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Particle Rendering advice

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Damn i dont got it !!!

while creating stuff in Houdini is fairly simple and well tutorialized - i dont find enough ressources to get Mantra

to help me understanding the Output of Particles...

So i have a bunch of questions and hope for answers that lighten my mind.

What i have understand so far is that i have to use the copy node to put some material on to the particles.

So particles are only a P in space that is then used to hold the geo - correct me if i am wrong.
To get round particles i have to use a sphere or is it enough to use a circle? 
How do i get Color Variation to the sphere or a ramp Alpha ? - i saw that it colorize the spheres when

i check use template attributes - but what does it do ?

and what about the radius ? is this an Attribute that i have to create as Pop property and how do i Transfer it

to the Spheres and one last - do one have to use very tiny spheres if one wishes to match the particle.

so next:
if this is the way to go - what about volumes ?
i would think that i take the sphere and iso Offset it and copy that to the particle Position - but how do i get Variation

then and what about Rendering times ? ...or is there a way to shade the copied Sphere with some kind of cloudy

volume ? but same Problem how to nice fluffy Variation so it doesnt look like spheres with a volume ;] and again

what about Rendering times.

i know that there are a lot more questions, but this seems to be a good start if one can help me with this questions.


de -HEAVY-


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Particles will render as points, by default (not sure if they are spheres or discs aligned to the camera).  You control the size with the float attribute "width".  You don't need to copy any geometry onto particles to get them to render as round shapes.  There's some attribute you can assign to control whether they render as spheres or discs, and control orientation, etc., but I can't remember that off the top of my head.


Color variation is done in any number of ways... Either assign color values (vector "Cd" attribute) to the particles or let the shader do the work.  Important point is that the shader is able to use the "Cd" attribute and pipe that into its color output.  Both the constant shader and the mantra surface shader from the material palette have that set up by default.  Same goes for alpha.  There are ways you can just let the shader handle all the work there, but it may be too complicated for you at this point.


Using copy/stamping for instancing objects onto particles... yes, using template attributes is the right thing.  It copies the attributes from the points and uses them to transform, scale, rotate, etc. the instances.  Check the manual for copy/stamping and instances for info there.  Should be pretty explanatory.


As for your volume question, there are a few ways to go... I would start by looking at this asset on the orbolt site: https://www.orbolt.com/asset/SideFX::pointcloudvolume

You can also just use a fluid source dop to create a single volume from the points.

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Some further tips for rendering particles -


If you're looking to render particles as little volumes, for say dust or sand, the simplest way is to build a material using the "lighting model" shop and set the lighting model parameter (the top one) to "Isotropic Volume". That will render each point as a volumetric sphere. Render times are pretty fast.

You can control the size of these with the pscale attribute. As a general rule things like Cd\alpha\pscale should be adjusted after the sim itself. eg; run your particle sim, cache out, then use a vopsop to adjust the attributes for rendering. This way you don't need to re-sim everytime you want to change something.


To see which attributes are on the particles middle click on any node below the particle sim, this will show you what each point has. By default you'll probably have P, age, dead, life, v and a few others.


I've attached a little file to help illustrate these techniques. The material is inside "shopnet1" in the particles object and is applied at the /obj/ level.





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So after reading over your initial questions it occurred to me you will also very much benefit from copy stamping.


So if you're wanting to get some other type of geometry onto the particles you do use the copy sop, setting "use template point attributes" does pretty much exactly what it says, it reads attributes from the incoming points and applies them to the object being copied.


So attributes such as Colour (Cd), Velocity (v), Uniform Scale (pscale) are either "Set", Multiplied by existing values or added\subtracted, the four options on the copy sop.


What is particularly cool about the copy sop is the ability to stamp variables up the chain. So in the attached file i've given you a few examples under the "stamp" tab on the copy sop. These variables can be called pretty much anything you want, i've tried to make the pretty logical :)


If you then look on the "xform1" sop you'll see the stamp expression in the different transforms -


stamp("../copy1","scale_x",0) for instance. It's saying for each copy of this box, scale it in x by "rand($ID)" (which returns a random number between 0 and 1, based on the "id" attribute) of the incoming points.


You can stamp nearly anything which makes this incredible powerful. You don't have to use particles as the input, any points will do. But you would need to change the stamp variables from "$ID" to "$PT" or point number. ID is a particle attribute.


Anyway... info dump, but hopefully you can find what you're looking for in the file and definitely look into copy stamping tutes etc...




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