check coordinate exists between two points of line

Question

I have two points, lets say x1,y1 and x2,y2. and i have one more point x3,y3, here is the question, i need to check is x3,y3 coordinate occurs in the line between x1,y1 and x2,y2 or not occurs in javascript.

Thanks in advance.

Answer 1

Try this:

var point_a = [0, 0];
var point_b = [100, 200];
var coord = [50, 100];

function checkCoordinate(coord) {
    var slope = (point_b[1] - point_a[1]) / (point_b[0] - point_a[0]);
    var newSlope = (point_b[1] - coord[1]) / (point_b[0] - coord[0]);
    if (coord[0] > point_a[0] && coord[0] < point_b[0] && coord[1] > point_a[1] && coord[1] < point_b[1] && slope == newSlope) {
        alert('Yes! they are in the same line.');
    } else {
        alert('No :/');
    }
}
checkCoordinate(coord);

Fiddle

Answer 2

First, verify x2,y2 do not (numerically) equal either of the end points:

You can create function to check this like (pseudocode):

IsNearZero(value) 
   if(abs(value) < tolerance) return true
   return false

So: // Assuming a false return means (x2,y2) is NOT between them.

if((IsNearZero(x2-x1) && IsNearZero(y2-y1))
   return false 
if(IsNearZero(x2-x3) and IsNearZero(y2-y3))
   return false

Now check if (x2,y2) is between the points:

// Left Edge
if((x1 < x3) && (x2 < x1))
   return false
if((x3 < x1) && (x2 < x3))
   return false
// Bottom Edge
if((y1 < y3) && (y2 < y1))
   return false
if((y3 < y1) && (y2 < y3))
   return false

Do similar tests for the right and bottom edges...

Finally, take the cross product of the two lines:

The first segment: (x1,y1,0) -> (x2,y2,0) --> (x2-x1, y2-y1,0)

The second segment: (x1,y1,0) -> (x3,y3,0) --> (x3-x1, y3-y1, 0)

if(not IsNearZero((x2-x1)(y3-y1) - (y2-y1)(x3-x1)) return false

Now, return true...