Question

I used standard hough transforms to obtain the straight lines in an image using OpenCV. Now, I need to find the x and y coordinates of the point of intersection of most of the lines in the image. My idea was to divide the image into several 8*8 pixel segments( I haven’t decided on the size yet) and then search the region for lines. But I am not sure as to how I count the lines in the segments. I am struck at this point. So the things I am unable to do( using openCV) are
1.Divide the image into 8*8 pixel segments( i don’t know if there is a function for it and if not what do I do)
2.Count the number of lines in each segment.
Any reading material or hints for the code will be of really helpful.

Answer 1

Your way of detecting intersection points is totally wrong. There is a simple mathematical formula for that. I am giving you an example code in plain C. :

// This is a point
typedef struct{
   int x,y;
} MYintPOINT;

// This is line
typedef struct {
    MYintPOINT  pStart;
    MYintPOINT  pEnd;
} MyLine;

#define PointMinusPoint(P,Q,R)      {(P).x = (Q).x - (R).x; (P).y = (Q).y - (R).y;}
#define PointCross(P,Q)             (((P).x*(Q).y)-((P).y*(Q).x))
#define SIGN(X)             (((X)>=0)? 1:-1 )
#define ABS(a)              ((a) >= 0 ? (a) : (-(a)))
#define ROUND(a)            ((SIGN(a)) * ( ( int )( ABS(a) + 0.5 ) ) ) 

// Given 2 line segments, find their intersection point
// rerurns [Px,Py] point in 'res' or FALSE if parallel. Uses vector cross product technique.
int findLinesIntersectionPoint(const MyLine*l1, const MyLine*l2, MYintPOINT *res){
    MYintPOINT  p  = l1->pStart;
    MYintPOINT  dp;
    MYintPOINT  q  = l2->pStart;
    MYintPOINT  dq;
    MYintPOINT  qmp;            // q-p
    int         dpdq_cross;     // 2 cross products
    int         qpdq_cross;     // dp with dq,  q-p with dq
    float       a;

    PointMinusPoint(dp,l1->pEnd,l1->pStart);
    PointMinusPoint(dq,l2->pEnd,l2->pStart);
    PointMinusPoint(qmp,q,p);

    dpdq_cross = PointCross(dp,dq);
    if (!dpdq_cross){
        // Perpendicular Lines
        return 0;
    }

    qpdq_cross = PointCross(qmp,dq);
    a = (qpdq_cross*1.0f/dpdq_cross);

    res->x = ROUND(p.x+a*dp.x);
    res->y = ROUND(p.y+a*dp.y);
    return 1;
}