Class A surface from scanned lines/point cloud

[Berserk]

Hello,

I have a project where we have to create a surface model for a scanned lines of a clay model for an ATV.

The scan is on X, Y, and Z, every 10mm.

I created a point cloud out of this scanned lines and I am using "From point cloud" to generate some of the surfaces.

I would like to get some advise on how to go about modelling the surfaces. Best practices.

This is my first time doing this kind of job.

Also what is the relation of the degree of a surface and the number of patches. How does this affect the smoothness of a surface.

Would a reversal of curvature acceptable on a Class A surface?

Whenever I look at the surfaces I generated, I want to smoothen it out, but when I do, it deviates from the points.

What is the acceptable deviation of the surface from the point cloud?

Thanks.

Roark

UGNX 1.0.4.2/WinXP SP1/GMPDL1-c.1.2

 

[HDS]

You decide what an acceptable deviation is.

Lower order and less patches is ususaly smoother.

 

[rossly]

First time Class A, big learning curve no doubt about it. Just remember it takes time to get things to look good, its not rocket science.

XYZ scan every 10mm, very detailed scan, maybe too detailed. Here is some quick advice, you will need to just jumpin and figure out what works best for you and your situation.

Create splines over the scan lines, not every one. You need to reduce the amount of primary and cross lines. When receating the splines make sure the cross lines touch the primary, that can be tricky and keep a good looking surface. Don't be surprised if you create surface to get me curves to create more surfs. It is a very time consuming process. Make sure that curves from one to another are curvature continous, VERY important. You then can contrain the surfaces by curvature.

As for deviation, we can keep +/- 0.5mm from surface to scan that we receive from the studio.

Hope this helps.

 

[Azrael]

You need to divide the model in subsizes, identify main surfaces and blended surfaces

 

[Berserk]

Hello,

Thank's for the replies.

I have concentrated on the front right corner of the scan.
We actually have an STL file. I have cut sections every 10mm.

rossly,
How do I make sure that the curves are curvature continuous?
What I useally do is a comb analysis, and make sure there are no reversal in the comb (the comb is on one side of the curve only). Is that what you mean by curvature continuous?

I have tried creating the surface as close as possible to the point cloud, but when I do, there are some dips on the surfaces. I use deviation gauge and grid section analysis to analyze the surface and use X-form to move the poles around to make the surface smoother.

 

[Azrael]

when you do a comb analysis you should check for dips in the comb, a smooth comb indicates curvature continuous

 

[rossly]

To change curvature, use Edit Curve. Pick the curve you want to edit. The next dialog will give you options for edit, pick Change Curvature (4th pick). Pick the other curve and the original curve will have the curvature of the other.

As for the dips, you don't want dips so removing the dips are correct but just don't modify the "look" of the surface. It is cat and mouse game at that point.

 

[tinhombre]

A couple of suggestions (sorry that they are on later releases)

NX2 and beyond has Edit -> FreeForm feature -> Refit face where you can specify how far a refit surface (SMOOTHER) can deviate from the original (Also toggle on Preferences->Modeling->FreeForm tab -> Associative Edit Freeform

It only seems to work on certain types of surfaces, on some I've had to do an Extract Face, B-surface option first, then edit that surface.

NX3 has when creating a Join Curve, and advanced refit option where you can specify tolerance to smooth within (if your existing curves are single curves, perhaps you could add a small curve to one end of existing to take advantage

I've found both work nicely.