Two line segments collinearity

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I've faced with an simple problem: I can't write a function which tells me whether two line segments are collinear or not. There is a piece of code:

 
double isVectorsCollinear(Vec4i segment_1, Vec4i segment_2)
{
    /* First vector /
    double dx_1 = segment_1[2] - segment_1[0];
    double dy_1 = segment_1[3] - segment_1[1];
    / Second vector */
    double dx_2 = segment_2[2] - segment_2[0];
    double dy_2 = segment_1[3] - segment_1[1];
//
    return fabs(dx_1 * dy_2 - dx_2 * dy_1);
}

The segments come from HoughLinesP which detects segmenst perfectly on simple image manually generated. But the result of this running this code is 10836, which is obviusly not correct. I guess it is because of pixel's coordinates integer values, but I'm not sure. It is very weird for me. What am I doing wrong? Also I've tried another way to test collinearity which gave me correct result. Just consider the difference between dot and vectors lengths product

 
double isVectorsCollinear(Vec4i segment_1, Vec4i segment_2)
{
    Point vec_1 = Point(segment_1[2], segment_1[3]) - Point(segment_1[0], segment_1[1]);
    Point vec_2 = Point(segment_2[2], segment_2[3]) - Point(segment_2[0], segment_2[1]);
    return fabs(norm(vec_1) * norm(vec_2) - vec_1.dot(vec_2));
}

I can use this code, but firstly, it is more computationally demanding (because of norm function calling), and secondly, I am really interested in why my first solution doesn't work?