I have set up a Linux build environment for OpenSCAD, and can successfully
build out of the box.

I'm now looking at unplugging CGAL, in order to try different backend.

I see the code uses "#ifdef ENABLE_CGAL" in several places. I naively tried
commenting "CONFIG += cgal" in openscad.pro, but this gives build errors.
Inspection shows files like GeometryEvaluator.cc refer to CGAL without any
guards.

Perhaps this is a build option that used to work and has decayed, or maybe I
am doing something wrong, but I couldn't find any mention in build guides or
google searches.

Any tips?

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I decided that GeometryEvaluator.cc is CGAL specific and should be in the
cgal {} group. I created a stub version to act as a placeholder for CSG API.

There are several places where CGAL dependency has an impact, notably
mainwin.cc and the OpenCSG preview. The preview requires CGAL to evaluate
some geometry OpenCSG can't handle.

After a bit of hacking, I got it to build and run, the GUI doesn't do
anything but the command line runs ok and can generate a CSG file.

There are also some miscellaneous dependencies, e.g. OFF import. I think
there is no problem using parts of CGAL where useful, and replacing the CSG
part, but the separation between openscad and CGAL is less clean than I had
hoped.

I'm not sure how preview/render modes are best done with alternative CSG
backends, it would be nice if the OpenCSG preview was self-contained.
Ideally I think the CSG engine and GUI would be independent.

For the moment I will continue with my hacked version, but longer term some
refactoring to create a cleaner interface will be needed. This could be done
independently of developing alternative prototype CSG implementations, if
anyone is interested? :)

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I had a quick look at pyOCC - Python wrapper for OCE. The quick example I
tried worked quickly and simply, might be a useful interface for test
purposes.

However, I am going to look at Carve first, this library has been in Blender
for a while and seems to have got positive comments from users. Carve has a
GPL2 or GPL3 license, so should be OK.

I have most of the OpenSCAD gui working without CGAL, except for the preview
mode which I have been able to get working. CGAL has hooks in quite a few
functions, it will take a little while to tease out the dependencies.

I will try to make some notes as I go along, which may serve as a "How to"
guide for replacing CGAL.

I've realised that CGAL has some unique features and quirks, which may be
difficult or impractical to replicate with a different CSG engine, which may
mean a hard choice whether to support multiple backends, or drop certain
features that users currently depend on.

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It's been a while, but I have been slowly working away at this. There is a
quite a lot of stuff to pick through. I've now got to the point of trying do
a union with carve. This seems to work ok for cubes, but not spheres :( I
get an assertion error in the carve lib, not really sure what it means.

There is quite a bit of stuff I have taken a best guess at, probably went
wrong somewhere. Latest effort :
https://github.com/bobc/openscad/tree/carve_csg

http://forum.openscad.org/file/n13249/screen1.png
http://forum.openscad.org/file/n13249/screen2.png

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Good news, I sorted out previous problems with strange results. Basically I
was passing a bad mesh to carve...

Also some slightly exciting news, on this particular benchmark carve is over
10 times faster rendering than 2015.03, 4 seconds vs 48 seconds.

http://forum.openscad.org/file/n13262/screen3.png

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