# Function poly2cw not working?

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Xiaohan Du il 12 Nov 2017
Risposto: Veda Upadhye il 15 Nov 2017
Hi all,
Matlab says:
[x2, y2] = poly2cw(x1, y1) arranges the vertices in the polygonal contour (x1, y1) in clockwise order, returning the result in x2 and y2.
However, if I have the (x1, y1) coordinates as:
x1 =
-1 -1 0 0
y1 =
-1 0 -1 0
obviously the poly2cw result should be:
>> x2
x2 =
-1 -1 0 0
>> y2
y2 =
-1 0 0 -1
However, if I run
>> [x2 y2] = poly2cw(x1, y1)
x2 =
-1 -1 0 0
y2 =
-1 0 -1 0
The result is not clockwise? Why?
Or
>> [x3 y3] = poly2ccw(x1, y1)
x3 =
0 0 -1 -1
y3 =
0 -1 0 -1
It's not counterclockwise either.
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### Risposte (1)

Veda Upadhye il 15 Nov 2017
Hi,
This is actually the expected behavior of the "poly2cw" function. This function expects its input to be a counter clockwise ordered polygon and simply reorders the vertices bottom to top to change the ordering to clockwise.
If you wish to plot points in the clockwise direction, I would suggest using the "convhull" function before the "poly2cw" function. The "convhull" function converts the unordered set of points to counter-clockwise ordered points. You may then use the "poly2cw" to convert them to clockwise order. Here is an example demonstrating this:
K = convhull(x1,y1);
ccx = x1(K);
ccy = y1(K);
[cx,cy] = poly2cw(ccx,ccy);
Documentation for the "convhull" function can be found here:
Hope this helps!
-Veda
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