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Perpendicular Split


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how could I get this effect using POPs only?

In the attached file a circle is copied to a particle. The circle emits new particles - like shockwaves.

I guess it would be possible to do this with just one popnetwork - resulting in a setup easier to control.

A cross product might help, but I neither know how to get a plane from a crossproduct - nor if this plane will help ...

I would highly appreciate, if you find the time to explain this!



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yes it helped.

1: you build a vector N perpendicular to T and V

2: you normalize V

3: you split the particles

4: ??? you randomize a angle 0-360 and build a rotation matrix from this angle and the velocity ... and multiply this by N ??? and feed this as new velocity.

I see it, I understand it partly.

thanks so far.


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I create 2 vector attributes in the initial particle.

One perpendicular to the velocity vector and I normalize it.

The other is the normalized velocity. I called it "vel".

So with this 2 vectors, I go to VOPs and use the Rotate VOP. This vops creates a rotation matrix using the axis (a, b, c) and x angles. If I multiply this by any vector, It will be rotated x degrees in the axis (a, b, c). So if I use as an axis my "vel" attribute and "rand($ID)*360" degrees it will give me a random rotated vector for each particle and ever perpendicular to the velocity of the initial particle. I set it as the velocity and the most important, I set the group to the group created in the split and not preserved, so the veocity is only applied the frame it borns.

Anyway, I don't know if this is the best method. It just occurred to me.

Edited by MIguel P
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do you happen to know what to multiply with what to create an additional circular motion to the emitted circles?

they should keep their expanding motion but rotate about their splitcenter.

I can add a attribute $TX,$TY,$TZ to the emitter that the split particles inherit.

But any orbit/torque/force after the voppop shows no effect.

I think we need an addition vector that is perpendicular to the linear motion and add/multiply/??? this to the velocity.

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Yes, my vector could do that.

(cannot work with h9 - as every voppop is filled with your 1st version :D)

Can't you see my scene? To get the vector do de cross product between "vel" and the normalized velocity of the new particles created in the split.

Edited by MIguel P
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yes I can see your file and fixed my 'bug' (I can edit vopops again ...).

The way we set things up it looks like its impossible to add generic forces:


After a few test with my h8 scene I think that the new vector must not be perpendicular to the velocity.

The angle needs to be modifyable to have controll over the angular motion.

In the meantime I played with my original scene and added some colors.


Need more time to understand this whole particle and matrix thing.

thanks for your time!


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Hehe, cool scene.

I think that is the vector to use. I attach a scene (houdini 8) showing it. I generate the tangents from the circle using the cross betwen P (a vector pointing outwards the origin) and a vector pointing to y positive direction (the circle is in the xz plane). The idea is the same for the other scene, but instead of using a vector (0, 1, 0), using the direction vector of the first particle.

Thanks yo you for the challenge. I really enjoy this aspect of houdini.


Edited by MIguel P
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