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Working with splines

steve_shubinsteve_shubin Member Posts: 1,061 ✭✭✭✭

The PDF below gives one method of working with the spline tool.

I’m not gonna say it’s the best method or the only way one should work with splines. But, it has come in handy from time to time

To use this method in Onshape, you will have to connect a number of splines, And wherever you want a smooth curve, you’ll have to make that connection tangent — Indicated by the teeter totter type of handle — see the GIF

I learned this method at a class I took in junior college around 1995. The instructor was accredited by Adobe for teaching Illustrator and Photoshop

One thing about this method is that it’s quite good for tracing over scanned images

And it can produce some very fluid and smooth — maybe even elegant French curve type of line art

But as with a lot of artsy type of things, it will boil down to the operators ability in the long run

Anyway, I hope it helps maybe at least some people

The PDF is a truncated version of some posts I made in another forum 8 years ago, under the handle How_goes_it. So by wild chance, if somebody recognizes this, well I’m not plagiarizing it. I’m the one that wrote those posts back then

There is a PDF directly below that you could download



  • michael3424michael3424 Member Posts: 673 ✭✭✭✭
    Nice - the PDF makes a good reference for future use long after I've forgotten the forum post.
  • steve_shubinsteve_shubin Member Posts: 1,061 ✭✭✭✭

    Here is some more about this —

    I’ve never played with curvature before because I’m primarily a mobile Onshape user. Unfortunately, we don’t have curvature on mobile at this time

    But, I decided to give it a try on my MacBook Pro.

    I found curvature to be quite compatible with the 3 RULES of working with bezier as outlined in the PDF

    1. Approximately 1/3
    2. Change of direction
    3. Appreciably tighter or looser

    Now I’ve only tried it for this first time. So who knows. Maybe this turns out to be a complete bust otherwise.

    But who knows. Maybe Adobe was onto something regarding how to work with Béziers

  • bruce_williamsbruce_williams Member, Developers Posts: 842 PRO
    good stuff!  thanks for expanding our knowledge.

    I would like @Evan_Reese to chime in here.  I am wondering how tangent as used here for spline differs from arcs in curvature results .  I am surprised by how smooth the transitions Steve has with minimal effort.
  • steve_shubinsteve_shubin Member Posts: 1,061 ✭✭✭✭

    I appreciate that Bruce — but thanks should probably go to Adobe because it was one of their accredited instructors that divulged the 3 rules to me. It’s the only thing I can remember about that class, but still makes me happy as a lark that I was able to hear that info

    Of those three rules, the 1/3 rule is used LOOSELY. But so far, it looks like a great starting point for dialing in curvature

    On the other hand, to start with one handle being something like 1/8th the distance and the other handle 7/8’s the distance — well I’m guessing that’s probably gonna be harder to work with right from the get-go as far as curvature goes

  • steve_shubinsteve_shubin Member Posts: 1,061 ✭✭✭✭
    edited August 2020

    I was aware of SHOW CURVATURE & CURVATURE VISUALIZATION. I’ve seen  screensaves of the combs and pictures or videos of the visualization in this forum.

    But as far as the curvature CONSTRAINT, it just didn’t come to mind until a few hours ago

    So I started playing with it

    For those interested — here’s my initial feelings about it.

    If you’re working with splines like I’ve been showing in this discussion, don’t use any curvature constraints for setting up curvature. It should only be used at the very end of your setting curvature up, in order to lock things in &/or to make micro adjustments

    To start with, when you employ curvature constraints up front, it’s like working with Jell-O. You move one handle, and things start moving all over the place. And you’re probably not gonna like the results

    Here’s the way I figure is a good way to engage curvature constraints. 

    First, keeping in mind the 1/3, change of direction and tighter/looser rules, draw your basic shape using consecutive splines. Apply dimensions and/or vertical and horizontal constraints to locate the pivot points along the spline. Adjust the handles to get things to look relatively smooth. In this first step you haven’t applied any tangent constraints.

    Now apply tangent constraints  (there is a reason for waiting to do this until after you’ve done the above)

    Next turn on SHOW CURVATURE so you could view your combs and get them to look the way you want.

    Once you’ve made relative minor adjustments to adjust your combs, then lock all the teeter totter handles down using dimension and angular constraints. You’re going to have to use construction lines to do this

    Then one at a time, delete one of the tangent constraints and replace it with a curvature constraint.

    Eventually you’ll get to a point where things go into an overconstrained mode.

    At this point, you’ll have to remove that particular curvature constraint you just applied. You’ll also have to remove one of the dimension or angular constraints at that same point. Then try reapplying the curvature constraint. If things go red again, then remove the other dimension or angular constraint at that same point, and reapply the curvature constraint, and it should work.

    Look at the picture below and notice the two DRIVEN dimensions. Those were the ones I had to remove before I could apply the curvature constraint without it going into an over-constrained condition 

    And note that when you are applying the curvature constraint, you’re not actually applying it to the pivot point, but you’re applying it to the two splines on each side of the pivot point

    This post does seem long.

    It makes you think that it’s gonna take a long time to do all this

    But in reality, once you get the hang of all this, it actually goes fairly fast, because when you’re setting the dimention and angular constraints for the handles, you’re not keying in any numbers. You’re just accepting what shows

  • steve_shubinsteve_shubin Member Posts: 1,061 ✭✭✭✭


    You start by getting close to the final shape (easy to do)

    1. connect splines
    2. add dimensions
    3. play with handles


    1. add tangents
    2. tune comb
    3. constrain handles
    4. micro adjust/fully define by curvature constraints
    • Everything is very well-controlled all along the way
    • No wild excursions in watching a spline get all wacky
    • But you got to follow the steps

  • michael3424michael3424 Member Posts: 673 ✭✭✭✭
    Thanks for very much for this thread, especially these last two posts of yours.

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