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- test1/README: added pointer to 2nd series of tests - test2/README: described findings and theories so far
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354 lines
12 KiB
Plaintext
Comparison of Free scripted 3D CAD systems, part 1
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==================================================
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Werner Almesberger <werner@almesberger.net>
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This is a brief evaluation of the scripted 3D CAD systems OpenSCAD
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and Cadmium, comparing the workflow, resource consumption, and the
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quality of the results.
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Introduction
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============
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This file and the sources of the models can be found in
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http://projects.qi-hardware.com/index.php/p/wernermisc/source/tree/master/cad/test1/
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Later experiments showed that small changes to the model can lead to
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quite different results. The continuation is at
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http://projects.qi-hardware.com/index.php/p/wernermisc/source/tree/master/cad/test2/
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Objectives
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----------
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This test aims to determine the general suitability of currently
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available Free scripted 3D CAD system for the construction of
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real-life objects.
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Aspects considered were the ease or difficulty of model development,
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the clarity of the modeling language, resource consumption during
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rendering, and the quality of the resulting mesh.
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A second objective was to evaluate the suitability of CSG as the only
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means for constructing models suitable for large-scale industrial
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production.
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Object description
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------------------
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The object to model is a simple button/key cap shape. The shape
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consists of a top part shaped as a 10 x 15 mm rectangle with rounded
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corners and at height of 1.5 mm. The top part rests on a base that's
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0.5 mm thin and has a border of 1 mm on each side.
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The corners of the rectangle are rounded with a radius of 2 mm. All
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other external edges are rounded (chamfered) with a radius of 0.2 mm.
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The edge where top and base meet is filleted with a radius of 0.4 mm.
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Note that a real button would typically have an internal cavity,
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possibly some depression or other structure on its top, and on the
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bottom side a pusher in the middle and possibly other support
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elements.
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Also, if the design was to be used for injection molding, sidewalls
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would be slightly tilted.
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The rounding of the bottom plate is not strictly necessary and was
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added for visual appearance.
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Candidate 1: OpenSCAD
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---------------------
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OpenSCAD [1] uses its own language, somewhat similar to POV-Ray's, to
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describe 3D objects. It has an IDE with a quick preview capability
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using OpenCSG [2].
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High-quality rendering, e.g., to STL, is done with CGAL [3] and can
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also be run non-interactively.
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OpenSCAD and OpenCSG are licensed under the GNU GPL v2. Parts of CGAL
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are licensed under the GNU LGPL v2.1 while others are licensed under
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the incompatible QPL. See [4] for details.
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The version tested was the openscad 2011.06-1+lucid1 Ubuntu package.
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OpenSCAD front-ends
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-------------------
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There also a number of Python-based scripted front-ends for OpenSCAD,
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namely OpenSCADpy [5], PyOpenSCAD [6], and pySCAD [7].
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Furthermore, there is Mecha [8, 9] for Haskell.
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Cadmium (see below) appears to be on par or better in terms of syntax
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clarity and tidiness than the OpenSCAD Python bindings. Therefore,
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only pure OpenSCAD was considered for this comparison.
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Candidate 2: Cadmium
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--------------------
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Cadmium [10] is similar in concept to OpenSCAD, but uses Python
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instead of a homegrown language. Open CASCADE [11] (via pythonOCC
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[12]) provides the 3D operations here.
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The respective licenses are GNU AGPL v3 for Cadmium, GNU LGPL v3 for
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pythonOCC, and a homegrown "LGPL-like" license [13] for Open CASCADE.
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The Cadmium version tested was Sun Jul 10 16:04:07 2011 +0530 commit
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d4ff63b150ee060a8179a74e369b5df3d0a4a3fc, with pythonOCC 0.5.
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Results and observations
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========================
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Model development was efficient with both systems, with most of the
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difficulties coming from the task of making the model, not from
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inadequacies of the tools.
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Both systems also also produced correct-looking meshes.
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Notable differences exist in the time the rendering takes, where
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rough previews with OpenSCAD are instantaneous and proper rendering
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takes minutes, while Cadmium has no preview and the rendering takes
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hours.
