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Old 27th March 2017, 02:06 PM   #1
coldby
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Join Date: Jun 2006
Posts: 102
Default Extrude along a path: an ancient mystery solved

The question came up already long ago: when you extrude an object along a path, which point on (or off) the object runs exactly along the path?
The topic is far from academic: every so often two different objects extruded along the same path must match perfectly along one edge - or at least well enough to allow a further "Snap Together by Distance" to weld them together without collapsing also legitimate vertices. And the only accurate way to do that is to set that point where it suits us.

Thus after long, frustrating and little satisfactory manual trimming (this time it was an old, gloomy and damn complicated Victorian building) I resolved to invest a little time finding out what that confounded ghost point really was.
A few tests showed immediately that it wasn't the object centre (that, whatever the manual's opinion, seems to be merely the centre of the bounding box), nor the real gravity centre of the object. But what the hell then?

As it popped out after a little fumbling, the answer was disarmingly easy: it's the gravity center of the vertices.
The difference to the gravity centre of the object is blatant: assuming the object made of a homogeneous material the local complexity is irrelevant, whereas if each vertex has an own weight and the object itself is made of thin air, then of course the most complex parts will be much heavier than the simple ones, thus shifting dramatically the centre towards the highest vertex concentration.

As such in order to extrude an object exactly the way we want the most immediate solution is to add a "counterweight" (to be deleted after the extrusion together with all the extra-surfaces it generated) that shifts the centre to a position of our choice.
Ten minutes to half-an-hour on the calculator adding up all vertex coordinates along each axis and dividing the results by the vertices count, it could be worse...

Quite a pity though, AC3D could make it in a fistful of ÁS...

Last edited by coldby; 27th March 2017 at 02:14 PM. Reason: typo
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