Re: 4D Geometry

On Sun, 2005-09-04 at 21:45 -0700, Paul Ramsey wrote: <blockquote type="CITE"><pre>
<font color="#000000">Chris,</font>

<font color="#000000">PostGIS already has 4d geometry, though few functions that work with  </font>
<font color="#000000">that fourth dimension (how far is 8am, in Paris from 4pm in  </font>
<font color="#000000">London?).  Have you checked if there is some room to meet your needs  </font>
<font color="#000000">with some PostGIS enhancements?</font>
</pre></blockquote><br /> I haven't checked it out, but definitely will. However, from your brief description, it
appearsthat it uses clock time. If so, that won't meet my needs, as I'm more interested in ordinal time (t=0, t=1,
etc).Also, part of the reason for my interest is that my application may need to deal with >4D in the future (if all
goeswell), and as we all know, 2D is cute, and fairly straightforward, but it rests upon some assumptions/shortcuts
thatjust don't hold, when you start moving to more complex analysis.<br /><br /><blockquote type="CITE"><pre>
 

<font color="#000000">Paul</font>

<font color="#000000">On 4-Sep-05, at 6:55 PM, Chris Traylor wrote:</font>

<font color="#000000">> Please excuse any stupidity, as although I've used postgres for  </font>
<font color="#000000">> quite some time, this is my first foray into developing for it. I'm  </font>
<font color="#000000">> working on converting the geometry stuff in adt to support 4  </font>
<font color="#000000">> dimensions. For my own use, I plan on patching 8.0.3 with the files  </font>
<font color="#000000">> I edited in the 8.1beta source, right away. I really ownly need the  </font>
<font color="#000000">> ability to store 4D (I do all the geometry functions outside of the  </font>
<font color="#000000">> db), so for the public, I'd really rather do this properly and  </font>
<font color="#000000">> completely. Since, optimistically, this won't see the light of day  </font>
<font color="#000000">> until 8.1.X/8.2, there's plenty of time to discuss/debate things.  </font>
<font color="#000000">> Any and all questions/comments/criticisms are welcomed and  </font>
<font color="#000000">> encouraged. Here are my questions.</font>
<font color="#000000">></font>
<font color="#000000">> 1.) Is anyone else currently working on this?</font>
<font color="#000000">></font>
<font color="#000000">> 2.) 75% of the changes were trivial and most of the remaining 25%  </font>
<font color="#000000">> are complications due to the way "line" is implemented.   </font>
<font color="#000000">> Particularly, the fact that it uses the 2D specific Ax + By + C =  </font>
<font color="#000000">> 0, and not a vector style storage. Obviously, I would have to  </font>
<font color="#000000">> change the line functions in geo_ops.c, and its spec in pg_type.h,  </font>
<font color="#000000">> but I've noticed that it only seems to be used internally, so other  </font>
<font color="#000000">> than those, I can't see any other changes that would be necessary.  </font>
<font color="#000000">> Can anyone, more familiar with the source, think of any good  </font>
<font color="#000000">> reasons that would make them leery of me changing the structure to  </font>
<font color="#000000">> reflect the parametric form, to say Point *A, Point *B, double p.  </font>
<font color="#000000">> [Normally, the parameter would be "t", but I call the 4th  </font>
<font color="#000000">> coordinate "t", so I figured "p" would be a little less confusing.  </font>
<font color="#000000">> Also A & B should be a vectors, but I'll get to that in a later  </font>
<font color="#000000">> question.]</font>
<font color="#000000">></font>
<font color="#000000">> 3.) As it stands now, I added support for the extra dimensions to  </font>
<font color="#000000">> pair_encode, pair_decode, and pair_count. Do you think that it  </font>
<font color="#000000">> would be better to:</font>
<font color="#000000">>     a.) leave the original signatures, and use those routines to  </font>
<font color="#000000">> work with the old style (x,y) coordinates, and setting (z,t) to  </font>
<font color="#000000">> (0,0), when necessary.</font>
<font color="#000000">>     b.) create a new set of functions called quad_encode,  </font>
<font color="#000000">> quad_decode, and quad_count to work with the new (x,y,z,t)  </font>
<font color="#000000">> coordinates, and use them in the code. I'm more thinking of outside  </font>
<font color="#000000">> stuff, (i.e. libpqxx, etc), that might use/depend on those  </font>
<font color="#000000">> signatures. I'm not sure if anything does, that's why I'm asking.  </font>
<font color="#000000">> Also, I'm trying to look ahead for when people that already use the  </font>
<font color="#000000">> geo types go to upgrade.</font>
<font color="#000000">></font>
<font color="#000000">> 4.) If changing the signatures for these routines presents  </font>
<font color="#000000">> problems, will the fact that I changed other signatures to support  </font>
<font color="#000000">> the additional coordinates, also present any problems?</font>
<font color="#000000">></font>
<font color="#000000">> 5.) As it stands now, I'm just using the Point structure to denote  </font>
<font color="#000000">> vectors in component form, and LSEG for stpt-endpt form. Does  </font>
<font color="#000000">> anyone see any reason I shouldn't do this. I realize that having a  </font>
<font color="#000000">> separate VECTOR structure would probably be more readable, and  </font>
<font color="#000000">> probably more useful, but it would more than likely be more work  </font>
<font color="#000000">> initially.</font>
<font color="#000000">></font>
<font color="#000000">> 6.) Are there any objections to breaking up geo_ops.c into separate  </font>
<font color="#000000">> sources?</font>
<font color="#000000">></font>
<font color="#000000">> 7.) Can anyone think of any issues that I'm missing?</font>
<font color="#000000">></font>
<font color="#000000">></font>
<font color="#000000">> Chris</font>
<font color="#000000">></font>
<font color="#000000">> --</font>
<font color="#000000">> Sometimes I wonder whether the world is being run by smart people  </font>
<font color="#000000">> who are putting us on or by imbeciles who really mean it. -- Mark  </font>
<font color="#000000">> Twain</font>
</pre></blockquote><table cellpadding="0" cellspacing="0" width="100%"><tr><td> Chris<br /><br /> --<br /> Sometimes I
wonderwhether the world is being run by smart people who are putting us on or by imbeciles who really mean it. -- Mark
Twain</td></tr></table>