12_watertight STL pt.1

I had a meeting with Bruce Gernand (3D pathway tutor) & Ben Knezevic (3D technician) at Central St.Martins. Lots of advice was given about all elements of my project but in this post i’d like to concentrate on Rapid Prototyping (RP).

Firstly, they have an RP Machine which can construct models 200*200*300mm. A model this size would cost approximately £160. This is a service open to all UAL students and the cost is based on something like £7.50 per hour about half the commercial rate. These models can take 10+ hours to render. I wont go into detail about the materials – suffice to say it is possible. Any larger and you’re talking different techniques.

The machine needs a 3d model as a watertight STL binary file. They have industrial software Magics 7.5 which can be used to prepare the STL file for the printer. I gave Ben one of my previous files (men walking in a ring) to check. The results were approximately:

87415 Bad Edges
8187 Bad Contours
3151 Bad Shells

All these should be at zero!

The program highlights the holes and my model was riddled with them. Further on taking slices across the whole model it was clear that thousand of inner hidden surfaces were present – and these should not be there at all. The whole thing was a mess – there are features in Magics to repair such problems (filling holes, stitching bad edges etc) but there were too many problems.

Ben suggested i send him a single figure exported from Poser and converted in Rhino to a STL to see what the program made of that. We suspected that the problem is with Poser which isn’t designed for use with RP or even 3D particularily, more with creating pictures/animations of 3D figures. I’ve sent it to him and we shall see.

In the meantime to get rid of the hidden surfaces i need to learn more about Combining Meshes in Rhino.

Select Mesh > Mesh Boolean > Union

This process should remove hidden polygons (ie: those inside both meshes), and this works OK when you use the inbuilt primitives in Rhino. A to B. You can see the border between the two meshes is welded together with some additional polygons.

When you try the same thing with a Poser figure problems occur.
Firstly export an OBJ into Rhino creates a figure where different parts of the figure eg. Head, Right Shoulder, Chest, Left Toe…are each stored as a mesh. If you Select All (CTRL ‘A’) in Rhino it tells us there are 91 meshes . To join all these meshes into a single mesh type Join in Rhino’s command line.

Now if you try and Union this with a primitive ball you get some bizarre results:

The legs are gone but the shoes have been left behind!

Also you can see where the hand just touches the ball. The hand has disappeared (A) leaving some holes in the ball (B) where there a few suspicious looking jags, and worse a few fingertips (C) have been left behind.

Zooming way in on the original single mesh object throws some light on this. The James Casual figure in Poser is clothed and below the clothes there aren’t complete body parts. for example the hand ends in a Naked Edge (the magenta loop). This figure isn’t watertight. Water could be poured down its sleeves to fill up the model. These naked edges will have to be filled in – which i can’t find an easy solution for.

Looking at one of the other models (Ben) solves some of this because Ben has no clothes and the only naked edges appearing are around the mouth and eyes. The union of the boy and the ball works fine.

Mouths & Eyes in Poser

Poser treats mouths and eyes differently than the rest of the model – this is to allow for animating the eyes and mouth for speech. These features have much more detailed 3d meshes, which sit behind the face mesh. The eyes can be removed by not exporting them in the first place. I think this should just leave a hole to fill for each eye – which would be fine for my purposes. Here’s an image of the features.

Looks to me as if the inner mouth parts need removing altogether, but as they aren’t seperate meshes this looks tricky. So two problems – 1) removing meshes 2) filling holes.
I’ll look into these later.

Here’s a first attempt to make a Union of two walking poses – the before and after. As you can see there is a lot of problems like half of the figures disappearing, and various sets of toenails /fingernails being left behind! The problem surely has to do with the meshes not knowing what is inside and what is outside because of the holes and naked edges. I’ll have to research this more. I’ve installed a piece of software called PowerShape which is designed to fix STL files but it tends to grind my machine to a standstill.

Another possible solution as suggested by Bruce & Ben, find better 3D models to work from in the first place. There are various websites where people design and sell figures – it may be a case of paying for something a little more watertight.


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