Point of intersection

[SteveD73]

Hi

Given 4 sided polygon defined by four points:

CPoint p1
CPoint p2
CPoint p3
CPoint p4

How can I calculate the point of intersection between the diaganols (p1,p3) and (p2,p4)?

Thanks for your help.

 

[jtm111]

rectangle is:

p1 p2

p3 p4


Intersection of diagonals:
x coordinate = p3.x + ((p4.x - p3.x)/2)
y coordinate = p3.y + ((p1.y - p3.y)/2)

x is horizontal axis
y is vertical axis

p1.x is the x coordinate of p1
p1.y is the y coordinate of p1

etc.

This seems right doesn't it?

 

[SteveD73]

The above only works when the shape is a rectangle. I need a formulae that will work with any 4 sided shape.

 

[Zyrenthian]

it is a mathematical formula that you need. First you need to put the lines into slope intercept form (y=mx+b)

lets say a simple square
p1(0,0)
p2(0,4)
p3(4,4)
p4(4,0)

and we draw lines from p1 to p3 and from p2 to p4

slope(m) = rise/run so the slope of the line from p1 to p3 is 1 rise = 4 run = 4

and 2 to 4 is -1 rise = -4 run = 4

now we have formulas for the lines
y = x + 0
y = -x + 4

we know the y coordinates will be the same so we can:

x+0 = -x+4

solve for x and we get 2x = 4 or x = 2

2 is the x intercept of the 2 lines
substitute 2 into an original equation

y = 2 + 0
y = -2 +4

either one give interseckiton of (2,2)

Matt

Sorry if this was already posted, i was away from my desk for a bit

Matt