View Full Version : opacity scale randomize in nparticle - misunderstanding or bug?

04 April 2011, 09:32 AM
I was just doing a quick test (maya 2011 sp1 linux64) and I noticed something odd.
I always thought the "opacity scale randomize" works like a multiplier with the opacity ramp. And apparently the documentation says the same:

Opacity Scale Randomize
Sets a random value multiplier for the per-particle opacity scale values.
This is not happening for me. Just to be sure, I changed the particle render type to numeric and set opacityPP in the attribute name to check the value.
So if I have my opacity ramp setted in nomalized age, everything works fine, and I see your opacity changing from 1 to 0 during the particle life.
Then I put 1 in the "opacity scale randomize" and my values go random. The problem is that I see values bigger than one (the input max is still 1), some even 2.7. So it seems like the randomize works adding (sort of..) instead of multipling!

Am I missing something? or is this an other bug to report?
Can anyone else confirm or contradict me, please?


04 April 2011, 04:19 AM
The input value in the Scale randomizer appears to be multiplied against your full opacity which is then added to or subtracted from the full opacity to arrive at its final value. That will be why you are getting values higher than your full opacity. So yes they are multiplied to retrieve an intermediate value but then addd or subtracted to arrive at final value.

The input max refers to whatever you have in the scale input. I find these things very non-intuitive to be honest and difficult to use myself.

04 April 2011, 01:34 PM
Hi Jeremy, thanks for the reply. And..... I don't really understand what you're saying (I read it many times :argh: ).
The all point, if your are right, is that you can't have precise numbers!?! Obviously everything can be done quickly with expressions...

Last time I forgot to mention that if I do something similar in the colour, everything works fine. But wait! I just did the same with the radius, and I have wrong values even there (bigger that 1). So now I'm more confused than before.

I guess the only way is as usual to report the odd behaviour to Autodesk and wait for an answer?

04 April 2011, 06:19 PM
Im bad at explaining most likely so Ill explain myself with an example. Also this is my observation only.

Lets use radius. Say our radius is set to 5 and scale input is ageNormalized. If we changed the scale randomizer to .75, Maya then multiplies .75 x 5 = 3.75 internally. At this point nothing has happened. But now it wants to generate a min and max value so Maya then subtracts 5 - 3.75 = 1.25, and adds 5 + 3.75 = 8.75 to get our 2 values. Now Maya can generate a random number between 1.25 & 8.75 to arrive at your final radius per particle.

The max input appears to refer to the scale input but it doesnt seem to work when i use ageNormalized. The ramp seems to affect radius time though.

Personally I dont think its a bug, I just think they are not very intuitive to use. For example what about random radius at birth but still have them get bigger over lifespan? It seems the old way of doing things is a lot simpler and easier to use.

Log a bug if you feel it is one, it would be good to have this cleared up. Perhaps Duncan might chirp in and uncover the math behind it all.

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04 April 2011, 06:19 PM
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