What's the most efficient way to fair lines from a table of offsets?

Discussion in 'Software' started by adt2, Mar 9, 2012.

  1. adt2
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    adt2 Senior Member

    I've got an old set of lines I purchased from the Mystic Seaport Museum. Included on one of the sheets is a table of offsets, presumably put there by the original designer (Beebe). I'm wondering, what is the best way to turn those offsets into a set of lines on the computer? Specifically, how would you fair them?

    I've entered the points defined by the TOO into Rhino, each station on its own layer. I'm guessing the thing to do is to fair the individual station curves and then fair a new sheer on the faired stations, but that's a wild-assed guess. Just wondering how you'd do it.
     
  2. DCockey
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    DCockey Senior Member

    "Most efficient way" depends in part on your knowledge and skills.

    There are several different approaches to "fairing" lines on the computer. One is the traditional method of fairing each curve (waterlines, buttocks, sections, sheer, rabbet/bearding line, and diagonals) individually and adjusting the curves so they intersect. First create each curve from the appropriate points using "interpolating" splines which go through the appropriate offsets. Don't assume that a set of individually fair sections (curves at the stations) will by themselves insure a fair surface. The other curves also all need to be fair and intersect. You'll need to go back and forth between the curves to get them to be both fair and intersect. Methodology is essentially the same as used when fairing the curves while lofting.

    Another approach is to create a surface based on the table of offsets and curves it implies, fair the surface, and then extract the curves and offsets/coordinates needed. First step though should be checking the offsets for large errors using the traditional approach above. Then create the surface using the revised offsets and/or initially faired curves. There are a number of different ways to create a surface from a table of offsets and/or set of curves. Which ones are appropriate depends on the shape of the hull, and also on your knowledge and skills. How to fair the surface depends in part on how the surface was created.
     
  3. DCockey
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    DCockey Senior Member

    It's critical when creating a set of lines and/or surface from a table of offsets to use curves and/or surfaces which go through the points from the offsets, and not use the points as "control points" for the curves and/or surface. In general control points do not lie on the corresponding curves and surfaces.
     
  4. adt2
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    adt2 Senior Member

    Thanks, David.

    You touch on exactly the problem I had (have). I input all the points, created curves through them, and faired them (the sections). I ended up with ten nice, fair sections. But when I put a surface through them, it looked awful.

    I don't really know enough Rhino to "individually adjust the curves so they intersect." I'm actually not that interested in reproducing the hull exactly; I'd like to get a nice, fair approximation of the hull described in the offsets, then use the faired surface to extract new sections. This isn't a historic reproduction.

    So maybe my next question ought to be, how does one fair an unfair surface? Is there a command similar to "Fair" (for curves) that applies to surfaces?
     
  5. adt2
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    adt2 Senior Member

    Yes, this is how mine are done. I draw all the points for a particular station, then draw a curve through the points. Then I typically copy the curve to a new layer, turn on "Analyze Curve" so I can visualize how bad it is, and run it through the "Curves>Curve Edit Tools>Fair" command a few times until it smooths out.

    Finally, I'll turn on both the "Original" layer and the "Faired" layer and compare the two to make sure fairing didn't do something really funky.
     
  6. DCockey
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    DCockey Senior Member

    When judging fairness of curves in Rhino use the curvature graph.

    For surfaces use the curvature analysis color shading, zebra stripe analysis, and surface reflection. The reflection of a "fluorescent tube" (single white stripe) can be very effective in revealing unfair areas.
     
  7. adt2
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    adt2 Senior Member

    Oddly (or at least interestingly), surface curvature analysis shows all lime green surfaces (which I thought meant smooth/fair, but clearly I was wrong). When I render the surface though, it looks like frozen ripples of water across the top of a pond.

    The surface was created using Sweep 2 Rails, with the sheer and the keel being the two rails, and the sections being the intermediate curves. Maybe I need to try again and skip several sections at a time.
     
  8. DCockey
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    DCockey Senior Member

    Curves can be adjusted by moving either control points or knots. Get the curves to intersect is an iterative process and can be tedious.

    Spend time going throught the appropriate sections of the Rhino tutorials and then experiment on your own. Nudge is very useful when moving points.

    Understand, and that generally simplifies the process.

    No, and the Fair command for curves is very easy to overuse.
     
  9. DCockey
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    DCockey Senior Member

    Learn to fair curves by moving control points and observing the corresponding changes in the curvature graph.
     
  10. DCockey
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    DCockey Senior Member

    You need to adjust the curvature range in Surface Curvature Analysis. Also look at the different curvatures.

    I generally use zebra stripe analysis and surface reflection more than surface curvature analysis when assessing fairness of surfaces.

    Check and adjust the mesh in Properties. The default "Jagged and faster" can cause a fair surface to appear unfair.

    Remove enough sections and you will obtain a "fair" surface. How close it will be to the desired surface is a different question.
     
  11. DCockey
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    DCockey Senior Member

    Effective use of Rhino to effectively and efficiently develop and fair arbitrary boat surfaces requires a investment of several days or more in learning and experimenting with the software. I expect this is also true of other 3D design software.
     
  12. adt2
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    adt2 Senior Member

    Agreed. I spend my days using AutoCAD and SolidWorks, and while they're great for many things, boat design ain't one of them. I was hoping Rhino would come a little more naturally to me, but so far, no such luck.
     
  13. jehardiman
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    jehardiman Senior Member

    Why do you need to put the lines into a computer?

    If you are going to build a hull, just loft them, it will be far less time than you will spend trying to get a fair set of lines from the computer.

    If you are not building a hull, then who cares if the lines are fair to the offsets, not anyone just looking at an CG image.
     
  14. adt2
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    adt2 Senior Member

    Not that this answers any of my questions, but I'll humor you. I have a large-format CNC machine that I'd like to utilize to cut frame patterns, among other things. They have to be computerized to go through the CNC machine, and patterns of unfair frames aren't much help.
     

  15. Submarine Tom

    Submarine Tom Previous Member

    How large is large?

    Humour me, really.

    -Tom
     
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