Does anyone use the Sketch Spline Tool?

todd_brous

Does anyone out there use the Sketch Spline Tool?  Basically, I think it is an unusable tool.  Its behavior appears unpredictable and genuinely unproductive.  Am I missing something?

For example, if one were to adjust a section of a curve, it will affect other areas.  This makes no sense to me.  (I think it may be time for Onshape to consider rebuilding the Spline Tool.)

Please note, that I was provided a workaround by Onshape tech support (6 months ago).  In a nutshell, rather than creating a "single spline", create multiple separate splines, and then add "tangent constraints" between the splines.  After considerable work, this can help build a desired curve shape, but this is really not a productive solution.  This is a workaround.

Also, if one needs to make any adjustments (creative or otherwise) one may find themselves wading through puddles of constraints that need to be managed/deleted/replaced.

Can we please have Adobe Illustrator-type Spline tools?

konstantin_shiriazdanov

todd_brous said:

In the below image I compare the behavior of moving a spline point in Illustrator vs Onshape.  I drew a similar "M" curve in both applications.  First, note where Onshape places the curve above and around the Spline Points.  Second, as the middle point is moved up and down, note how Onshape changes the curve shape and handle locations.  Illustrator's curve is stable and does not move or wiggle.

from my knowledge of the math behind the cubic splines - each point position should affect the whole spline shape and it is normal how spline behaves in OS when you move one of it's points. In illustrator you have more control under the tangency and the weight of intermediate spline points so internal spline regions are harder constrained than in OS and the tangency conditions don't affect adjacent regions since they are defined on the border points - in some situations it could be pretty unwanted since it may cause some overcomplicated and unnatural curve shapes.

billy2

@todd_brous I tend not to use the mid node control like you show. I've had it in the past, but don't really miss it.

On a crazy day I: add construction lines to the node handles, than add symmetry. You can also add dimensions to handles for magnitude control. 

I'd never do this for real. This curve doesn't interest me.



Now this one is one that I'd use:

The center node is horizontal making the curve symmetrical and allowing mid node placement definition. 


And of course, one curve mirrored in both directions:


I use this one a lot. Most of the time it's the 1st sketch in a design. FYI this works better than any other cad system in that each handle works equally. Other systems you have to figure out which are the primary handles meaning you'd have to pull on all of them to make it work. In onshape they all work nicely.



I believe that between the nodes is called a segment and it's evaluated from 0 to 1. Having a 3 noded spline causes a blending matrix to evaluate what's going on between segments and computing the coefficients. Possibly if you could assign curvature continuity between segments, you could contain movement between 2 segments. I'm not a mathematician, possibly @konstantin_shiriazdanov can explain the blending matrix between multi-noded splines. I avoid them, too much math creating complex surface patches.


Honestly, I'm just trying to get it to look right. 


Try to use construction lines & constrain handles making 2 noded splines. This is my typical workflow.

philip_thomas

Todd - as a Pro user, you can submit enhancement requests like this directly to Onshape through the 'Contact Support' option on the help pulldown. Please add a few notes about the specific capabilities that you are looking for. Note also that we have added several new curve types over the past year and if there are specific cases you would help with, either tech support can help you, or a public link here will get you multiple solutions from the community at large.
Have fun!

todd_brous

Thanks, Philip!

billy2

I believe Onshape uses a b-spline as most CAD systems do.

It's a basic spline which is a 3rd order polynomial.



The top is one spline segment which is what I use most of the time.

The bottom is a mirrored spline which I use often.



I'm interested in what more do you need/want?

todd_brous

Bill-

Thank you for your reply!  

First off, using a mirrored spline is a super brilliant idea!

I think the problem that I am experiencing is with a longer spline curve that contains multiple points and segments.  In Onshape, moving one side of the spline affects the other side.  Does that make sense?  

Take a look at how Adobe Illustrator deals with splines.  To me, that feels like a better behavior.

(As an aside, Onshape's auto-constraints can sometimes get in the way while "designing" a spline curve.  If you are just working out a design shape, and you accidentally place a spline point on or near an auto-constraint location, then the whole spline behavior changes until you can find and delete that constraint.  Sometimes, I think it would be nice to have an "ignore all constraints" mode while drawing in a sketch. Then one could add any needed constraints afterwards.  I, for one, do not always have my "design intent" well defined before I start working.)  

