[postgis-devel] wkb parsing

[David Blasby]

strk wrote:
> I'm hardly trying to get wkb parsing to work, but it is not
> as easy as I expected (at least for me).
> 
> I've seen the 3d extension brought to wkb by postgis, but I'm
> not sure about some other compatibility issues. For example:
> is a WKBMultiPolygon composed of WKBPolygons OR as for the
> postgis object of Polygons ? OGC spec is really loosy.


According to the spec::


wkbMultiPolygon {
byte byteOrder;
uint32 wkbType; // 6
uint32 num_wkbPolygons;
WKBPolygon wkbPolygons[num_wkbPolygons];
}

WKBPolygon {
byte byteOrder;
uint32 wkbType; // 3
uint32 numRings;
LinearRing rings[numRings];
}


So, in the byte stream you'll see a header ("wbkMultiPolygon")::
byte byteOrder;  //NDR or XDR
uint32 wkbType;  // 6
uint32 num_wkbPolygons;  // number of polygons

Followed a set (num_wkbPolygons) of WKBPolygons.

byte byteOrder;
uint32 wkbType; // 3
uint32 numRings;
LinearRing rings[numRings];

byte byteOrder;
uint32 wkbType; // 3
uint32 numRings;
LinearRing rings[numRings];
...

There's a parser and creator in postgis, so you can look there.  You can 
also use unix commands like "od" and "hexedit" to look at the actual wkb 
  byte stream.

dave