center og polygon!! ahhhhhh!!! - Xtreme Visual Basic Talk

05-12-2005, 06:19 PM

obot74639

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Join Date: Oct 2002

Location: Arizona

Posts: 308

center og polygon!! ahhhhhh!!!

yet another reason why AutoCAD should cease to exist, j/k...
does anyone know how to find the center of a 2d polygon, not only rectangle, but an irregularly shaped, multipoint ploygon. Like I need to send a list or array of polygonal points like x=this and y=that (1,23)(2,45)(1.5,77.3) for many many many points, and get the center of the polygonal blob which the points will define. The point list will be from a plyline or collection of line segments, (which are changing, and always different from eachother). What math would I need, please super brains- help!!!!!!!!! thanks

05-12-2005, 06:45 PM

DaftasBrush

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Location: A room without Windows

Posts: 896

Depends what you mean by "CENTRE"...

You can find the Max and Min, X and Y values.... essentially draw a rectangle around the whole shape... then find the centre of that

centre.x = (max.x + min.x)/2
centre.y = (max.y + min.y)/2

Or there's the "Centroid".. which is effectively the Centre of Gravity
which is a more complicated calculation... but I've got it around somewhere.... I can look it out if that's what you need.

HTH

__________________
Dan B
"Don't anthropomorphize computers. They don't like it."
Trig is for wuss's - Coordinate Geometry RULES!

05-12-2005, 07:21 PM

obot74639

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Join Date: Oct 2002

Location: Arizona

Posts: 308

that may be it

the center of gravity, I think that would be it- if you could post that- thanks alot, I can't wait to start testing it- thanks again

05-12-2005, 07:41 PM

DaftasBrush

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Location: A room without Windows

Posts: 896

Just to be clear... with certain shapes you will look at the Centroid Position and say "That's not the centre"... so just be sure what you want.

The advantage of the Centroid
Its position always remains constant with respect to the vertices through rotation operations.... this is not true of the "Bounding Rectangle" centre point.

See the theory

Anyway, this was written using the UDT coord I like so much...
It's also written to handle LOTS of coords (ie. everything's in longs)

Code:

Public Type coord
   x as Double
   y as Double
End Type

Public Function Centroid(ByRef c() As coord) as coord
    ' Computes the Centroid (Centre of Gravity) of the polygon
    ' described by the array of coords c()
    ' Note : c(ubc) need not be equal to c(0) - "Closing Line Implicit"

    Dim i As Long, i1 as Long, ubc as long
    Dim Area As Double, b As Double
    Dim Cx As Double, Cy As Double
    ubc = Ubound(c)
    i1 = ubc
    For i = 0 To ubc
        b = (c(i1).x * c(i).y) - (c(i).x * c(i1).y)
        Area = Area + b
        Cx = Cx + (c(i).x + c(i1).x) * b
        Cy = Cy + (c(i).y + c(i1).y) * b
        i1 = i
    Next i
    If Area <> 0 Then
        Centroid.x = Cx / (Area * 3)
        Centroid.y = Cy / (Area * 3)
    End If

End Function

One final note... if the vertexes describe a shape with 0 Area.. the function will return (0,0) for the centroid.

Have fun.

__________________
Dan B
"Don't anthropomorphize computers. They don't like it."
Trig is for wuss's - Coordinate Geometry RULES!

Last edited by DaftasBrush; 05-13-2005 at 05:07 AM. Reason: Added link to the theory