Grass Tool spawns below Landscape

[anonymous_user_9c1d21ab]

Hi,

I’ve followed the [Grass Tool][1] tutorial and used it for my landscape. The grass spawns in a very grid like fashion and sometimes spawns below the landscape and looks like this:

A closeup of the grass below the landscape.

I’ve tried several options for the grass type, with jittering, scaling etc. but I always get the same results. What am I missing?

[anonymous_user_9c1d21ab]

I did some more testing and realised this behaviour is only evident because I used a high scale factor for the landscape (2500 times for x and y. I need to do this because in my heightmap 1 pixel represents 25 meters).

I looked at the Unreal Source and found something in the file [LandscapeGrass.cpp][1]. The z values of the grass meshes are computed with bilinear interpolation:

OutLocation->Z = DrawScale.Z * FMath::Lerp(
	FMath::Lerp(Sample11, Sample21, LerpX),
	FMath::Lerp(Sample12, Sample22, LerpX),
	LerpY);

which is not the correct way to compute the z coordinates within arbitrary four vertices. For landscapes with a high vertex count the difference is minimal but with my landscape the problems mentioned in the original post arise. I changed the code to this:

FVector P11 = { (float)X1, (float)Y1, Sample11 };
FVector P12 = { (float)X1, (float)Y2, Sample12 };
FVector P21 = { (float)X2, (float)Y1, Sample21 };
FVector P22 = { (float)X2, (float)Y2, Sample22 };

float Z;
if (LerpX > LerpY) 
{
	FVector N = FVector::CrossProduct(P22 - P21, P11 - P21);
	Z = P21.Z - ((TestX - P21.X) * N.X + (TestY - P21.Y) * N.Y) / N.Z;
}
else
{
	FVector N = FVector::CrossProduct(P22 - P12, P11 - P12);
	Z = P12.Z - ((TestX - P12.X) * N.X + (TestY - P12.Y) * N.Y) / N.Z;
}

OutLocation->Z = DrawScale.Z * Z;

Which fixed the grass position.

Without my changes the scene looks like this:

The grass spawns either below the landscape. Or sometimes above (not visible here). With my changes the same scene looks like this:

Am I still missing something? Or is this a bug in the landscape grass node?

Cheers

[eyosido]

Definitely something that should be looked at by Epic, any feedback?

[Reousa]

Did you somehow extend the said file, or recompiled your own version of the engine?

[eyosido]

@bneukom : could you make a bug report at Unreal Engine Bug Submission Form - Formstack so this will get fixed for everyone?

[anonymous_user_9c1d21ab]

I had to recompile the engine and replaced the code as mentioned in my reply above.

[Reousa]

Are you facing the same problem? @eyoli

[Reousa]

thank you.

[eyosido]

Not yet since I am not using upscaled landscape, but if this is a bug it needs to be fixed and the best way for this is to report it into the bug tracking system.

[Reousa]

Mhmm, I’m facing a slightly different issue where my grass spawns WAY above the actual landscape. Tried everything, can’t figure out the cause.

[eyosido]

this may be due to your grass mesh. Make sure in your modeler it has its coordinates at 0,0,0 and make sure you applied all transforms. Same goes for rotation/scaling.

[anonymous_user_9c1d21ab]

Sometimes it also spawned above for me. Is it always the same distance above the landscape or does it vary (as in my screenshots)?

[Reousa]

Was always the same space from the landscape, i tried messing with the Z scaling, somewhat hacky fix for it but it’s still not giving 100% proper result.

[Imagine-Games]

i have this same bug, i was stumped for a couple of days until I stumbled across this post , video for reference… bug report, foliage spawns underneath landscape on scaled plane - YouTube