> Quote - Hmm unfortunately this seems to not be it. It creates very beautiful artifacts at the seams, though :-). > Goddamned, there must be a solution for this :D

You might try calculating both parent and child deltas separately, then averaging the two and setting the same for both vertices, perhaps. 

I don't know if this might help show somehow what Poser is doing with welds at render-time, but maybe.  Apologies if I'm veering OT and emphasizing the abstract question with this.

The cape in the attached image is an unusual figure, which has the inner cape (front) welded to the outer cape (back).  In the full figure render at the left, the dark lines showing up on the cape are places where the front and back have been welded, to create the effect of seams.  Some unusual things happen here, as shown by the various renders on the right.  The back is the weld parent here, the front is the weld child.

In the full image on the left, there is no back-lighting and the cape back is in shadow, creating the dark lines on the front.  When a back light is added, the top-left image of the four on the right shows that the new lighting on the back actor bleeds through to the front at these weld locations.  The lighting for the weld parent vertex is dominant.

The top-right image shows what happens when the "Remove backfacing polys" render option is used on the figure.  The cape front polygons at the weld seams disappear.  Poser considers those to be back-facing polygons.  The weld parent has dominated fully in terms of how the normals are determined for rendering.

The bottom left image shows the cape back hidden.  The weld seams are completely relaxed and now they render properly as facing forward.  This is similar to the effect seen in the bottom right image, where "Bend" has been turned off for the front actor, the weld child.  The seams vanish completely.

Apologies again if this trends off topic.    I thought it might help present some new, potentially useful  information about how Poser handles welds.  :unsure:

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Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.