estimateRigidTransform()

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After reading several posts about getting the 2D transformation of 2D points from one image to another, estimateRigidTransform() seems to be the recommendation. I'm trying to use it. I modified the source code (to change the RANSAC parameters, because its hardcoded, and the hardcoded parameters are not very good)(the source code for this function is in lkpyramid.cpp). I have read up on how RANSAC works, and am trying to understand the steps in estimateRigidTransform().

// choose random 3 non-complanar points from A & B
...
// additional check for non-complanar vectors
a[0] = pA[idx[0]];
a[1] = pA[idx[1]];
a[2] = pA[idx[2]];

b[0] = pB[idx[0]];
b[1] = pB[idx[1]];
b[2] = pB[idx[2]];

double dax1 = a[1].x - a[0].x, day1 = a[1].y - a[0].y;
double dax2 = a[2].x - a[0].x, day2 = a[2].y - a[0].y;
double dbx1 = b[1].x - b[0].x, dby1 = b[1].y - b[0].y;
double dbx2 = b[2].x - b[0].x, dby2 = b[2].y - b[0].y;
const double eps = 0.01;

if( fabs(dax1*day2 - day1*dax2) < eps*std::sqrt(dax1*dax1+day1*day1)*std::sqrt(dax2*dax2+day2*day2)
|| (fabs(dbx1*dby2 - dby1*dbx2) < eps*std::sqrt(dbx1*dbx1+dby1*dby1)*std::sqrt(dbx2*dbx2+dby2*dby2) )
        continue;

Is it a typo that it uses non-coplanar vectors? I mean the 2D points are all on the same plane right?

My second question is what is that if condition doing? I know that the left hand side (gives the area of triangle times 2) would be zero or near zero if the points are collinear, and the right hand side is the multiplication of the lengths of 2 sides of the triangle.