Help Aligning Grass with Vellum Grains Simulation

[Masoud]

Hi everyone,

I'm trying to copy some grass onto the top of my Vellum grains simulation. However, the grass blades aren't aligning with the direction of the grains — they always point straight up instead of following the grains' orientation.

Does anyone know how I can make the grass follow the grains' axis properly?

 

Thanks in advance for your help!

Grain Grass_01.hip

[Librarian]

Check rolling debris file  in qLib Master files (workFlow) . You need orient Att..  @Masoud

[Masoud]

Hi Tesan,

I've downloaded and installed qLib, but I haven’t been able to find any clear documentation or guidance on how to use it.

Is there a simpler way to achieve this effect, or could you point me toward any resources that might help?

Thanks in advance!

[Librarian]

I just used last vex after simulation that you have in That file . For more advance Trick and Study if you search Grains orientation you gonna find some Studies on that Topic and Vex Codes.
@Masoud
 

[Masoud]

I opened the "rolling_debris.hip" file and confirmed that the POP Solver already aligns particle orientation to the surface correctly, without requiring any additional code. What I actually need is the same behavior for Vellum Grains, but it seems that Vellum doesn't orient the particles properly by default.

I searched for "Grains Orientation" but couldn’t find any helpful tips. Since the Vellum solver doesn’t compute orientation by default, I manually applied an initial orient attribute before the solver. This stabilized the orientation, but now I need it to align properly with the animation.

Edited May 28 by Masoud

[Masoud]

I added the "up" and "orient" attributes before sim, and included these lines of code after it. The alignment has a little improved, but it's still not exactly what I need:

vector axis = v@v * v@up;
float angle = @Time * $PI;
vector4 spin = quaternion(angle, axis);
@orient = qmultiply(@orient, spin);

 

The issue is that the grain's orientation resets to its initial state after they reach to the rest (zero velocity).

Any ideas to improve it?

 

 

[Masoud]

@Librarian Could you share the codes, please?

Edited May 28 by Masoud

[Librarian]

I just copied All from that file(rolling_debris )@Masoud without to take consider all attributes that have no Impact on your Setup. Just search "Grains Rotation" , you have some Vex solution and some Heavy Study on That Topic.


 

[Masoud]

Here’s my scene using the same network, but it doesn’t work:

Grain Grass_10.hip

[hannes603]

Grain Grass_11.hip

 

 

maybe you can just turn the grain to geo and use the normals for orient the grass?

[Masoud]

@hannes603 @Librarian

Converting the grains to a surface and extracting normals from it is a solid approach, but I’m specifically trying to achieve this effect using VEX code, as Tesan suggested. The issue is that when I implement his solution, it doesn’t produce the same results in my scene as it does in his.!