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Is there any automatic way??


not to my knowledge....

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How do i know which vert needs to be welded to which vert???


Well, the meshtester selects the areas that needs to be fixed (it helps to be in vertex mode, so it selects verts, not faces), and in most cases it will suffice if you select the verts that are problematic and veld them one by one, the trick is, that at first glance, they do not appear to be unstitched (no apparent missing gaps...) to my knowledge this happens only, if you have empty spaces in a model or if you imported one, in which case some or all faces are partially detached, which causes this, so you have to veld them back

it is easier for me to explain this in picture...





Make a test, import a simple cube and use meshtool, then select verts on each corner one by one and veld them. thats the easiest way you will know firsthand where the problem is and how to fix it

i hope this makes sense
have fun,
ego