[postgis-devel] ForceRHR doesn't?

Paul Ramsey pramsey at refractions.net
Sat Mar 11 22:22:17 PST 2006

Hunting through Google, it looks like the preponderance of entries  
referring to 3D graphics use RHR to mean anti-clockwise (as a way of  
defining which direction the surface normal of a polygon is).

There are a small number of entries on GIS data capture (a lot of  
them from BC, but I also found one from Australia) indicating that  
RHR means "object on right of linestring defined in direction of  
capture" (clockwise).

So I think in a GIS data context I am correct. I think. (It seems to  
be a common issue for capturing bounded areas in GIS software that is  
oriented primarily to linear features, like CAD programs.)


On Mar 11, 2006, at 10:06 PM, Michael Fuhr wrote:

> On Sat, Mar 11, 2006 at 09:49:04PM -0800, Paul Ramsey wrote:
>> "Right hand rule" means "if I walk along the boundary of the polygon
>> with my right hand extended, it is always inside the polygon".  So
>> the external rings go clockwise, and the internal rings go anti-
>> clockwise.
> Is there disagreement in the field on that or does the definition
> vary depending on context?  I've found that definition as well as
> the opposite; hence my confusion.  Maybe the documentation should
> explain exactly what "Force polygons of the collection to obey
> Right-Hand-Rule" means.
>> And it looks like you found a silly bug, since ForceRHR should not be
>> calling an unconditional reverse.
> That's the main point: ForceRHR doesn't seem to be doing what it
> should be.
> Thanks.
> -- 
> Michael Fuhr
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