Calculating the Moment Of Inertia for a concave 2D polygon relative to its orgin

Question

I want to compute the moment of inertia of a (2D) concave polygon. I found this on the internet. But I'm not very sure how to interpret the formula...

Formula http://img101.imageshack.us/img101/8141/92175941c14cadeeb956d8f.gif

1) Is this formula correct?
2) If so, is my convertion to C++ correct?

float sum (0);
for (int i = 0; i < N; i++) // N = number of vertices
{
    int j = (i + 1) % N;
    sum += (p[j].y - p[i].y) * (p[j].x + p[i].x) * (pow(p[j].x, 2) + pow(p[i].x, 2)) - (p[j].x - p[i].x) * (p[j].y + p[i].y) * (pow(p[j].y, 2) + pow(p[i].y, 2));
}
float inertia = (1.f / 12.f * sum) * density;

Martijn

Answer 1

#include <math.h> //for abs
float dot (vec a, vec b) {
   return (a.x*b.x + a.y*b.y);
}
float lengthcross (vec a, vec b) {
   return (abs(a.x*b.y - a.y*b.x));
}
...
do stuff
...
float sum1=0;
float sum2=0;
for (int n=0;n<N;++n)  { //equivalent of the ?
   sum1 += lengthcross(P[n+1],P[n])* 
           (dot(P[n+1],P[n+1]) + dot(P[n+1],P[n]) + dot(P[n],P[n]));
   sum2 += lengthcross(P[n+1],P[n]);
}
return (m/6*sum1/sum2);

Edit: Lots of small math changes