View Full Version : Soft Particles?

09 September 2008, 05:45 PM
i'm working on a scene which contains some particles that pass through other objects. the problem is when they collide with other objects a sharp line appears at the point of collision that doesn't look very nice (as you can see in the picture below). (

can anyone help me get rid of them?

09 September 2008, 04:51 PM
your using a shape facing operator (it looks like.)
when it intersects with geometry it will look like this
try a sphere with fall off or somethings else
either way this will happen when it intesects
why do u want it to go through the plane anyway?

09 September 2008, 05:22 PM
i tried sphere but it still looks the same. (

09 September 2008, 09:00 PM
As far as I can see you have possible solutions. One may not be possible with standard max and may require a plugin like PFTools Box3. That way would be to make the particle opacity ramp down BEFORE it gets to the surface, giving the effect of disappearing into something soft. The Falloff Map has some of this, but it works of point positions, not face proximity, you would need Box3 for that. or use a very create mapping setup. The other solution would be to make the object the particles are passing though volumetric. Use either Max's standard ones or for more control a tool such as Afterburn.

But simply softening the particles wont help because even a full volumetric puff will create a stark edge when passing through a hard surface.

09 September 2008, 07:55 PM
thanks charleyc.
its not exactly what i had in mind but looks alot better than what i was doing.

09 September 2008, 05:15 PM
Why don't you render them without a falloff and blur them in post? That way the "hard edge" (which occurs when penetrating another object) gets blurred as well...

Steve Green
09 September 2008, 07:47 PM
If it was just a flat plane, I guess you could use a falloff based on distance to mask the opacity, not sure how you'd do it on an arbitrary shape though.

I remember one of the Digic movies (Warhammer?) specifically mentioned that they wrote a shader to do this, but it was a Maya/Renderman pipleine IIRC.

I just wonder if there's a pass that you could render which could be screen mapped back onto the particles to blur the edges.

- Steve

09 September 2008, 07:22 AM
Well, the "hands on"-way would probably be copying the object several times and putting a "push"-modifier on the copies. Then each of the copies would get a slightly higher push-value, creating several "layers" of outer "shells". Then those objects get pure black materials with 100% opacity for the inner-most object, and 0% opacity for the outer-most object. (The in-betweens of course get the appropriate opacities) Now the particles get a complete self-illuminating white material, and the whole thing is rendered as an alpha for compositing the particles.

Steve Green
09 September 2008, 12:47 PM

it would work up to a point, but only as a silouhette, you'd still have to get round the situation where you have geometry that self-occludes?

Not sure if there's a mental ray shader that can use an arbitrary surface to define a falloff which can be applied to a material for another object.



Steve Green
09 September 2008, 02:59 PM
Maybe I'm barking up the wrong tree here, but would it be possible to use an ambient occlusion shader in the opacity slot?

- Steve

09 September 2008, 03:36 PM
what if i render the scene without particles once and then give the whole scene a matte/shadow matterial and render the particles. now if i somehow get max to only render the intersection lines i can use this render as a mask or something in a composting app to delete the intersection areas from the particle render softly.
i think it will work i just dont know how to do it ( :surprised ) , so is it possible?

Steve Green
09 September 2008, 03:41 PM
I think your problem there is going to be what it should look like where you've got particles overlapping, or say, an arm of a character overlapping the body.

I just did a quick test using the ambient occlusion shader from mental ray in the opacity slot of a standard material or the cutout slot of an Arch Design material and it looks promising.

I stuck a noise map with negative output value in the dark slot of the a/o shader to give it a more definite falloff to nothing, but it looks like it could work once you've got the right amount of falloff/spread.

I've got to nip out now, but I might have a proper play with it tomorrow, see if I can get it working in a real world example.

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