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    Line length on circle scaled to ellipse is very wrong

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    • pbacotP Offline
      pbacot
      last edited by

      if you split the circles before the length of the two half curves remains the same when scaled regardless of sides...

      MacOSX MojaveSketchUp Pro v19 Twilight v2 Thea v3 PowerCADD

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      • bazB Offline
        baz
        last edited by

        @pbacot said:

        if you split the circles before the length of the two half curves remains the same when scaled regardless of sides...

        Sorry, not quite following you there.

        But to restate the question: If I draw a line at 100, Entity Info will tell me its 100.
        If I draw an arc and explode, I will get an accurate line length.
        However, if I scale a circle to make an ellipse, Entity Info gives me nonsense, and not even consistent nonsense. I just went back to my example model as shown at top and all the reported numbers are now different...

        It's to do with scaling, I reckon. It confuses SU's little brain.

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        • ely862meE Offline
          ely862me
          last edited by

          This might be a bug..

          Elisei (sketchupper)


          Before no life was done on Earth it was THE LIFE ITSELF...GOD
          Come and See EliseiDesign

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          • bazB Offline
            baz
            last edited by

            @ely862me said:

            This might be a bug..

            According to previous posters, it is a limitation of SU. But I can't see why a line shouldn't be a line, whether curved, stretched or otherwise abused.
            Shall I load the skp?

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            • G Offline
              Garry K
              last edited by

              Circle1 144s at 400 mm - scale by factor of 2
              Circle2 288s at 400 mm - scale by factor of 2

              entity information reports ~5026.5 mm for both - I don't know where this number comes from

              Then pushpull each ellipse into elliptical cylinder
              Select arc and explode
              entity information now reports ~3872.6 mm for Circle1
              entity information now reports ~3875.3 mm for Circle2

              Elliptical Circumference calculates out at 3875.38

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              • bazB Offline
                baz
                last edited by

                @garry k said:

                Circle1 144s at 400 mm - scale by factor of 2
                Circle2 288s at 400 mm - scale by factor of 2

                entity information reports ~5026.5 mm for both - I don't know where this number comes from

                Exactly! Or inexactly as it happens. Could you open that file again and see if the numbers have changed, they have on mine.

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                • ely862meE Offline
                  ely862me
                  last edited by

                  Once you get to copy them around you get all kinds of lengths.

                  Elisei (sketchupper)


                  Before no life was done on Earth it was THE LIFE ITSELF...GOD
                  Come and See EliseiDesign

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                  • pbacotP Offline
                    pbacot
                    last edited by

                    @baz said:

                    @pbacot said:

                    if you split the circles before the length of the two half curves remains the same when scaled regardless of sides...

                    Sorry, not quite following you there.

                    But to restate the question: If I draw a line at 100, Entity Info will tell me its 100.
                    If I draw an arc and explode, I will get an accurate line length.
                    However, if I scale a circle to make an ellipse, Entity Info gives me nonsense, and not even consistent nonsense. I just went back to my example model as shown at top and all the reported numbers are now different...

                    It's to do with scaling, I reckon. It confuses SU's little brain.

                    It doesn't make sense. Really CAD programs should have an ellipse tool.


                    Screen Shot 2016-02-09 at 8.35.22 AM.png

                    MacOSX MojaveSketchUp Pro v19 Twilight v2 Thea v3 PowerCADD

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                    • S Offline
                      slbaumgartner
                      last edited by

                      I did some probing via Ruby, and it seems there is a chain of bugs causing this. First, when you scale a circle to make it an ellipse, SketchUp should drop the metadata defining it as a circle and treat it as just a closed loop of segments (a Curve). It does not do so, it continues to carry the center point, radius, and normal vector as if the loop is still a circle, and the edges still say they are part of an ArcCurve. This evidently triggers a second bug in the method that calculates the circumference of the loop. In some cases it still calculates the mathematical 2pir as if the circle hasn't been scaled. In other cases it calculates a strange value that has no evident relationship to anything. In no case does it return the sum of the lengths of the edges!

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                      • Wo3DanW Offline
                        Wo3Dan
                        last edited by

                        Lengths of (part of) an ellips do not show rigtht. This has been mentioned before.
                        I hope you can see this thread. See posts around 17/4/2008 and on.
                        https://groups.google.com/forum/#!searchin/sketchup3d/ellips$2Barea%7Csort:relevance/sketchup3d/z70T5xdUm_I/g-ChcSEcL2YJ

                        *"Taff, Google SU team,

                        Very nice approach Taff, almost precise. Here again there is the
                        lack of a precise ''snap to" with these rotations. But it's the best
                        you can do!

