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