Why we build our surface models from scratch

Well, it’s time to dust off this blog and get it going again!  It’s been way too long.  Let’s just say life happens sometimes, but I’m glad to be back at this again.  So, to kick off the first post, I’m going to talk about something that we spend a lot of time talking to customers about when we build surfaces from laser scanned data.  We’re often asked, “Why do you use Rhino3D to do reverse engineering work, instead of one of the purpose made reverse engineering software platforms?” The short answer comes down to one word – quality. While these purpose made software platforms do quite a good job on mechanical types of assemblies, where they fall short is in the area of freeform surface modeling. Since the vast majority of the work we do falls under the category of freeform surface modeling (that is complex shapes that cannot easily be described with straight lines or arcs), to get a high quality model that we are proud to deliver to our customers, we build them “from scratch” using Rhino3D. Take for example this laser scanned aircraft nose:

NoseLaserScanned

This is acceptable laser scanned data, certainly good enough for the purposes of building a surface model off of. Another vendor took this data, and using purpose made reverse engineering software created this:

Nose_NotSSDC

What our customer needed was a watertight (gap free) exterior surface model of the macro shape, minus all the bits like antennas and other small details, so that CFD analysis could be run on it. What you see above is a collection of surfaces that are patched together to form this nose, except that they are not actually watertight. The alignment of the “isocurves” – those lines you see along the surface, does not make any sense in the context of the actual object. Areas like the transition from the nose to the windshield exhibit some really strange surface defects that give it a lumpy look, defects which are not in the laser scanned data. In short, this surface model was useless to the customer, because it could not be imported into his CFD software, did not accurately capture the essence of the shape, and was such a collection of small patch surfaces that making it watertight would have been extremely time consuming. Here is the surface model we created “from scratch” from the very same laser scanned data:

Nose_SSDC

The vast majority of the nose is one single surface, and the entire thing is watertight, so that it can be imported into the customer’s CFD package without any further work.  Using the laser scanned data and photos of the actual aircraft, we were able to give the windshield and windows the proper shape.  There are no strange lumpy defects, and the isocurves are aligned in a way that actually make sense.  In the overall scheme of things, an aircraft nose is really not a complicated shape.  It’s quite telling that a reverse engineering software package does such a poor job of making what is frankly not the most taxing of tasks. From what we’ve seen, this is the rule and not the exception when it comes to shapes like these with reverse engineering software.  Did it take longer to do this from scratch?  Probably, we can’t know for sure, since we didn’t make the first surface model.  But, we’re not exactly comparing apples to apples here, since the original surface model was not actually usable by the customer, so it’s like saying “we could do this faster if we deliver something that’s totally useless and causes you nothing but frustration.”  We take pride in our work and we like to make our customer’s lives easier, and so we take the time to do it right, and build our models from scratch.

Comments
14 Responses to “Why we build our surface models from scratch”
  1. Frank Jarratt says:

    Welcome back.

  2. Rex says:

    Hi Sky
    Good to see you back up on line, I have a lot of experience with high end scanning packages and I agree with you, Rhino, Rhino Reverse and T splines are fantastic your T spline Utube are awesome.
    cheers
    Rex

    • Thanks Rex! I’m going to be doing a bunch of posts on Autodesk Shape (formerly VSR Shape). Have you played with it? They do a very bad job of showing people that it’s an insanely powerful RE tool. Check it out if you haven’t, just realize that the power of the tool is not at all apparent at first! -Sky

  3. Welcome back Sky. Glad you’re back! Can you tell us what CFD package would be next in this workflow? Thanks.

  4. Great article and good to hear from you again Sky. I wonder how long it will be until Autodesk buys Rhino as they seem to have bought up just about everything else in the last few years.

  5. Sky, welcome back! Glad to see you’re OK and back at it. I always enjoy your posts.

    Question. On the first model there appears to be a fairly smooth transition between the body and the windshield. On the second its a sharp crease. It’s also showing also showing facets with 3, 4 and 5 sides. Does Rhino allow all of them or is it two models adjacent to each other?

    • Thanks Billy! The actual aircraft has a pretty sharp break between the nose and windshield, as you can see from the laser scanned data. As for it being “fairly smooth” – I assure you if you looked at it up close it looks more like oatmeal. As for surfaces with 3 or 5 sides – ALL NURBS surfaces are “quad” in that they must follow a rectangular layout. Yes, you can force a rectangle into a triangle, but that’s where bad things start to happen. If you look closely at the isocurves of the bad model, you’ll see they are actually still all quad, but they have more than four surfaces meeting at one point in some instances. The overall point here is that the surface “layout” (where the surfaces are broken up, and how the points are aligned) is completely non-sensical, and leads to unusable models. This is but one example of this type of problem – one that I got permission to show, but I can tell you I’ve seen variations on this over and over and over….

    • One more point to make – if the client wanted to have a blend between the nose and the windshield, adding it to our model would be easy. On the other model, the blends are sort of “baked in” to the surfaces themselves – there is no way to take that model and tweak the transition between the nose and windshield. That’s another issue we see with models like this – by making the models from so many surfaces, doing any sort of editing to the surface is either impractical or impossible.

  6. Rex La Rance says:

    Hi Sky
    I have been checking out SHAPE you’re are so right it is a very cool RE Tool I thought T Splines had some power full features.
    I would like to contact you out side of this forum by email is that ok!
    If so you have my email flick me a line.
    Cheers
    Rex

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