View Full Version : Particle Falloff??

03 March 2007, 12:54 AM
Hey guys i am working with afterburn in max 9 i am doing a meteor crash. I got smoke using particles but i am trying to get the particles to fadeoff after a certain time is there any way you get that effect? because right now they just cutoff. heres a shot

03 March 2007, 01:06 AM
You have to use the Density values. The first value is the Density at birth and the second value is the density at death (if you're using PFlow make sure you have a Delete operator set to Age). So for soft smoke a start Density of 1-2 is pretty good and a end Density of 0.0 will cause the volume to be invisible. Now use the AFC control (button inbetween parameters) to adjust the linear spline interpolation (by default) so that the density falls off in a quadratic curve. Basically, you want the Density to decelerate towards the final value, otherwise it will "pop off."

Hmm, that sounded confusing.

03 March 2007, 02:03 AM
to add to what BrandonD said, a delete operator set to by particle age is a MUST otherwise afterburn wont be able to read in the life of each particle and interperlate between start and end values.

** An age test linking to a delete even wont cut it. **

03 March 2007, 02:33 AM
thanks for the info i am going to give it a try:thumbsup:

03 March 2007, 07:48 AM
Too bad that lowering densities makes for longer renders...

Makes me wish afterburn had an 'opacity' control too :)

03 March 2007, 07:38 PM
yea that would be awsome

03 March 2007, 10:29 PM
ok..... in what part of Afterburn am i adjusting the density values in the color or????

03 March 2007, 12:15 AM
in the bottom rollout called 'Noise Animation Parameters' the first setting is Density. its clearly labeled 'Density'

03 March 2007, 10:56 PM
Too bad that lowering densities makes for longer renders...

Makes me wish afterburn had an 'opacity' control too :)

over at Blur we had pretty strict render time limits per frame,

AB density falloffs combined with selfshadow raytracing were often over those limits.

To solve this we would render two AB passes:

1, the beauty pass: this includes lighting, shadows and color detail, but a constant density value, and usaly higher step size value (speeds up render)

2,hpow the Density pass: this is the same system with lights shadow and everything turned off,
low step size (removes stepping) and a nice density falloff.

combine in comp

using the alpha of the density pass on your Beauty pass

u get a very nice falloff with much lower rendertimes!


03 March 2007, 11:02 PM
Daaaaaaaamn, thats such a great idea superhype, i really wish i knew that a couple weeks ago. I was getting murdered on my rendertimes. I was had to use a super low density, lots of particles and shadows. Uhg 2 hours a frame. Tooo gross. I'm tottaly gonna use this trick next time.

03 March 2007, 06:43 AM
indeed that is an awesom trick. thanks for imparting that piece of brilliance upon us

03 March 2007, 06:57 AM
Great idea, Sam!

03 March 2007, 05:58 PM
Hey Sam thats truely a great concept to get things production worthy!!

(BTW: give Yariv Newman a huge hand shake from me when and if you see him there..)


03 March 2007, 06:30 PM
Yup, Sam knows what he's talking about. There are some important basics to understand about how volumetrics work, in particular the concept of Raymarching. It's like Raytracing in that you're firing rays from the camera and testing what they hit. Where it's different is that instead of bouncing around the scene collecting color, they "walk" through the volume in Steps collecting opacity (density) along the way. Think of it like a hammering a nail into a piece of wood and collecting a sample every time the nail stops. You can swing the hammer a couple of very hard times and get the nail through the wood quickly, but the nail can only sample the inside of the wood two or three times. Now if you were to softly tap the nail, driving it deeper and deeper into the wood bit by bit, you'd be able to collect a much broader amount of information from inside the wood.

Think of this as the trade-off of speed vs accuracy.

Density is one of the most important factors that drives this equation. When you have a very dense volume like thick smoke, it's a waste of time to sample all the way through the volume because it's so think anyway, you're never going to see things inside or through it. For that reason you can limit the depth of sampling by the raymarching engine, otherwise you're just wasting samples (time). But with thick volumetrics you often get lots of fine detail, so you need to decrease your Step Size so that you're not skipping through the volume, missing that detail.

Conversely, if you have a soft steam cloud, you can often see through it to a degree, see things inside it and it's often devoid of detail. In that case you can increase your Step Size and your Limit, forcing the Raymarcher to hop through the volume quickly, but thoroughly.

Where this all gets tricky is dealing with a stream of volumetrics that goes from High to Low density. That's just something that takes some experimenting and experience to find the "sweet spot" in the raymarcher's controls.

03 March 2007, 08:33 PM
Thans guys, this is really helpfull!

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