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157 lines
4.6 KiB
Plaintext
157 lines
4.6 KiB
Plaintext
Comparison of Free scripted 3D CAD systems, part 2
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==================================================
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Werner Almesberger <werner@almesberger.net>
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This is the continuation of my evaluation of scripted 3D CAD systems.
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It shows new findings and improvements since the last test.
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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/test2/
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New findings and hypotheses
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===========================
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The first test revealed several problems in both OpenSCAD and
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Cadmium. It turned out that most of them could be resolved by making
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small changes to the models.
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The road to hell is paved with good intentions
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----------------------------------------------
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The original model tried to prevent the creation of artefacts through
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rounding errors. It turned out that this isn't only unnecessary with
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Cadmium, but it massively slows down the rendering.
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Also, after removing these preventions the bug the "noise" parameter
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worked around no longer appeared.
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This is a comparison of run time (in seconds) and mesh size:
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real user system faces vertices
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Before: 4901.0 4904.4 1.5 8362 3183
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After: 316.3 315.9 0.3 2942 1473
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This makes Cadmium only about 4-5 times slower than OpenSCAD's CGAL
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rendering, a marked improvement from the previous factor of 57. The
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resulting mesh is now less than half the size of the mesh generated
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by OpenSCAD.
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Furthermore, the model is much easier to design and more
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understandable without these extra tolerances.
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Background: preventing artefacts
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--------------------------------
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When subtracting volumes from each other, surfaces that coincide in
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both the original volume and the one subtracted from it can remain
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as arbitrarily thin artefacts.
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This 2D example illustrates the effect:
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- given an original shape like this
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+---------------+
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+---------------+
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- we want to subtract the following shape from it:
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+--------+
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+--------+
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- expecting this to result in something like
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+---------------+
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| +--------+
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| +--------+
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+---------------+
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- however, if the subtraction isn't perfectly exact, we may get this
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instead:
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+---------------+
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| +--------+
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| | |<--- very thin surface
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| +---^----+
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+----------|----+
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empty space
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- to avoid this problem, we could enlarge the shape that is
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subtracted:
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+---------+
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+---------+
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Artefacts in OpenSCAD's mesh were rounding errors
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-------------------------------------------------
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The artefacts found in the mesh produced by OpenSCAD were also caused
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by rounding errors. In this case, the problem were parts that were
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fused along a common surface. As in the above example, this could be
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solved by making the parts overlap a little.
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This change affected the rendering performance as follows:
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real user system faces vertices
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Before: 85.5 85.0 0.4 7798 3351
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After: 68.8 65.4 0.5 6508 3258
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The resulting mesh looks as good as the one generated by Cadmium.
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OpenCSG problems may be an OpenGL compatibility issue
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-----------------------------------------------------
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It could be that OpenCSG showing volumes to be removed just with a
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different color without actually removing them is caused by using
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OpenGL emulation in Mesa. This needs further investigation.
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Conclusion
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==========
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The model used in the first test contained constructs that were meant
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to help avoid ambiguous situations. It turned out that Cadmium did
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not need this "help" and removing the constructs substantially
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increased the rendering speed.
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OpenSCAD is the exact opposite: it produces clearly visible
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artefacts in ambiguous cases and rendering speed also benefits from
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manual resolution of these issues. The cost of doing so is an
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increase of the complexity of the model.
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Performance after the model changes (times are in seconds):
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real user system faces vertices
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OpenSCAD: 68.8 65.4 0.5 6508 3258
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Cadmium: 316.3 315.9 0.3 2942 1473
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The sources of the models used are in
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http://projects.qi-hardware.com/index.php/p/wernermisc/source/tree/master/cad/test2/
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The meshes can be downloaded from
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http://downloads.qi-hardware.com/people/werner/cad/test2/
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---------------------------------------------------------------------
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