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# Using Sine to drive Opacity, but between a range of values

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 Using Sine to drive Opacity, but between a range of values on Jun 29, 2017 at 7:47:25 am

I basically have a shadow underneath a hovering object that I want to increase and decrease in opacity as a function of a sin wave. The hovering object already has this expression on it's y position:

```amp = 5; freq = .75; y = amp*Math.sin(time*freq*Math.PI*2); value + [0,y,0]```

The tricky part is, getting the max and min values to fall between something like 20 and 50. I understand it might be done with something like a clamp, but I don't understand the syntax.

Thanks

 Re: Using Sine to drive Opacity, but between a range of valueson Jun 29, 2017 at 1:13:08 pm

This is probably the simplest:

freq = .75;
s = Math.sin(time*freq*Math.PI*2);
linear(s,-1,1,50,20)

You might have to swap the 50 and the 20.

Dan

 Re: Using Sine to drive Opacity, but between a range of valueson Jun 29, 2017 at 11:22:03 pm

Brilliant.

Thanks Dan!

p.s. my colleagues and I are trying to figure out what the purpose of the Math*PI function is?

 Re: Using Sine to drive Opacity, but between a range of valueson Jun 30, 2017 at 12:23:26 am

Math.PI*2 is the number of radians in one complete cycle of the sine wav. Including it lets the freq variable specify the frequency in complete cycles per second.

Dan