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