For the past couple of months I've been playing around with Blender, with the goal of making my own Poser/DAZ Studio content. I'm beginning to get the hang of the terminology, and have been given some good leads for understanding all the triangles/quads polygon flow stuff. Vertex normals and single/double-sided faces also seem to be making sense (took me a while to spot that Blender defaults to rendering double-sided, while Poser/DS do it single-sided, and thus vertex normals are important and need to be exported to the .obj.

I'm playing with a fairly simple morphing chess piece at present, and I've noticed that Blender, Poser and DAZ Studio all seem to render the same .obj slightly differently. I'm fairly sure it's to do with crease angles and shading groups. Any comments?

Pointers to threads on this 3D Modeling Forum that might help me would be most appreciated.

Quote - ... vertex normals are important and need to be exported to the .obj.

Actually, no - Poser ignores normals within the OBJ file (I don't know if D|S does too). Poser determines which way a facet is facing by examining the "winding order" - that is, the order in which the vertices are defined in the file. If they appear in anticlockwise order, then the facet is facing you. Fortunately you don't have to worry about this, it's all taken care of if you have the normals right in Blender, but you don't need to export them.

Quote - I'm playing with a fairly simple morphing chess piece at present, and I've noticed that Blender, Poser and DAZ Studio all seem to render the same .obj slightly differently. I'm fairly sure it's to do with crease angles and shading groups. Any comments?

It's true. Poser tends to ignore a lot of the stuff that can be present in the OBJ file format (see normals discussion above), such as material definitions and smoothing. These things are taken care of in the Poser library file (PP2 or whatever).

Thanks for the correction English Bob (you weren't kidding on your site when you said you can be found hovering at most poser-related forums).
I'd noticed that some .objs intended for Poser have the normals explicitly defined and some don't (should have realised at that point!) - I'd actually scribbled down a query "f 1,2,3,4 ≠ f 4,3,2,1 ?", so I think the fact was there in the back of my head.

On the subject of single/double sided faces - I've noticed with dynamic cloth simulations that I often end up with faces from the inside of the cloth towards the camera. These, of course, are not rendered (a double-sided mesh would render correctly).
I suppose that if you created a mesh with all normals pointing outwards, then duplicated it, inverted the normals, merged duplicate vertices and exported an .obj then, when rendered in Poser/DS, it should appear as if it was a double-sided mesh? Is this the a sensible way to get over the problem?

Regarding the .pp2/.cr2, I'd already come across Kuroyume's unofficial file CR2 format spec (.pp2, etc seem to be a subset of this?). Although perhaps not quite up-to-date it seems to be a very good explanation.
Guess it's time to consider the .obj and .pp2 files together., and try adding it as a prop rather than importing it as an .obj

In the Poser Material Room make sure that the "Normals Forward" box is ticked and both sides of the mesh will render.

We may be at cross-purposes here, but real double sided polygons should be avoided when making stuff for Poser. They will cause all sorts of horrible artifacts when rendering, and also probably choke the cloth room. As Mark says, there are techniques for rendering the "inside" of polygons. If you do need to model the inside of a mesh, for example to get a lining inside a coat or something like that, you should give it some thickness to avoid the rendering problems. It won't work very well in the cloth room, though.

Just to be sure of the terminology - a true double-sided face is created from a single face in the object file? A double-sided mesh could be made up of double-sided faces, or pairs of single-sided faces, each pair having the same vertices but opposite winding order?

Thanks dadt - I'd searched the manual and could find nothing.

English Bob - "Horrible artifacts when rendering" doesn't sound nice. So now that I know how to enable double-sided polygons, here's a good reason to avoid doing it! Isn't there a note somewhere about only doing cloth simulations "with single-sided, uncapped objects" - I'm sure I read it recently, but I can find it again! (That's one of my big problems - I've read loads of stuff over the past few years, but don't know what's good and what's rubbish, and can't put my finger on where a particular nugget came from to double-check it!)

I read somewhere about "giving thickness" to items, as you suggest, if you wanted them double-sided. It said that the thickness should be added "in the direction of the normals" (or inverted normals, depending on whether you want your original surface to be the inside or outside) which makes good sense - so it's back to the modelling tool's manual for that.
I see your point about it not working in the cloth room, what with the item turning itself inside out as it slides down the figure.

Thanks again - all input greatly appreciated!

As far as Poser Firefly rendering is concerned, there is more than one way to get double sided artifacts. Whether there are two faces sharing the same vertices, but with opposite winding orders; or whether there are two distinct faces with independent vertices, but coincident in space, the effect is the same. And again, as long as the inner and outer faces are moved apart, the artifacts are avoided; this can be done either by modelling the mesh with "thickness", or in the renderer by adding a small displacement to one face. DAZ|Studio doesn't mind double sided facets, by the way, it's just Poser - and only when using the Firefly renderer, at that.

Quote - Isn't there a note somewhere about only doing cloth simulations "with single-sided, uncapped objects"

Sure there is. Here it is again. Don't do cloth simulations with double sided objects, or with objects which intersect either them selves or the figure they collide against. :)

I have a hypothesis that the cloth room is coded to believe that the "inside" of a single sided mesh is the only thing that should be involved in figure collisions - so it gets confused when faced with the outside of a facet, as I discovered when trying to cinch clothing in with a belt, for instance. This also implies that clothing meshes should be modelled with their outsides facing the right way, regardless of whether you can get them to render or not, although I haven't tested this. The cloth simulation isn't accurate enough to "roll" a mesh with thickness down the figure, perhaps unfortunately. :)

I made a simple prop (obj,pp2,texture) with 28 vertices, 8 texture vertices and 12 faces.
It appears as six stacked items, top to bottom:

One face - square, planar.

Two faces - square, planar, same vertices, opposite winding orders.

One face - quad, non-planar.

Two faces - quad, non-planar, same vertices, opposite winding orders.

Two faces - rectangular, planar, at 90 degrees to each other (2 vertices shared).

Four faces - you can work it out...

Each face is texture mapped to make it easy to work out what's what on the renders.

The phrase "SINGLE FACE" on a red background

"DOUBLE FACE (FRONT)" on green

"double face {back}" on yellow

I've numbered each render - here's the key:
1 - Poser6, with default settings
2 - As render 1, but with "Normals Forward" set for the prop.
3 - As render 1, with "backfaceCull" set to 1 in the .pp2 file for the prop.
4 - As render 1, close-up to show the effect on the middle face.
5 - DAZ|Studio 3 render. Similar effect, very obvious on the warped face.

Suffice to say that in every case, two coincident faces with opposite winding orders cause a problem.
Neither "Normals Forward" nor "backfaceCull" seemed to have any noticeable effect, which was a big surprise.

The last two renders were done to try and understand "Normals Forward". The Poser Reference Manual states:
"Checking the Normals_Forward checkbox will flip the surface normal to point towards the camera (or towards the ray in case of raytracing). Use this option to avoid shading artifacts on doublesided polygons."

Renders 6 and 7 show that it does something. These are the props for these two renders, top to bottom:

Single square face

Single square face rotated 180 degrees about the y-axis

Two square faces, same vertices, opposite winding order.

Two square faces, same vertices, opposite winding order, rotated 180 degrees about the y-axis

Texture map:

"FRONT" in red on a white background

"BACK" in blue on a white background

 
6 - P6, default settings, but with a reddish lamp to light up the front, and a bluish lamp for the back.
7 - As render 6, but with "Normals Forward" set.

Any thoughts or comments anybody?

Next step, try English Bob's displacement suggestion.