There is an obscure, oft-overlooked rule in the Poser advanced techniques manual (advtech.pdf at the MC web site, or in the P4 manual: chapter 15) that says you can't have more than one possible parent for each child part in your Poser figure. For an example, it has the hind leg of the Poser horse, but a better example might be the cat's shoulder.
On the left, we have the Poser cat, as is. The "shoulder" is really the elbow-area of the cat's leg. On the right is a more logical definition, that breaks the forelegs into the three anatomical parts of the leg. Why didn't Zygote design it that way? Well, you can see that the new "upper arm" piece touches both the shoulder and the chest, meaning it could be the child of either one. And having two possible parents in a Poser figure is against this rule.
What does that mean? Well, you won't get any PHI file errors for doing this, or a Caught C Exception, or an Error Code 50, or even a crash. All that will happen is, when you go to pose your anatomically correct cat, the upper arm will split its seams where it touches the chest.
This is not good. But, happily, there are ways to get around this rule. (Note: I am not a cheater, I just use creative problem solving!)
Note: I have done this to the poor cat, and have had great success! You can find Cat2 in my cheetah package, located under characters at Poserworld.
Here's the mesh for my swan figure, with the chest highlighted in red. Uh oh! The wing touches not only the chest, but the abdomen as well. Actually, due to a fluke (and an undiscovered Ray Dream technique), when I built the swan, the wings weren't actually touching ANYthing: they were floating off the body altogether. Here is an angle picture showing the gap between the wings and body. (Hint: press the numberpad keys in the MFM.)
At the point I discovered this problem, I was very carefully following the advanced technique, and so managed to avoid having actual illegitimate children. It doesn't matter that much, as the wing and body meshes are completely separate, anyway. To "fix" the gap, I just did a morph that stretches the wing cover over into the body. Here are two views in Poser, with the un-morphed wing on the left.
This method is good for disparate body parts that happen to meet, or pieces you can intersect to cover up any split seams that might appear. However, it isn't good for every illegitimate child occasion.
Important Note: This method is still in the experimental stages. Use at your own risk.
This method is for people who are comfortable messing around in the actual code of the CR2 files. I recommend using CR2Editor by John Stallings to help.
Here's a classic example of something that is typically impossible to do in Poser, a bat/dragon style wing. This isn't too hard of a case, if we eliminate the Whand part. Then each fold can be a new piece and have only one parent/child relationship. But what if we wanted to do something even more bold, like make all the wing fingers children of the hand instead of each other, or divide the wing fingers into two or three joints each?
Here you can see it isn't impossible to have more than one child part, but that Poser simply isn't equipped to deal with it. The top figure shows the Poser default. Because section W2 can be the child of either W1 or Whand, it will split at the seams. In this case, it is the child of the Whand, so the seam appears between it and W1.
In the CR2 excerpt on the right, each part gets one Weld line -- it is welded to it's parent. Now what if we help the CR2 by adding in our own Welds? This produced the lower figure in the above illustration. Same PHI, same Hierarchy, but the seam is gone, due to one added line.
Here is the excerpt of the edited CR2, showing two Weld lines for W2.
Note: I've been able to spot Weld simple pieces like this together, but I have not had as much luck doing more complex objects, yet (like double jointed wing fingers!). There may be limitations on the effectiveness of this method, but it needs further experimentation. Good luck!
Note 2: IMPORTANT! This may make Poser crash. Sometimes
these extra welds will cause a 'lost in space' vertex effect when rendering.
I believe (this has not been verified) it occurs when you use two
body parts that meet, but weren't actually one mesh before you divided them.
For example: this occurred in my wing, at the point where all the vanes
met the hand. The hand (and bone segments) were separate meshes from
the membrane mesh. The membrane had one point at it's apex that was
in common to all the 'fingers,' but did not exist in the hand (which had
no membrane section).
Everything will appear fine, until you render. Then, you will see looooooong triangles sticking out of your mesh, all meeting at some distant point somewhere. After this renders, you will not be able to start another render, you have to End Poser's Task. Ouch!
Update: This seems to occur because the vertices to be welded are not exactly lined up, and Poser is looking out to infinity to try to find a mate for the unmatched vertices.
I have a theory for this method, but no hard experimentation for it, yet. The basic premise goes: What if you built the illegally touching pieces at an angle, so they don't touch, then give them a permanent bend so they do in Poser? In other words, to do the cat example, you would build the cat laid flat out like a bearskin rug, with the forelegs flying out of the chest to the sides, so the upper arm doesn't touch. Then, once in Poser, you just bend the whole leg down 90 degrees and it is in place, all nice and legal.
Stay tuned for further development on this idea!