Geometry Boost List: [ggl] confused by point_order clockwise/counterclockwise

Subject: [ggl] confused by point_order clockwise/counterclockwise
From: feverzsj
Date: 2011-04-14 12:25:38

hi, list
While wrting a function to check the ring/polygon orientation by using the 
signed area algorithm, I also took a look at ggl's area function(with rings 
always presumed clockwise). The ggl area() only returns negative area for 
wrongly oriented ring with respect to the expected orientation, while the 
signed area algorithm returns negative area for counterclockwise ring.

To my understanding, ggl algorithms are always based on "clockwise". So, in 
the area() function, counterclockwise(determined by template parameter) 
rings are reversed as clockwise. And also in many other scene. This seems 
kinda different from the common computational geometry, which may bring some 
surprising, and possibly better to be mentioned in the first place.

Furthermore, correct() is very useful, but the related dynamic check 
utilites(like detect orientation, close/open) are not presented. Will it be 
a good idea to provide them?

regards, zsj