How would I change an equation to modulate a number on a line, to one that that modulates on a sine curve?

I'm trying to modulate an alpha value in a开发者_如何学Go Java application I'm building on Android. Right now it goes like this:

if (goingUp) {
    newAlpha = oldAlpha + rateOfChange;
    if (newAlpha > maxAlpha) {
        newAlpha = maxAlpha;
        goingUp = false;
    }
} else {
    newAlpha = oldAlpha - rateOfChange;
    if (newAlpha < minAlpha) {
        newAlpha = minAlpha;
        goingUp = true;
    }
}

Where rateOfChange is an arbitrary int that cannot be greater than maxAlpha. The equation is evaluate every tick in a thread and is independent of time.

Is there a way using only the variables given + Math.PI and other Math elements (I'm assuming Math.Sine will be in there) to get newAlpha to be a number on a Sine?

I'm thinking min and max would be the amp of the wave and rateOfChange would be a product of the Sine function, I just can't figure out how it all goes together.

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Your equation will look like this:

y is vertical position at time t, A is the amplitude, f is the frequency, and t is the time (or ticks of your Android clock).

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Why don't you consider this?

...at the top of your class definition, include:-

import java.lang.*;

...and within your function, after the assignment to newAlpha,

newAlpha = Math.sin(newAlpha%(2*Math.PI));

if you would like newAlpha to be in the range [-1,1] as the sin() function is

OR

...within your function, after the assignment to newAlpha,

newAlpha = Math.asin(newAlpha%3 - 1);

if you would like newAlpha to be in the range [-1,1] as the sin() function is

I'm not sure what datatype your newAlpha is, but I'm going to assume it will not affect the answer for this expression - like newAlpha is of type double.

It is generally the way to get a number within a certain range that you apply modulus to whatever expression you have ie. expr%N results in a number in range [0,N-1].

Hope this helps.

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Based on duffymo's general equation I had to go all the way back to my TI-83 days (literally, put the app on my phone). But I was able to put all the pieces together so it ended up looking like this:

newAlpha = (int)((alphaMax - alphaMin) * 0.5 * Math.sin(rateOfChange * ticks + randomPhaseOffset) + (alphaMin + (alphaMax - alphaMin) * 0.5))

FMI: http://en.wikipedia.org/wiki/Sine_wave