# Why is a simple curved lofted surface flat not curved over a large span?

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I'm trying to figure out why when I loft a nurbs surface with the railings both being identical nurbs curves ( which were created by converting a 2d arc into a nurbs curve ) and then connecting the the ends of the curves with a straight nurbs curve, the finished loft surface is more like two flat planes not a soft curve? I even tried adding more degrees which adds more vertices to the curve but that doesn't really change the result either. In wireframe view the curves look correct but when I view in opengl or final renderworks all I end up seeing is very flat gable roof not a curve. If I lay the nurbs surface over a extruded polyline having the same shape, they only intersect in the middle and the ends. The is with an arc having a length of 50' and radius of 220'.

Any thoughts on why this is happening? I this a bug to 2015 or does large lofted nurb surfaces become simplified the larger they get?

Can you post pictures of what you are talking about? Or even better post a VW file with the original are and the NURBS curves so we can look at what you actually have?

Pat,

Here are some quick screen shots:

Here is a 2013.vx file with an example of the shapes I was lofting compare to an extruded polyline.

I discovered this problem trying to attach a surface over some trusses using the same curve as the top of the trusses and after I would loft the nurds surface the truss struck up thought the created surface.

I can see a few things I would watch for -

- for this type of operation make sure to turn up the quality level of OpenGL using View>Rendering>OpenGL Options just to make sure you're really seeing what's going on.

- remember that because your roof surface is sloping from side to side wedge shapes portion of the trusses will always show unless you shape the tops of them. I would shell the roof surface and solid subtract it from the tops of the trusses to do that.

Edit: I get the same result, a slightly facetted surfaces, by lofting in the loft tool's first mode using the two edge curves, and by lofting in the third mode using the two edge curves as rails and the front curve as the profile.

I think there's an anomaly with the VW NURBS engine. Your curve is clearly based on an arc and technically an arc is a degree 2 curve yet VW converts it to a degree 3 curve. When I recreate your scenario in Rhino it looks nice and smooth. When I import it back into VW it exhibits facetting even worse than your example. Maybe Jim can look into this for you.....

Kevin

Edited by Kevin McAllister

Kevin,

Thanks for reminding me to check my OpenGL settings, they were set to low. You forget that the setting isn't saved from file to file unless you are using a saved template file. Increasing them and checking anti-aliasing helps but the nurbs surface still only shows as a curve in hidden line render.

Yes, you are correct about the top of the trusses being flat compared to the sloped roof. But if you look at the file from the right or left view in wire frame you will see that the roof surface is above the flat top of the trusses and they still show thru! I need to raise the roof surface nearly 2.75" above the trusses for it to not be visible in OpenGL? That seems like a very large gap, considering if the track that the roofing material slides into is 6", that a very large error to have in the file just to render correctly. Right now I'm in the rough layout phase, I should be able to look at the roof surface just touching the top of the trusses and then move it up 6" and know that later when I add the channels I don't have the same visualization problem all over again.

I guess I should submit the nurbs anomaly as a "bug" to be looked at.

Dennis

Kevin, I hope you don't mind me copying your footer, I thought it would provide some of the background operating parameters if that matters or helps.

___________________________

VW 2015 Designer w/Renderworks SP4 | Since Minicad 6 | Mac OS 10.10 | Retina Display Macbook Pro 8GB Ram |

Kevin,

After taking another look and adjusting the file, I believe I didn't have the roof set correctly. I was using the upper flat edge of the roof as viewed in wire frame and aligning it to the top of the trusses not the part of the roof that sits directly over the trusses, so that where the spacing error came in. Sometimes I think one begins to lose perspective on distances when working with large shapes.

Working with curved shapes and angles in 3d space certainly takes more effort than flat shapes.

Dennis

___________________________

VW 2015 Designer w/Renderworks SP4 | Since Minicad 6 | Mac OS 10.10 | Retina Display Macbook Pro 8GB Ram |

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