                        But I noticed an (important) bug in the "Entity Info" and "Text"
                        tool output.
                        The curve length shown is only half of what it should be.

                        Before you exploded the 1/4 ellipse curve the Entity Info reports
                        141.371669' as the curve length (also through Text tool)
                        It should however be: 282.743338'

                        16 segments out of the exploded curve add up to: 281.923143'
                        just a bit shorter which is obvious.
                        The more segments the more precise Taff's approach will be.
                        Even better when there were a "rotate snap" option to a curve.

                        Regards,
                        Wo3Dan "*

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                        • bazB Offline
                          baz
                          last edited by

                          Pilou. The actual line length is 2155 +/- 5. measured in the real world.

                          This thread is starting to make my brain hurt...so, respected elders, is it a Bug or not?

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                          • pilouP Offline
                            pilou
                            last edited by

                            @unknownuser said:

                            Using Approximation 2, the perimeter would be 2118.265712279433

                            A nurbs program 400 * 900 ellipse gives 2118,2799332 length line perimeter

                            Sketchup gives 2118,276442 with circle 1000 segments

                            Frenchy Pilou
                            Is beautiful that please without concept!
                            My Little site :)

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                            • S Offline
                              slbaumgartner
                              last edited by

                              I think it is a bug. I looked up the formula for perimeter of an ellipse and compared that with what SketchUp's entity info returns. For a 10x20 ellipse, the result should be 96.88. SketchUp variously returned 62.83 (20*pi), 111.3, and 113.27.

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                              • ely862meE Offline
                                ely862me
                                last edited by

                                There was already a bug submitted for this, yes it is a bug. Also changing lengths when copying an ellipse is a bug. They are in the database and most likely sometime in the near future will be fixed.

                                Elisei (sketchupper)


                                Before no life was done on Earth it was THE LIFE ITSELF...GOD
                                Come and See EliseiDesign

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                                • M Offline
                                  mac1
                                  last edited by

                                  There is no closed form equation for perimeter of ellipse
                                  it is approximated by p=2PIsqrt[[[ a^2+b^2]/2]^1/2]
                                  The correct value p=4aE where E is the elliptic integral where k=1/a
                                  sqrt[a^2-b^2]

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                                  • bazB Offline
                                    baz
                                    last edited by

                                    @mac1 said:

                                    There is no closed form equation for perimeter of ellipse
                                    it is approximated by p=2PIsqrt[[[ a^2+b^2]/2]^1/2]
                                    The correct value p=4aE where E is the elliptic integral where k=1/a
                                    sqrt[a^2-b^2]

                                    Yeah, what Mac said...

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                                    • S Offline
                                      slbaumgartner
                                      last edited by

                                      Mac is right, though there are several other approximate formulas that are better than the one he gave. Also, there are power-series that can be run to enough terms to get arbitrarily close. The point here is that even the sum of the lengths of the edges in the oval comes much closer than the strange values that SketchUp sometimes provides.i

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                                      • C Offline
                                        CAUL
                                        last edited by

                                        The best estimation of the circumference I've found is this:

                                        
                                          def self.circumference(major, minor)
                                            a = major / 2.0
                                            b = minor / 2.0 
                                            d = b / a
                                            
                                            x0 = d**0.5
                                            x1 = d
                                            x2 = d**1.5
                                            x3 = d**2
                                            x4 = d**2.5
                                            
                                            s0 = 3929 * x0 + 1639157 * x1 + 19407215 * x2 + 24302653 * x3 + 12892432 * x4
                                            s1 = 86251 + 1924742 * x0 + 6612384 * x1 + 7291509 * x2 + 6436977 * x3 + 3158719 * x4
                                            c = 4 * a + b * (s0 / s1)
                                            return c
                                          end
                                        
                                        

                                        It's extremely accurate, unfortunately I don't know where I found it so I can't give credits.

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                                        • pilouP Offline
                                          pilou
                                          last edited by

                                          And what is its result in this case ? (Ellipse 400*900)

                                          Frenchy Pilou
                                          Is beautiful that please without concept!
                                          My Little site :)

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                                          • C Offline
                                            CAUL
                                            last edited by

                                            @pilou said:

                                            And what is its result in this case ? (Ellipse 400*900)

                                            2118.2799288074484

                                            The result above is for major = 900, minor = 400, if your input refers to the semi axes multiply the result by 2.

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