random rotate expression

# 1 08-10-2012 , 04:07 PM

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Join Date: Oct 2005

Posts: 1

Hi all, I have been ripping my hair out trying to get this correct. What I have is an airplane that is instanced to particles and moved in formation with an air field. Simple enough, but what I need now is to randomly rotate them like if they are individually rocking back and forth (twists where wngs go up and down).
Have a creation expression of: particleShape1.RandRotate = <<0,0,rand(-3,3)>>; because I want the planes to be created with a rotation, randomly, between -3 and 3. Again simple enough, but now where the problem comes in, I need them to twist smoothly and randomly between -3 and 3 during runtime.
Have a runtime expression of: particleShape1.RandRotate += <<0,0,sin(time*2)*1.1>>; which works, but it does not sweep between the full positive and negative degrees. Would not make any sense to have them only rotate between 0 and 3 say haha.
Anyone have any ideas? user added image

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# 2 06-11-2012 , 04:06 AM

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Join Date: Feb 2004

Posts: 2,988

You can do this by the following function: (3/2) * (sin(x) + 1)

You also have to remember that in Maya, sine expects units in radians, meaning that 2*pi = (~6.28) = one full cycle of sine. Therefore, in 6 frames, you make a complete cycle of sine. In your expression, you have halved that (remember that multiplying inside the brackets does the opposite transformation), so it now takes 3 frames to cycle completely. The minimums and maximums of sine are found at pi/2 and (3*pi)/2. (Green lines) Since (2*pi)/3 = ~2.1, you are jumping past pi/2, and again past (3*pi)/2. In the example where you are currently hitting are the red lines.

user added image

In short:
float $speed = 0.02;

pCube1.rotateZ = (3.0 / 2.0) * (sin(frame * (1.0 / $speed)) + 1);

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Last edited by NextDesign; 06-11-2012 at 04:32 AM.

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# 3 03-02-2013 , 05:15 PM

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Location: London, UK

Posts: 3,198

The noise() function is handy for these kinds of situations. It takes in time and outputs values between -1 and 1. So something like this would work:

Code:

float $fFreq = 1.0;
float $fMag = 3.0;

myObject.rotateZ = $fMag * noise(time * $fFreq);

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# 4 03-02-2013 , 07:43 PM

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Join Date: Aug 2007

Location: The Netherlands

Posts: 88

You can do this by the following function: (3/2) * (sin(x) + 1)

You also have to remember that in Maya, sine expects units in radians, meaning that 2*pi = (~6.28) = one full cycle of sine. Therefore, in 6 frames, you make a complete cycle of sine. In your expression, you have halved that (remember that multiplying inside the brackets does the opposite transformation), so it now takes 3 frames to cycle completely. The minimums and maximums of sine are found at pi/2 and (3*pi)/2. (Green lines) Since (2*pi)/3 = ~2.1, you are jumping past pi/2, and again past (3*pi)/2. In the example where you are currently hitting are the red lines.

In short:
float $speed = 0.02;

pCube1.rotateZ = (3.0 / 2.0) * (sin(frame * (1.0 / $speed)) + 1);

Originally Posted by NextDesign

trying to combine the commands shown here with a previous one about a flickering light
would you be willing to explain this a little more further....
or maybe i have to open a new thread with an explanation of my goal......???

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