Mesh

When you create an object in a CAD program .. be it Blender, Sketchup or another it is a 3d mesh composed of triangles. A cube can be considered ‘solid’ in the virtual world of the CAD program. If you plan ahead you can often hollow such a 3d object or mesh .. a cube is very easy to do so. Problems arise when we want to hollow objects with curves. I’m going to address that.

Note: I will be using Sketchup and MeshLab for this tutorial. I’m sure some who read this will respond with other software .. Blender rather than Sketchup .. MeshMixer rather than MeshLab. Great .. you want to do that then go and write a tutorial or search for those written. Go you.

I will be using Hollowing a Model for this tutorial. 1

A Shape

I threw together this shape quickly .. spun a bottle shape, then added a pie shape and cylinder.

This is simple enough to understand and complicated enough to make for a useful demonstration on hollowing the mesh.

The height is shown as 11.583 .. which is completely by accident. In fact the units for this model are decimal inches. That doesn’t matter as exporting the STL file will only export the numbers .. not the unit of measure. I will simply accept them as mm as that will be what MeshLab will expect.

Note: I added a height to the orangy colored piece .. 3.637 .. again .. means nothing. This is only as a reference. When we start exporting and importing .dae and .stl files the dimensions get screwy. We will use this later to re-size our hollowed model correctly.

Strictly for Uber Geeks: If you are curious as to WHY we get export errors .. here is the explanation .. DAE Export Error

Note: Also notice the X,Y and Z axes. Exporting a mesh and then importing into another program these will sometimes appear different .. nothing to be worried about but just something to keep in mind.

With the ‘housekeeping’ out of the way .. let’s continue

Export STL

I created a folder for the STL files named .. STL .. (go figure) and saved the mesh as shape.stl

I have found that it helps to be very precise when creating folders/sub folders. In this case the STL folder is a sub folder of the Hollowing folder which itself is a sub folder of Articles. Planning your folder hierarchy allows finding specific files much easier

MeshLab Layers

Open us MeshLab. We are going to be working with two meshes .. one is the mesh we will import and another is a copy which will be offset negatively to create an inner shell .. the inner surface of the hollow.

Click the icon circled .. this is the “Show Layer Dialog” which will open up a Layer panel

Layer Dialog Panel

Here we see the Layer Dialog Panel. With this we can show or hide layers, merge layers and so on. This will be explained further later.

Import Mesh

The next step is to import a mesh into MeshLab so we have something to work with. This is pretty simple .. File –> Import Mesh (Ctrl+I)

Import shape.stl

Remember we exported our CAD file as shape.stl .. now we simply locate the file, select it and then click ‘Open’.

Unify Duplicated Vertices

Just click ok. Without getting too complicated you may have duplicate faces, duplicate vertexes .. this simplifies the mesh. There are times when you may not want to do this but that is beyond the scope of this tutorial and my brain.

Source
  1. Hollowing a model[]

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