Falling paper and instanced particles.

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Old 10 October 2009   #1
Falling paper and instanced particles.

Hi there,
I am trying to solve a problem of falling dollar bills in the wind. I am planning on baking out animations per frame and using them as an obj sequence. My main concern is getting a little more movement out of the particles.

I can randomize their creation rotation and make a runtime expression to rotate them. But this is a little too random. I can also use the velocity to aim them with the aimDirection in the instancer but then they never flip or rotate. In short, what I want to do is make a randomized but somewhat directed simulation where:

-The paper slows down when it is falling with the up vector of the paper geometry facing along the y-axis
-The paper travels faster downward when the up vector of the paper is facing perpendicular to the y-axis

I did something like this in Houdini where I was taking a normalized dot product of the object normals and the world up axis, but I have no clue about creating this in maya. I'm posting my scene file so if someone could check it out that would be super awesome.

Cheers,
Jason
Attached Files
File Type: zip dollar.zip (50.8 KB, 14 views)
 
Old 10 October 2009   #2
Well, I did some stuff which may be what you're looking for:

1: change the rotation of the instanced geometry so its x-axis points along the bill (so you get better rotation since the order is xyz)
2: added some control-attributes (rotSpeed for rotation speed multiplier, gravPara and gravPerp for the gravity magnitudes when the normals are parallel and perpendicular respectively to scene up).
3: added the pp-attribute gravityField1_magnitude controling the gravity on a per-particle level.
4: made the rotation speed velocity-dependent (it seemed like something you wanted to do anyway)
5: moved the rotation calculations to runtime after dynamics to better control where in the pipe they happen - so the order of things is 1) what's the rotation now - update gravity magnitude 2) what's the velocity now (which is dependent on the gravity magnitude of course) - update rotation.
6: added the stuff you actually asked for in the creation and runtime before dynamics expressions -
vector $randRotPP_rad = particleShape1.randRotPP;
   // convert to radians:
   $randRotPP_rad = <<deg_to_rad($randRotPP_rad.x), deg_to_rad($randRotPP_rad.y), deg_to_rad($randRotPP_rad.z)>>;
   // rotate the vector <<0,1,0>> about the euler rotations to get the direction of the bill's "normal".
   vector $up = <<0,1,0>>;
   // rotate about x:
   $up = rot ($up, <<1,0,0>>, $randRotPP_rad.x);
   // rotate about y:
   $up = rot ($up, <<0,1,0>>, $randRotPP_rad.y);
   // rotate about z:
   $up = rot ($up, <<0,0,1>>, $randRotPP_rad.z);
   
   // project on scene up:
   float $parallel = abs(dot ($up, <<0,1,0>>)); // if this is 0 it's perpendicular, if it's 1 it's parallel
   
   // map to proper range
   particleShape1.gravityField1_magnitude = $parallel * particleShape1.gravPara + (1.0 - $parallel) * particleShape1.gravPerp;


Hope that does what you want.
Attached Files
File Type: zip dollar1.zip (49.3 KB, 39 views)

Last edited by AndersEgleus : 10 October 2009 at 07:35 PM.
 
Old 10 October 2009   #3
Sounds like a job for nCloth. Duncan posted a confetti example on his blog.

EDIT: I just reread your question and this
Quote: In short, what I want to do is make a randomized but somewhat directed simulation
probably means the nCloth will be unsuitable.

-- David
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Last edited by djx : 10 October 2009 at 11:28 PM.
 
Old 10 October 2009   #4
Thanks for the replies guys. That isn't exactly what I am looking for Anders, but you definitely have set me off on the right foot. I know what I am going to do now so thanks for that. Lots of good info in that file.

Thanks for forwarding that example David. I was thinking of doing this in cloth but I didn't want the sim to get to heavy. Especially with the mesh motion in the wind. I am going to go the dot product route and just make a multiplier with the absolute value of a normalized dot that scales the effect of a particle moving downwards. I will also add some ncloth sims for forground stuff to make it look extra nice. I'll try to post the scene file here when I'm done!

Thanks for your responses.

I also have another question to throw out there if anyone is reading. I created a rolling ball with ncloth and I want the ball to collect small sheets of ncloth as it rolls over them(to create a giant dynamic rolling ball of paper). I tried turning on the stickiness attribute for the various objects but everything just sticks to the ground plane. I also tried using a negative stickyness attribute for the ground plane and a positive one for the ball and sheets of paper but still couldnt get what I wanted. It would be pretty simple make a small script that just dynamically constrains objects to each vertex of the ball but I'm not sure I want to do that because it will look kind of odd. If anyone has any Ideas please let me know!

Cheers
Jason
 
Old 10 October 2009   #5
About the falling bills: You may have better luck using a wind field than a gravity field, since a wind field forces particles to reach a certain velocity, rather than applying a (constant) acceleration to them.
 
Old 10 October 2009   #6
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