have a dark halo around the lower edge of the objects in HDR. The used effect is just stock model-combined without any adjustments. The halo goes away moving camera lower than the object that displays a halo.
The only I got to work was FXAA from driver, and that don't get shown in screenshots. Also tried --prop:/sim/rendering/hdr/antialiasing-technique=1 or 2 to no avail.
This type of animation seems to not work in HDR.
Missing model features
Ignores canvas background alpha
Canvas background color alpha is constant at 1.0. So canvas HUDs wont work.
Ignores model alpha (AC3D material)
Constant at 1.0
It don't show.
It does something weird.