Just to warn you…long post incomming
So first of all, about your LPV crash. Its not crashing because you have mat IDs that use translucency. Its crashing because these mat IDs are casting shadows. Just deactivate shadow casting on these parts in the static mesh/skel mesh viewer and you should be fine
However, when talking about LPVs vs. Lightmass I just want to mention some things that you might not know (or maybe you do and decided to artistically work around them, which is totally fine^^)
So first of all, LPVs are super cool, but for now, they only support ONE feature. And that is to make dynamic GI from the main directional light. While that is already a cool thing, its impossible to get convincing results with that because some features that are missing are so elemental to the consistency of a rendered image, that it “almost” makes no sense to really use them right now.
The thing is, the material system for example needs reflections, because without them, they dont look the way they should and that is for all types of materials, but its the most important thing for metals. Right now, LPVs dont support the whole reflection environment from UE4 which means, all materials will look broken. The reflection environment in UE4 right now is FULLY static.
Yes, there are SSR, but they are not really a part of the reflection environment and are also way to weak/buggy/limited to provide full reflections.
The thing is, you could still use a skylight to use it for reflections (and that only works if you dont deactivate Lightmass which you then need to build with the main light deactivated to get no lighting from it), but since it will not only make reflections, but bounce lighting as well, you will get double GI
If you reduce the intensity, you will also reduce the intensity of the reflections…so thats not so cool^^
What you also have to be aware of, even if you say “force no precomputet lighting” in the world settings, static lighting stuff is still active in the shaders and causes permutations, slow downs and texture streaming to hang. So if you really want to use the LPVs, you should also add “StaticLighting=0” into the console variables. That makes **** a loooot faster (and it could help you with your 3 lights issue for the backlights. Also for those, I would use 3 movable lights with shadows off, and only the one in the middle will get shadows on…if that makes a difference for the looks. So you can guarantee some nice shadows while maintaining performance)
But as mentioned, then you will force deactivate anything static, so no skylight reflections anymore. You will only get SSR and metals will look white.
As an interesting information…since Lionhead is developing the LPVs for UE4…this is what they have right now:
all the cool stuff mentioned there, is sadly not in our build yet. So I would definitely use LPVs, but not until these updates are provided^^
In your case, I would totally use Lightmass with the skylight. Make an importance volume around your road, add the skylight with a radius that is tweaked in a way you will still capture the bg mountains and add reflection capture actors only where stuff is close to the road so you will get reflections of these when the car passes them.
You wrote on the answer hub that without LPV, your interior is way too dark. That could be caused by having no indirect lighting cache so your skel mesh (which I suppose the car is) wont get any of the prebaked bounce lighting. The importance volume should fix that. Also note that when you make black/dark materials, they should never be black. In our production, the darkes material we allow has a luminosity value of 60/60/60 rgb. Charcoal is around 50, but thats fine. Black leather or plastic is at about 75-80. So with that in mind, it should actually not get too dark inside your car
Hope that helps and is at least a bit informative for you If you already knew…just read over it and continue with this awesome project xD