Question

My game needs to move by a certain angle. To do this I get the vector of the angle via sin and cos. Unfortunately sin and cos are my bottleneck. I’m sure I do not need this much precision. Is there an alternative to a C sin & cos and look-up table that is decently precise but very fast?

I had found this:

float Skeleton::fastSin( float x )
{
    const float B = 4.0f/pi;
    const float C = -4.0f/(pi*pi);

    float y = B * x + C * x * abs(x);

    const float P = 0.225f;

    return P * (y * abs(y) - y) + y; 
}

Unfortunately, this does not seem to work. I get significantly different behavior when I use this sin rather than C sin.

Thanks

Answer 1

For your fastSin(), you should check its documentation to see what range it’s valid on. The units you’re using for your game could be too big or too small and scaling them to fit within that function’s expected range could make it work better.

EDIT:

Someone else mentioned getting it into the desired range by subtracting PI, but apparently there’s a function called fmod for doing modulus division on floats/doubles, so this should do it:

#include <iostream>
#include <cmath>

float fastSin( float x ){
    x = fmod(x + M_PI, M_PI * 2) - M_PI; // restrict x so that -M_PI < x < M_PI
    const float B = 4.0f/M_PI;
    const float C = -4.0f/(M_PI*M_PI);

    float y = B * x + C * x * std::abs(x);

    const float P = 0.225f;

    return P * (y * std::abs(y) - y) + y; 
}

int main() {
    std::cout << fastSin(100.0) << '\n' << std::sin(100.0) << std::endl;
}

I have no idea how expensive fmod is though, so I’m going to try a quick benchmark next.

Benchmark Results

I compiled this with -O2 and ran the result with the Unix time program:

int main() {
    float a = 0;
    for(int i = 0; i < REPETITIONS; i++) {
        a += sin(i); // or fastSin(i);
    }
    std::cout << a << std::endl;
}

The result is that sin is about 1.8x slower (if fastSin takes 5 seconds, sin takes 9). The accuracy also seemed to be pretty good.

If you chose to go this route, make sure to compile with optimization on (-O2 in gcc).