In the below image I compare the behavior of moving a spline point in Illustrator vs Onshape.  I drew a similar "M" curve in both applications.  First, note where Onshape places the curve above and around the Spline Points.  Second, as the middle point is moved up and down, note how Onshape changes the curve shape and handle locations.  Illustrator's curve is stable and does not move or wiggle.


To my knowledge, the way to avoid this in Onshape is to make splines with only 2 or 3 points each.  If you need a longer curve, then one will need to join and constrain multiple curves together.  I feel like this is an inefficient method, and I think it could be improved. 

Does any of this make sense?  Note, I am quite possibly doing everything wrong in the universe.  This is just my experience.

Thanks again for your reply!

Best,
-Todd

konstantin_shiriazdanov

todd_brous said:

In the below image I compare the behavior of moving a spline point in Illustrator vs Onshape.  I drew a similar "M" curve in both applications.  First, note where Onshape places the curve above and around the Spline Points.  Second, as the middle point is moved up and down, note how Onshape changes the curve shape and handle locations.  Illustrator's curve is stable and does not move or wiggle.

from my knowledge of the math behind the cubic splines - each point position should affect the whole spline shape and it is normal how spline behaves in OS when you move one of it's points. In illustrator you have more control under the tangency and the weight of intermediate spline points so internal spline regions are harder constrained than in OS and the tangency conditions don't affect adjacent regions since they are defined on the border points - in some situations it could be pretty unwanted since it may cause some overcomplicated and unnatural curve shapes.

billy2

@todd_brous I tend not to use the mid node control like you show. I've had it in the past, but don't really miss it.

On a crazy day I: add construction lines to the node handles, than add symmetry. You can also add dimensions to handles for magnitude control. 

I'd never do this for real. This curve doesn't interest me.



Now this one is one that I'd use:

The center node is horizontal making the curve symmetrical and allowing mid node placement definition. 


And of course, one curve mirrored in both directions:


I use this one a lot. Most of the time it's the 1st sketch in a design. FYI this works better than any other cad system in that each handle works equally. Other systems you have to figure out which are the primary handles meaning you'd have to pull on all of them to make it work. In onshape they all work nicely.



I believe that between the nodes is called a segment and it's evaluated from 0 to 1. Having a 3 noded spline causes a blending matrix to evaluate what's going on between segments and computing the coefficients. Possibly if you could assign curvature continuity between segments, you could contain movement between 2 segments. I'm not a mathematician, possibly @konstantin_shiriazdanov can explain the blending matrix between multi-noded splines. I avoid them, too much math creating complex surface patches.


Honestly, I'm just trying to get it to look right. 


Try to use construction lines & constrain handles making 2 noded splines. This is my typical workflow.

todd_brous

Billy-

Wow!  Thank you for this insight.  This demonstrates using Onshape in ways I never considered.  That is incredibly helpful.  In short, it appears that you are building a "rig" to adjust the design of the curve.  I will need to explore how this can be applied to my projects.  Regardless, this is brilliant.

Personally, I still think I would prefer Adobe Illustrator's behavior, but you have explained and demonstrated how Onshape's curves work.  I can hope that some day the developers will take a look at expanding the Spline functionality, but in the mean time you have helped tremendously.

My take away: 

• Use multiple 2-point curves and connect them together.
• Constraints are friends and enemies. 
• Construction Guide Lines can be used to shape the curve.

THANK YOU!

Best,
-Todd
  

michael_mcclain

@billy2 thank you for showing your workflow. There are definitely some points to take away from this discussion.

@todd_brous Hold "shift" to ignore auto constraints in sketches.

todd_brous

@michael_mcclain OMG! omg! omg... 
Thank you!  You have just changed my life.

michael_mcclain

@todd_brous check the keyboard shortcuts menu. There's a lot of good stuff in there to help make your workflow more efficient.

owen_sparks

Add in some variables / configurations / relationships that drive the construction geometry and you're controlling your part without ever editing the sketch.

O.S.