[postgis-devel] LWGEOM - XY,XYM,XYZ,XYZM

[strk at refractions.net]

News from the ZM front (still awake due to nightmare bugs!).

Four functions help you modifying dimensions of geometries:
	force_2d(geom)
	force_3dm(geom)
	force_3dz(geom)
	force_4d(geom)

The function do not change the semantic of the geometries,
rather they add or remove coordinate values.

Do you think a swapzm(geom) function would be useful ?
For example, I have a POINT(1 2 3) and want to change it
into a POINTM(1 2 3). Force_3dm would make it a POINTM(1 2 0).

force_3dm(swapzm(force_4d('POINT(1 2 3)'))) will do what I need:
	force_4d  --> 'POINT(1 2 3 0)'
	swapzm    --> 'POINT(1 2 0 3)'
	force_3dm --> 'POINTM(1 2 3)'

What do you think ?

--strk;

On Tue, Oct 05, 2004 at 09:53:26AM +0200, strk at refractions.net wrote:
> Ok. The ZM flags are now handled as for internal representation.
> If you have 3d or 4d data already within LWGEOM you'll probably
> need a dump/restore. Internal storage size is the same, but two
> bits gets interpreted differently (old:DD, new:ZM)
> 
> 	DD 	ZM
> 2d	00	00
> 3d	01	10|01
> 4d	10	11
> 
> Old 3d geoms will be considered XYM.
> Old 4d geoms will be considered XYZ (and will be messed up in coordinates).
> 
> The summary(geometry) function show all present flags for each geometry
> and subgeometry (ZMBS).
> 
> Creation:
> 
> WKT:
> 	Type(0 0)	<=> TYPE
> 	Type(0 0 0)	<=> TYPEZ
> 	Type(0 0 0 0)	<=> TYPEZM
> 	TypeM(0 0 0)	<=> TYPEM
> 
> WKB:
> 	#define WKBZOFFSET 0x80000000
> 	#define WKBMOFFSET 0x40000000
> 	WARNING ! new WKB input is incompatible with previous version
> 	where 3D flag was current WKBZ and 4d flag was current WKBM
> 	This means that if you now use an OLD 4d WKB to create
> 	a NEW geometry it will be interpreted as TYPEM (3dimensions)
> 	thus corrupting the output.
> 
> Questions ?
> --strk;
> 
> On Mon, Oct 04, 2004 at 11:22:31AM +0200, strk at refractions.net wrote:
> > The adt work is on its way. We now have an LWGEOM abstract data type
> > with deserialize/serialize/clone/release functionalities.
> > 
> > The GEOS<->POSTGIS conversion has been fixed to keep geometry structure,
> > and the summary() function has a new format for better debugging.
> > 
> > Before going on with constructurs I think it's important to decide
> > what to do with hasM, hasZ flags.
> > 
> > My idea was to use two flags so to support:
> > 	XY 	- hasM=false hasZ=false
> > 	XYM	- hasM=true hasZ=false
> > 	XYZ	- hasM=false hasZ=true
> > 	XYZM	- hasM=true hasZ=true
> > 
> > This would allow for space saving in the db and better semantic
> > specification.
> > 
> > In order to keep semantic in input/output WKT functions we might 
> > add a <GEOM>M type as a custom extension:
> > 
> > 	POINT(1 2) 	<--> POINT
> > 	POINT(1 2 3)	<--> POINTZ
> > 	POINT(1 2 3 4)	<--> POINTZM
> > 	POINTM(1 2 4)	<--> POINTM
> > 
> > Comments ?
> > 
> > --strk;
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