Two outer walls, plus two inner walls for the tube. - Revar > On Jun 13, 2019, at 11:41 PM, Troberg <troberg.anders@gmail.com> wrote: > > RevarBat wrote >> A line could possibly pass through a maximum of four walls of a tube, so >> convexity should be 4. > > I can't see how a straight line could pass through more than two walls. How > would it pass through four??? > > > > -- > Sent from: http://forum.openscad.org/ > > _______________________________________________ > OpenSCAD mailing list > Discuss@lists.openscad.org > http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org

         translate([0, 0, 20])
             cube(20, center = true);

     }

This is what OpenCSG says: "The *convexity* of a primitive is the maximum number of front (or back) faces of the primitive at a single position. For example, the convexity of a sphere is one and the convexity of a torus is two. " So it it seems 2 is the correct value for a tube and the OpenSCAD manual is misleading. On Fri, 14 Jun 2019 at 08:04, nop head <nop.head@gmail.com> wrote: > However if that was the case why doesn't it default to 2, because for a > convex shape 2 in the minimum. And why isn't it called concavity as it is > concave shapes that go wrong? > > On Fri, 14 Jun 2019 at 08:00, nop head <nop.head@gmail.com> wrote: > >> This is a distilled version of the code I was having a problem with: >> >> for(c = [1:4]) >> translate([c * 40, 0]) >> difference () { >> linear_extrude(height = 50, center = true, convexity = c) >> difference() { >> circle(5); >> circle(4); >> } >> >> translate([0, 0, 20]) >> cube(20, center = true); >> >> } >> >> This is how it looks on my old laptop: >> >> >> [image: image.png] >> >> And this is how it looks on my normal desktop machine, which also has >> Intel graphics but it never gives me problems. Goldfeather makes no >> difference. >> >> [image: image.png] >> >> It seems 2 is sufficient on this machine but the old machine needs 4. >> >> The manual says "Integer number of "inward" curves, ie. expected path >> crossings of an arbitrary line through the polygon." in one place and "The >> convexity parameter specifies the maximum number of front sides (back >> sides) a ray intersecting the object might penetrate" in another. >> >> Neither statement is clear to me. What does " front sides (back sides)" >> mean? >> >> If it is simply the maximum number of edges a line can pass through then >> I agree it should be 4 for a tube. >> >> On Thu, 13 Jun 2019 at 22:56, adrianv <avm4@cornell.edu> wrote: >> >>> I hadn't really paid attention to convexity and hadn't realized that >>> there >>> was a solution to my models having sections that disappear as I rotate >>> them >>> in preview. I did a test run and found that setting convexity to 10 had >>> no >>> effect on my run time and it increased the geometry cache by about 10%. >>> Basically, no obvious penalty. >>> >>> >>> >>> >>> -- >>> Sent from: http://forum.openscad.org/ >>> >>> _______________________________________________ >>> OpenSCAD mailing list >>> Discuss@lists.openscad.org >>> http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org >>> >>

         translate([0, 0, 20])
             cube(20, center = true);

     }

I found on my dodgy machine having convexity higher goes wrong again. Here is it swept from 1 to 10 [image: image.png] 4, 5, 8 and 9 work. The actual tube is correct for 2 onward, as it should be, but the back faces of the subtrahend get displayed for some values of convexity. That never happens on my good machine. On Fri, 14 Jun 2019 at 08:08, nop head <nop.head@gmail.com> wrote: > This is what OpenCSG says: > > "The *convexity* of a primitive is the maximum number of front (or back) > faces of the primitive at a single position. For example, the convexity of > a sphere is one and the convexity of a torus is two. " > > So it it seems 2 is the correct value for a tube and the OpenSCAD manual > is misleading. > > On Fri, 14 Jun 2019 at 08:04, nop head <nop.head@gmail.com> wrote: > >> However if that was the case why doesn't it default to 2, because for a >> convex shape 2 in the minimum. And why isn't it called concavity as it is >> concave shapes that go wrong? >> >> On Fri, 14 Jun 2019 at 08:00, nop head <nop.head@gmail.com> wrote: >> >>> This is a distilled version of the code I was having a problem with: >>> >>> for(c = [1:4]) >>> translate([c * 40, 0]) >>> difference () { >>> linear_extrude(height = 50, center = true, convexity = c) >>> difference() { >>> circle(5); >>> circle(4); >>> } >>> >>> translate([0, 0, 20]) >>> cube(20, center = true); >>> >>> } >>> >>> This is how it looks on my old laptop: >>> >>> >>> [image: image.png] >>> >>> And this is how it looks on my normal desktop machine, which also has >>> Intel graphics but it never gives me problems. Goldfeather makes no >>> difference. >>> >>> [image: image.png] >>> >>> It seems 2 is sufficient on this machine but the old machine needs 4. >>> >>> The manual says "Integer number of "inward" curves, ie. expected path >>> crossings of an arbitrary line through the polygon." in one place and "The >>> convexity parameter specifies the maximum number of front sides (back >>> sides) a ray intersecting the object might penetrate" in another. >>> >>> Neither statement is clear to me. What does " front sides (back sides)" >>> mean? >>> >>> If it is simply the maximum number of edges a line can pass through then >>> I agree it should be 4 for a tube. >>> >>> On Thu, 13 Jun 2019 at 22:56, adrianv <avm4@cornell.edu> wrote: >>> >>>> I hadn't really paid attention to convexity and hadn't realized that >>>> there >>>> was a solution to my models having sections that disappear as I rotate >>>> them >>>> in preview. I did a test run and found that setting convexity to 10 >>>> had no >>>> effect on my run time and it increased the geometry cache by about 10%. >>>> Basically, no obvious penalty. >>>> >>>> >>>> >>>> >>>> -- >>>> Sent from: http://forum.openscad.org/ >>>> >>>> _______________________________________________ >>>> OpenSCAD mailing list >>>> Discuss@lists.openscad.org >>>> http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org >>>> >>>