Understanding Gdp

[davedjohnson]

So, I'm fairly new to the HDK. Can someone explain what "gdp" is and where it comes from?

Dave

[Mario Marengo]

So, I'm fairly new to the HDK. Can someone explain what "gdp" is and where it comes from?

It's a pointer to a geometry detail, so I read it as "Geometry Detail Pointer"... but I could be wrong.

And, naturally, it came from Mars.

[edward]

That's what it stands for. But who came up with the word "detail" to mean container?

[Mario Marengo]

But who came up with the word "detail" to mean container?

Martians I tell you!!

(...but I don't think it was Ivan... )

[davedjohnson]

So how do I:

1) delete specific points

2) build polys out of existing points

3) add temporary groups and put points/prims into them

I assume it has to do with the gdp, right?

Dave

[Guest xionmark]

Hi Dave,

Yes you need a "gdp" for all of those operations. A quick glance through the HDK docs I find:

GB_Detail

int deletePoint (const GB_PointGroup &ptGrp, unsigned deleteFast=0)

GB_PointGroup * newPointGroup (const char *name, int internal=0)

As far as building the polys from the existing points, I guess that could get a little tricky because of the fact you have to keep track of which vertices are associated with each point. Shouldn't be too hard I guess, just a little attention to how things may get reordered when deleting the points.

Hope to see you at SIGGRAPH.

Take care,

Mark

[sibarrick]

Hi Dave, if you haven't done so already read all the hdk html help, there's not much of it but there are a few gems in there about building polys.

[davedjohnson]

Thanks. I'll poke around in there some more. All the information seems to be so spread out, I'm having trouble keeping track of where I read something. I've been grepping the /include directory a lot lately. I have the doxygen pages which help keep things a bit more organized, but there's a sorry lack of comments in the header files. No offense to the authors intended, but my mileage is varying.

[sibarrick]

I know the feeling. Best thing is probably to decide on something simple and just dive in, post any progress and those that can will help out.

[davedjohnson]

Thanks for the encouragement.

I'm trying to write my own version of a SOP that will find a convex hull of the given geometry. Probably not the easiest place to start, but it would be very helpful for what we're doing. And that gives me motivation (and funding) to learn what the HDK thing is all about.

I've figured out how to delete polygons and keep the points, and how to create new polys, but not with the old points. The new polys add new points (in the same place as the old ones). I suppose that I could consolidate points after adding the polys. But it would be nice to just say "connect points 1 34 67 and 2 into a poly and run with it".

Dave

[sibarrick]

Did you know edward posted a bit of code for doing that on this forum. If you want it I have it. It's not source code but a compiled exe, he used the technique of having it as a standalone C program which you can just call with a unix sop. I've wrapped it up into an otl if you would like it let me know. I can't remember where he posted it exactly but I expect a quick search will reveal it.

[davedjohnson]

Did you know edward posted a bit of code for doing that on this forum. If you want it I have it. It's not source code but a compiled exe, he used the technique of having it as a standalone C program which you can just call with a unix sop. I've wrapped it up into an otl if you would like it let me know. I can't remember where he posted it exactly but I expect a quick search will reveal it.

19673[/snapback]

Yeah, send it to me. I'll search too.

Dave

[sibarrick]

Yeah, send it to me. I'll search too.

Dave

19693[/snapback]

chull_1_.tar.gz

[edward]

BTW, the source for that can be found here:

http://maven.smith.edu/~orourke/books/ftp.html

All I did was take GPD and incorporated it into chull so that I could use it in Houdini via the Unix SOP. I also have some hacked source for qhull too so that it can output .ply files that Houdini can then read.

[sibarrick]

I'd be interested in the qhull stuff, been looking out for it for a while. Any chance of a link or better yet the code...

[davedjohnson]

The code for qhull is at www.qhull.org

but you probably already knew that.

I'm trying to do the same thing in HDK, rather than use the UNIX SOP. Partly because I want to learn how and partly because it would be cooler in our work flow to have "proprietary" software in the pipline.

[sibarrick]

I think I must be thinking of something else, it can't be qhull, damn. I remember checking it out now, it's not quite what I'm after. I'm looking for either code to do 3d paving and/or quadrangulation.

This is it CQMesh, ever seen code for that anywhere?

Actually, its ok I've found it, how weird Google came up with it straight away this time.

http://www.seas.upenn.edu/%7Emarcelos/cqmesh.html

Edited July 21, 2005 by sibarrick

[edward]

I'm trying to do the same thing in HDK, rather than use the UNIX SOP. Partly because I want to learn how and partly because it would be cooler in our work flow to have "proprietary" software in the pipline.

19706[/snapback]

You missed the thread I posted about chull. The only reason why I used the GPD is because I don't have MSVC at home and thus have been compiling using MinGW. GPD is the public domain version of the $GEO library. So basically, it's a cinch to integrate chull into a SOP if you want to.

The problem with qhull is that the code is really, really ugly.

Also, mentioned in the other thread was that I was going to write my own convex hull code as a side hobby. However, I've been distracted for quite awhile now and never got past the 1D case.

[davedjohnson]

You missed the thread I posted about chull. The only reason why I used the GPD is because I don't have MSVC at home and thus have been compiling using MinGW. GPD is the public domain version of the $GEO library. So basically, it's a cinch to integrate chull into a SOP if you want to.

The problem with qhull is that the code is really, really ugly.

Also, mentioned in the other thread was that I was going to write my own convex hull code as a side hobby. However, I've been distracted for quite awhile now and never got past the 1D case.

19708[/snapback]

Is there a way to go back and read the thread on chull?

I'm using Linux, and I'm not sure what GPD is. I thought you meant gdp at first glance and then I saw MSVC (which is Microsoft Visual C, right?).

Anyway, I'm not sure what the 1D case for a convex hull would be. Do you mean 2D?

Thanks for your insights,

Dave

[edward]

Sorry, yeah MSVC is Microsoft Visual C++ ...

If all the points are coplanar, then you get the 2D case where the convex hull is a single polygon. So by extension (whether I'm actually correct or not), I think of the 1D case as the situation when all the points are collinear. So the convex hull is a line (with 2 points).