Yep - that's the way the smooth brush works. If there is a gap or seam between the group, Poser seems to cover it up, so it's not noticeable. The smooth brush is the only tool where this occurs. The botton line is that if you are doing morphs, don't use the smooth brush - and in practice, with morphs - you are rarely actually doing that much smoothing - so it is not so much a problem. Use Std, Inflat and Nudge. Pinch can be handy too. Merging points is a no-no and will lead to all sorts of morph problems. Also, if you are doing Unimesh morphs - check out dburdick's tutorial on using PoserLittleHelper.

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Creator of OctaneRender for Poser
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