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On the other hand, some small anomalies could be found in the mesh
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generated by OpenSCAD while the Cadmium's mesh looks perfect.
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Model development
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-----------------
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Both systems offer the same basic CSG primitives and operations,
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which made the model development per se straightforward and the
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porting from one system to the other effortless.
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The very quick preview of OpenSCAD is immensely helpful during
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development. The usefulness of the preview is diminished by
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differences only being shown as unions of the solids involved, with
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color indicating their role. It was thus often necessary to isolate
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and simplify elements before the resulting shape could be guessed, or
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to render with slower CGAL.
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Given the slow rendering process, debugging non-trivial designs with
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Cadmium is currently quite time-consuming.
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Development of the basic model (without chamfers and fillets) was
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first done with Cadmium. I then switched to OpenSCAD to develop the
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more advanced features, and finally ported them back to the Cadmium
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model.
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Designing the model elements for filleting and chamfering was
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somewhat awkward with only CSG and - without understanding the
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entire construction process - it may not be easy to see what the
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resulting code does.
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Modeling language
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-----------------
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The limited programming language of OpenSCAD proved to be more than
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adequate for this simple design. To ease comparison and to reduce the
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porting effort, the Cadmium model has the same code structure as the
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OpenSCAD model.
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It should be noted that some redundancy could be avoided in Cadmium
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if all the "rbox_*" functions were placed in a common class whose
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objects could then remember the box's geometry for reuse with the
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fillet and chamfer functions/methods.
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One nuisance with OpenSCAD is that mistyped variable names merely
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generate a warning but let rendering proceed - often with confusing
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results.
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One difficulty encountered when making the Cadmium model was that
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there appears to be no null value for the "union" operation, which
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means functions that generate all their objects in a loop have to
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special-case the first element, making them look a bit awkward (e.g.,
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rbox_chamfer_top_corners). It should be easy to remedy this
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shortcoming.
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The Python language also introduces complications to Cadmium that
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OpenSCAD can avoid, such as the Python parser's limited ability to
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detect continuation lines, requiring continuations to be marked with
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a backslash, and the need to pay attention to the mixing of
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floating-point and integer numbers when using divisions.
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Cadmium's ability to use short operators instead of blocks generally
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yielded only marginally more compact code, since many operations
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ended up being longer than one line anyway. In fact, the code
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structure often looks a bit tidier in OpenSCAD.
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The placement of transformations before creation of the object in
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OpenSCAD e.g.,
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translate(...) rotate(...) cylinder(...);
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is slightly less intuitive than the reverse order Cadmium uses, e.g.,
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Cylinder(...).rotate(...).translate(...)
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Furthermore, if each step is placed on a separate line, Cadmium's
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syntax puts the object in a more prominent position than the list of
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translations.
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Bugs
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----
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OpenSCAD got stuck allocating excessive amounts of memory when trying
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to preview with OpenCSG from the IDE.
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Cadmium fails at line 113 of button.py if the "noise" parameter
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introduced to work around this bug is absent or set to zero.
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The mesh generated by Open SCAD appears to have some small anomalies,
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see section "Resulting mesh".
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Execution
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---------
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On a lightly loaded Intel Q6600, the "high quality" rendering time
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was as follows:
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real user sys
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OpenSCAD 1m25.491s 1m24.990s 0m00.410s
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Cadmium 81m44.408s 81m41.110s 0m01.540s
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This is consistent with the time the rendering of earlier stages of
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the design took: OpenSCAD with CGAL was always much faster than
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Cadmium with Open CASCADE.
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I didn't attempt to systematically search for costly operations, but
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observed that the crossed cubes/boxes forming the core of the rounded
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box took considerably longer than a run with one of them removed.
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Resulting mesh
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--------------
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The rendering results are available at
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http://downloads.qi-hardware.com/people/werner/cad/test1/
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The STL files are scad.stl.bz2 and cadmium.stl.bz2 for OpenSCAD and
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Cadmium, respectively. scad.png and cadmium.png show screenshots of
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the meshes rendered with MeshLab 1.2.2, with double side lighting and
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"flat" rendering.
