Function poly2cw not working?

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.

Answer 1

[Veda Upadhye]

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:

http://www.mathworks.com/help/matlab/ref/convhull.html

Hope this helps!

-Veda