Grass gets black when it's near to the camera

Development Rendering
1

Title says everything. I’m using grass from the Open World Demo. Everything was fine until I built the light.

2

This looks like a LOD problem.
Do the High poly (Black Grass) and the lower Poly (normal grass) use the same lightmap uvs?
UE shoudl warn you if they do not have matching UVs.

3

I guess dithered LOD transition is bugged. We did have some problems with it on mobile that caused black borders around alpha tested objects. Fix was to nuke all dithering code from MaterialTemplate.usf

4

Raildex_ - Yes, looks like all LOD levels have the same lightmap uv’s.

I rebuilded light once again and in attachment is Message Log.

5

The message says your UVs are not the same.

6

So how I can fix it?

7

You have to setup your LOD UVs correctly in your 3D modelling app.

8

Sorry for such a late answer.
There is 3 LOD levels. First and second - ok, uv’s looks similar, so I can probably change them to make them fit to each other. But third level is completely different mesh and don’t even have enough uv islands. By the way, what if for example first LOD level is sphere and second is cube? UV maps will not fit to each other, so what then?
Another question - why it’s a problem for UE4 at all? If I understand correctly, engine creates lightmap for highest LOD level and lower levels are just using this lightmap. So why lower LOD levels just don’t use this lightmap whatever it looks like? How engine know if it’s matching or not?
Third question - why I have to set this lightmaps at all? It’s asset from Open World Demo, hundreds of people are using it and don’t have this problem. Any ideas?

9
  1. The Lightmap UVs have to be the same because else you get artifacts. It’s the same when using a different Mesh with the same Texture, it won’t fit.
  2. You usually create a high poly mesh, and reduce the polys for LODs. It’s bad practice to create a whole new mesh for LOD. See my answer on the first question why is that.
  3. The Open World Demo does not use Baked Lighting like you do and thus doesn’t use Lightmaps.
10

So that was the problem for all the time. Now it’s all clear for me. Thanks!

But in that case I have one more question. Is it enough if I change Static to Movable in foliage settings? Or maybe something else? Isn’t it more expensive than baked, static lighting?

11

Dynamic Light is always more expensive. But Baked Lighting in an open world would mean

  1. very long (like multiple days) baking times
  2. a lot (and I mean a lot) of memory used for lightmaps
12

Hmmm… so looks like it’s compromise between baking time and optimization.
Anyway - thank you. All the time I was thinking that grass have to be static.