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The two meshes are of similar size, as reported by MeshLab:
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Vertices Faces
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OpenSCAD 3351 7798
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Cadmium 3183 8362
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Note that the OpenSCAD model uses a slightly larger number of circle
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segments (explicitly set with $fn) than the Cadmium model (which just
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uses whatever is the default behaviour).
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At earlier stages of the design, the Cadmium mesh was found to be
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significantly larger then the OpenSCAD mesh.
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Both meshes look clean and at a first glance show now major
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distortions (*).
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(*) Note that the model already takes care of avoiding situations
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where the subtraction of volumes could leave behind solids with
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the thickness of a rounding error.
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When viewed with MeshLab 1.2.2, with smooth rendering and
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"Fancy Lighting", some faces appear to be inverted. These faces are
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shown in red in
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http://downloads.qi-hardware.com/people/werner/cad/test1/scad-reversed.png
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A peek at the inside of the OpenSCAD-generated mesh reveals internal
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structures left over from the construction process, as shown on
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http://downloads.qi-hardware.com/people/werner/cad/test1/scad-inside.png
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No anomalies could be found in the mesh generated by Cadmium.
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Conclusion
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==========
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In the conclusions, I first consider the relative performance of the
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two CAD system and then reflect on the whether the CSG-only workflow
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as such proved to be satisfactory.
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OpenSCAD vs. Cadmium
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--------------------
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Both systems succeeded in handling the task. OpenSCAD impressed with
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fast response allowing highly interactive development, while Cadmium
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---------------------------------------------------------------------
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soon gets very slow. It is not clear whether this slowness is a
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general shortcoming of Cadmium or whether it is a consequence of poor
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choices made when making the model.
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The mesh generated by OpenSCAD shows some anomalies, but it's not
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clear whether they would affect further processing steps, e.g.,
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conversion to toolpaths.
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In terms of resource consumption and stability, even this relatively
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simple model exhausted both systems, with OpenSCAD exhibiting
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stability issues and Cadmium requiring excessive processing time.
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Both modeling languages can be used in very similar ways and were
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pleasant to use. Python-based Cadmium may be more suitable for tasks
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requiring structured building blocks.
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The CSG-only workflow
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---------------------
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With both systems, translating the mental models of the various
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components into correct instructions was difficult where more
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abstract operations were involved, requiring some amount of trial and
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error.
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Also, the resulting code does not easily reveal its purpose and
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textual comments are an unsatisfactory means of illustrating
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geometrical properties. (As an example, consider the above section
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"Object description".)
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A workflow that includes distinct steps with a visual representation
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of intermediate results, e.g., instead of CSG, using extrusion with
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shapes and paths generated by some 2D CAD system, may be less
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demanding.
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Also, while generating the basic shape was very easy, most of the
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work went into the addition of fillets and chamfers. Neither of the
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two systems provides operations to automate such tasks.
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The story continues
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-------------------
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Later experiments showed that small changes to the model can lead to
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quite different and generally better results. The continuation of
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this evaluation is at
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http://projects.qi-hardware.com/index.php/p/wernermisc/source/tree/master/cad/test2/
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References
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==========
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[1] http://www.openscad.org/
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[2] http://www.opencsg.org/
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[3] http://www.cgal.org/
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[4] http://www.cgal.org/license.html
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[5] https://github.com/hmeyer/openscadpy
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[6] https://github.com/etjones/MCAD/tree/master/PyOpenScad
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[7] https://github.com/kevinmehall/pyscad
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[8] http://hackage.haskell.org/package/mecha/
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[9] https://github.com/tomahawkins/mecha/blob/master/Language/Mecha/Examples/CSG.hs
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[10] http://jayesh3.github.com/cadmium/
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[11] http://www.opencascade.org/
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[12] http://www.pythonocc.org/
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[13] http://www.opencascade.org/getocc/license/
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---------------------------------------------------------------------
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