Howdy,
I worked on this model and after I finished the retopo I exported the obj to symmetrize (mirror) it in XSI, as Houdini's mirror SOP won't do. (as described here [sidefx.com])
So I loaded it in Softimage and after my eyebrows returned from the back of my head, I went back to Houdini to see what went wrong.

The problem is twofold: one is that there's very little visual aid, apart from the normals, to see what's going on there and two, assuming the issues weren't generated at export/import, why did Houdini let me do this?
I mean, the s/w should let the user do w/e he wants, but shouldn't be some type of warning, especially when creating non-maniflod geometry?
Not as intrusive as this one, but something.


If I only had Houdini and went in Zbrush to import this, after rejecting the .obj, what means do I have for correcting the issue in H?
Attached there is a comparison between what I see in Houdini viewport and Softimage.

These are not the only problems related to the Topobuild, I've submitted many RFEs on this topic thus far and if anyone's interested what they are, just let me know and I'll make a list.

How were the polygons with incorrect windings generated? Were they created with TopoBuild's new “Skin” mode or manually created with TopoBuild's “Build” mode? Do you have a reproducible scenario that results in TopoBuild creating polygons with incorrect windings?

Most of the topo work was done in “build” mode, I've used “skin” once or twice as well. I don't know how to reproduce it and I'm not sure if it matters, since I don't think this is a bug but a consequence for a lack of certain features. AFAIK in all programs you can produce this type of geometry, but the difference is that you'll know it.

I will do some more testing, if you think knowing how to reproduce this will help you, when time permits.

McNistor
If I only had Houdini and went in Zbrush to import this, after rejecting the .obj, what means do I have for correcting the issue in H?

I don't have answers in regards to Topobuild (haven't used it yet tbh), but you can try using the PolyDoctor SOP to fix the normals. Under the Topology tab, check “Correct Winding of Polygons to Majority in their Manifold Patch”.

Hi Derrick,
I think I've tracked down this bug, it's polyextrude at fault and is related to a setting part of a configuration I'm trying to make it work as a custom tool that invokes a preset (this thread) [sidefx.com]

The setting I'm talking about is Output Back. As you can see those polys have their normals inverted.
Now, if you output the side polys as well, you'd have a volume and it would make sense to have all the normals pointing outside. But here's the problem: having all three outputs checked is not a practical situation as it produces non-manifold geometry. Thinking out loud, this is probably why XSI has a Poly Duplicate command separate from Extrude.

So maybe Houdini should approach this the same - a separate tool for poly duplication? Or a check mark “keep normals” for “Output Back”? Honestly I'd prefer the former as these nodes are starting to get monstrous in size and by the looks of it, it's not a walk in the park to create custom shelf tools with which to invoke various presets.

I have filed other RFEs directly as well as indirectly related to Topobuild among which, better visual cues. Thanks.

edit: thanks DaJuice

Upon further digging, seems like there's a poly duplicate feature already, which is not related to polyextrude. It's on the “Modify” shelf, “Duplicate”. Works with both polys and objects.
That's the tool you want to use in these retopology cases, like I should've used when I duplicated the finger polys in order to save time.