Question

I have some object whish moves by sinusoid. I have to animate it each time it reaches the top (or the bottom) of the “wave”. I want to do this with derivative function: as we know it changes the value (from positive to negative or contrary) at that points. So the code is:

// Start value
int functionValue = +1;

// Function
float y = k1 * sinf(k2 * Deg2Rad(x)) + y_base;

// Derivative function
float tempValue = -cosf(y);

// Check whether value is changed
if (tempValue * functionValue < 0)
{
   animation = true;
}
functionValue = tempValue;

if I will output the tempValue it shows strange numbers:

0.851513
0.997643
0.0242145
0.690432
0.326303
-0.614262
0.892036
0.1348
0.709843
0.968676
0.0454846
0.920602
-0.423125
0.692132
-0.960107
0.0799654
-0.747722
-0.635241
0.148477
-0.98611
0.900912
-0.877801
0.811632
-0.362743
-0.233856
0.35512
-0.994107
0.885184
-0.468005
0.982489
0.675337
0.661048
0.870765
0.0312914
-0.319066
-0.602956
-0.996169
-0.95627

And animation is strange too. Not only at the top of wave. What’s wrong is there?

Answer 1

You have

y = a * sin(b * x) + c

derivative of that is

y' = a * b * cos(b * x)

not

y' = -cos(y)