[postgis-users] New Curve Types in 1.2
Mark Leslie
mark at refractions.net
Tue Dec 12 14:29:02 PST 2006
Ken,
You are correct that the compound curve example is missing a parenthese.
COMPOUNDCURVE((0 0, 1 1, 1 0),(1 0, 0 1)) is correct, but wasn't a very
good example in the first place, since it's equivalent to LINESTRING(0
0, 1 1, 1 0, 0 1).
A better example would be COMPOUNDCURVE(CIRCULARSTRING(0 0, 1 1, 1 0),(1
0, 0 1)). When a sequence of points is left unqualified in the compound
curve definition (or MULTICURVE and CURVEPOLYGON for that matter) it is
assumed to be a LINESTRING. In fact, LINESTRING can never be explicitly
stated in the SQL-MM defined WKT.
Similar rules apply to POLYGONS in MULTISURFACEs.
Mark
Ken Southerland wrote:
> This is very exciting news for us as we are just about to implement
> curves/arcs in our application. I was just about to write code that would
> store meta data to translate line strings into arcs/curves. But now maybe I
> don't have to.
>
> Is there any other documentation specific to the new curve types? I see brief
> mention of them in the html documentation accompanying the source code.
> Specifically I see the following line ...
>
> COMPOUNDCURVE((0 0, 1 1, 1 0),(1 0, 0 1)
>
> ...as an example of a mixed curve/straight line sequence. First off, I'm
> assuming it is missing a closed parantheses at the end of the string. In
> other words, I'm assuming it should read ...
>
> COMPOUNDCURVE((0 0, 1 1, 1 0),(1 0, 0 1))
>
> ... but then a further question I have is how this is read. I'm assuming that
> this means there is an arc defined by the 3 points in the first set of
> parantheses and then a short straight line sequence followed by the two
> points defined in the second set. But how is this determined? What if I had
> a 3-point LineString definition in the second part? How does the
> CompoundCurve know that I meant a straight line sequence and not a
> CicularString sequence?
>
> Also, are there java classes yet written to handle these, like the LineString
> class found in the java/jdbc/src/org/postgis/ subdirectory?
>
> Also, is it true that only circular arcs are currently implemented? What is
> the outlook for more complex curves like a Bezier curve?
>
> Cheers! I'm very thrilled with this news!
>
> Ken
>
>
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