FitCrv vs rebuild curve nonuniform


#1

Can anyone help me understand the difference in these 2 commands? Thanks.

Chad


#2

Hi, Chad

  1. make a CV poligon (Control point) curve (select it and hit ‘F10’ key to see the number of CV points; should be equal to the amount of clicks you have made during the creation of the curve command), select it again hit F3 → then hit Details…
    · see the Degree of the curve in Info window
  2. make two copies of the curve (place them at a distance)
  3. ‘Fit’ the first copy leave the Degree number the same as the original (as is usually), just hit Enter
    · again hit F10 to show the CVs (a whole much more bunch of them, right?), then F3 → Details… check the degree (should be the same)
  4. Rebuild the second copy
    · now a window appears, which tells you in brackets what the CVs number is and what the degree is
    · in the boxes next on the right you can choose the Rebuilt CV number and the degree
    · even if you leave them to be equal correspondingly there wil bee a deviation in the rebuilt curve (shown as a number below in the same window)!

Fit command makes a perfect fit of the original curve (even if the same Degree is applied) with the price of much more CVs (and spans in the curve → the flow of the curvature is not smooth)
· to see Curvature graph, select the curves and go to Analyze → Curve → Curvature graph

You will know when you see the graphs. (for comparision select all the curves and hit F10 to see the CVs also)


#3

FitCrv tries to reduce the number of control points on a curve while staying within the given user tolerance. It doesn’t always succeed in reducing the number of points, there are some types of curves it works better on than others.

Rebuild (normal) will rebuild a curve with control points more or less uniformly distributed along the curve, but it does not guarantee the rebuilt curve will be within any given tolerance of the original (you have to try and check).

RebuildNonUniform is mostly designed for attempting to rebuild sets of curves (although it also works on just one); it has 2 or 4 additional control “points” that you can drag around while in the command to see if you get a better result… You can “request” a tolerance but it won’t necessarily achieve it, depending on the max number of points specified, if it can get to that tolerance, it tries to do so with the minimum number of points.

Unfortunately, rebuilding in Rhino is still somewhat limited, you can often get closer with fewer points using hand editing, but of course that’s time consuming.

–Mitch