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Mouse in region

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Does anyone know where I can find any code that checks if the mouse is in a region that can be defined as any kind of region? Checking for a rectangle is easy, but checking for any polygon region I'm not sure how to go about doing that.

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This is a snippet I found on the internet a while ago, seemed to work fine for everything I threw at it.

Don't ask me about the math behind it ;)

xp and yp are point to an array of corners, x,y is the point you want to check:

template < typename T >
  bool is_point_in_poly( int num_edges, T *xp, T *yp, T x, T y)
    int i, j;
    bool c = 0;
    for (i = 0, j = num_edges-1; i < num_edges; j = i++) {
      if ((((yp[i]<=y) && (y<yp[j])) ||
           ((yp[j]<=y) && (y<yp[i]))) &&
          (x < (xp[j] - xp[i]) * (y - yp[i]) / (yp[j] - yp[i]) + xp[i]))

        c = !c;
    return c;
Edited by Endurion

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