[postgis-devel] Canonical binary form AND WKB

[Mark Cave-Ayland]

Hi strk, 

> -----Original Message-----
> From: postgis-devel-bounces at postgis.refractions.net 
> [mailto:postgis-devel-bounces at postgis.refractions.net] On 
> Behalf Of strk at refractions.net
> Sent: 17 December 2004 11:20
> To: postgis-devel at postgis.refractions.net
> Subject: [postgis-devel] Canonical binary form AND WKB
> 
> 
> Changed subject and started new thread to reflect discussion.
> 
> A 'canonical binary' representation has been defined for 
> LWGEOM, being the exact internal representation with an 
> additional inizial byte specifying byte order:
> 
> 	0 : XDR : big endian
> 	1 : NDR : little endian
> 
> These are available with a cast to/from bytea, and are fast
> and lossless conversions.
> Bytes flipping is unsupported in both forward and backward 
> conversions.
> 
> WKB is another binary representation, again encoded as a 
> bytea but different from the canonical binary representation 
> in that it does not contain a SRID. WKB can be produced and 
> accepted in both XDR and NDR form. It's implementation is 
> currently slow. It understands an 'extended' OGC WKB 
> including 3dz, 3dm and 4d geometries.
> 
> One question is wheter make output of asBinary() *always* OGC 
> compliant. This can be done internally forcing the geometries 
> to 2d form.

I think we should stick to the OGC standard, no matter how bad it is, purely
so we know that anything we export can be imported into another OGC
compliant application. One thing we could consider is to overload the
asBinary() function such as asBinary(geometry, 'EWKB') which would output
the extended WKB format for applications that need it, containing all the
extended attributes needed for 3dz, 3dm and 4d geometries.

I guess we should make this extended type the default for the LWGEOM_in/out
functions (if this isn't already the case) so that geometries can be
correctly dumped/restored to/from a database across endian architectures.

> Another question is how to distinguish between an OGC WKB 
> (extended or not) and a canonical form, which are currently 
> both of type bytea. 

The only option I can think of would be to change the canonical form endian
identifiers from 0x00/0x01 to something like 0x80/0x81; then if the first
byte > 0x80 then we know we are dealing with canonical form, otherwise we
assume it is (E)WKB. Having said that, is it possible to tell the difference
between EWKB and WKB?


Kind regards,

Mark.

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