Constraining splines in sketch mode
So here's a question that might border on heresy for some folks:
Why would you need to fully constrain and dimension a spline?
"You can also choose to only constrain the X and Y of the points but leave the handles unconstrained so you can adjust the curvature still."
This has caused be problems! I now understand that the handles require constraints, but in the past, simply constraining X-Y coords for points would turn the spline black, fooling me into thinking the spline was constrained. Silly me for thinking that!
So, add a line segment tangent to the end of the spline (or parallel to the end handle, same thing I think?), and adjust the distal line end point, and Shazzam! The spline goes out of shape. @$%$#!
I do see that if at least one handle has been adjusted, the spline turns blue again.
Suggestion to the UI team: ANY unconstrained spline should be blue!
Adding to the confusion: Sketches containing these pseudo-constrained splines get the red padlock ( https://forums.autodesk.com/t5/fusion-360-design-validate/what-does-a-red-quot-lock-quot-icon-mean-o... ).
When working on boat hulls, I may have a score of splines having a score of points each. That would be a lot of constraints to add.
@patmat2350 wrote:
When working on boat hulls, I may have a score of splines having a score of points each. That would be a lot of constraints to add.
It would indeed be, but that many fit points for a spline is also a surefire way to create splines with horrible curvature. Having as many splines as you have in the image you posted is equally a surefire way to create surfaces with bad curvature.
@patmat2350 wrote:
I agree it's an issue. But I'm recreating historic vessels here,
It is a mathematical issue. The NURBS math doesn't care how old the vessels are 😉
@patmat2350 wrote:
... but I often find that some points need tweaking within the rounded eighths to achieve decent fairness...
How do you judge fairness of a spline, or surface ? Are you using the tools in the inspect menu?
For example the curvature comb on splines of edges, or the Zebra stripes or IsoCurve analysis on surfaces?
@patmat2350 wrote:
...I'm afraid I don't know much about control point splines...
The old draftsmen and boat builders bent wooden sticks around pins to create smooth curves> These "wooden sticks" were the first splines. Then much later mathematical formulae were developed that resembled the behavior of such sticks. Those Bezier curves also were called Bezier splines. Fitpoint splines in Fusion 360 are 5-degree multi-span B-Splines. The more fitpoints you add to such a spline, the more complex that mathematical monster becomes.
While ultimate curvature quality is unlikely needed for projects such as these, I would really limit the number of fit points and number of splines and try to get the overall shape of the boat correct.
@patmat2350 wrote:
I wonder if they would do a better job in fairing the data?
No, because you can only manually control/move the CVs (control vertices.) They are simply easier to handle when creating gentle curves.
Other CAD software sometimes includes algorithms to fair splines for a best fit through data points. This does not exist in Fusion 360. If you want to go down that rabbit hole I would look at software specialized for boat builders. But I doubt that's really what is needed.
Math don't care... but I do!
And it's combs on splines. I confess, it's hard to appreciate the meaning of the comb magnitude... or to get the comb as smooth looking as I would like... but on, for example, this convex curve, I'm happy to get all the comb on one side with no curvature reversals.
@patmat2350 wrote:
Math don't care... but I do!
When you're using computer software math and algorithms have the last word, no matter how much you care.
Suit yourself.
"...but that many fit points for a spline is also a surefire way to create splines with horrible curvature. Having as many splines as you have in the image you posted is equally a surefire way to create surfaces with bad curvature."
Oh, I'm happy with my abundance of spline points...
That's a nice model. But unfortunately a rendering doesn't gives away to much information. Can you show an e.g. zebra analysis of the hull? As you can see, both bodies do look fine in the design view but not in the zebra analysis. And I only added 3 additional rail profiles.
And the bodies are nearly identical
@patmat2350 schrieb:Oh, I'm happy with my abundance of spline points...
Are all combs really that smooth?
Very rarely do you get such exact profiles for a boat.
You can't tell from the picture how perfect the hull is. Anyway, it looks very good, a nice job.
What kind of boat is this? It looks like some kind of lifeboat.
Can you post a link to it?
It's not a rough model- it's very true to the original.
I don't give a hoot about the analytical capabilities of some striped horse- the shape is correct, and smooth enough to appear fair in images. Maybe it could be smoother, who cares? If it's smooth but wrong because it has only three sections and two rails, it's of no use to me.
And I assure you, my boat's butt is near perfect- you're likely confused by the turn of the bilge versus the deadwood area.
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