How can I tell if a point belongs to a certain line?

Question

How can I tell if a point belongs to a certain line?

Examples are appreciated, if possible.

Answer 1

If you have a line defined by its endpoints

PointF pt1, pt2;

and you have a point that you want to check

PointF checkPoint;

then you could define a function as follows:

bool IsOnLine(PointF endPoint1, PointF endPoint2, PointF checkPoint) 
{
    return (checkPoint.Y - endPoint1.Y) / (endPoint2.Y - endPoint1.Y)
        == (checkPoint.X - endPoint1.X) / (endPoint2.X - endPoint1.X);
}

and call it as follows:

if (IsOnLine(pt1, pt2, checkPoint) {
    // Is on line
}

You will need to check for division by zero though.