CGTalk - basic question about nparticles (time of flight)

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notthere

01-30-2011, 05:48 AM

a couple of hours ago, i tried to replicate the arrows-scenario duncan showed during his tech talk at siggraph 2009. but when i watched the particles fly, their speed seemed off to me and i started reading about how units, mass, etc. could affect that.

i have read the nucleus white paper, maya help and various postings here but all the knowledge now about e.g. that mass is mostly just relevant in terms of collision and air resistance and that the default units of the nucleus don't match the default unit settings from maya doesn't help.

because the units in my setup match (on a sidenote: i have a roughly 4x2cm surface as emitter, shooting particles at 45°), the air density is 0, yet they take 4.58 seconds to land 1 meter from where they were shot. which is quite a difference from the 1.41 seconds it should take.

Albo

01-30-2011, 01:22 PM

Hi,
honesty I didn't seen the Duncan talk.... What I'll do is play with the time or space scale in the nucleus.
Alberto

notthere

01-30-2011, 05:07 PM

sure, i could do that but the problem with that is that then it becomes a horrible trial & error situation. and i would have to go through that every time i work with ndynamics.

i want to be able to do a reliable setup because a) it would be faster to do in the future, b) then i would be sure that i know how nparticles work, c) if i adjust the scales to fit the particle movement and not the particle movement to work with the scales it is supposed to, i bet i'd often have to go back and readjust the scales just because of a change in how the particles move.

still, thanks for the suggestion... i suppose if there's no accurate way, i'll have to do it like that.

Duncan

01-31-2011, 09:11 PM

If the air density is zero or the drag/lift are zero on the particles then the keys to the correct flight time are simply the emission speed and the gravity( and the launch angle ).

To get correct gravity make sure that your scene scale is adjusted correctly... 0.01 if you treat units as cm, 1.0 if meters. (the actual units setting is ignored by dynamics) (the gravity setting should be left at the default 9.8)

Duncan

notthere

02-01-2011, 05:51 AM

thank you so much! you mentioning that the unit setting is ignored made me realize what my problem was.

i was looking at 1 unit on the grid while i had the working units on meters. which obviously is "really" 100 units, since nucleus ignores the working units. now everything works perfectly - thanks again! :)

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