Beginner problems: How to use the pull command for locally fitting spline?

[dunderhead]

My questions tend to involve scenarios that are hard to explain, so there a five screen captures along with my text. But they didn’t paste into the composition window of the forum, so you’ll just find the text below. The text with pics are in the attached pdf file

Hi, I’m deeply intrigued by Fusion 360, but I’m stumbling a bit the sculpt workspace. Could you help me with my baby steps?

I’m working on a head for fun. It currently looks like a platypus (how that happened is another post J) but here I’m focused on creating an eyeball. In particular, I’d like to use a solid model, such as a sphere, to trim the grid I’ve already put in place (FIG 1):

With timeline magic, I put the construction of the sphere, the fitter or trim object, before the sculpting of the spline. So the first three actions below, the creation and the two moves, were done originally with the face in place, after the spline creation, so that I could align the sphere (FIG 2):

Problem 1: disappearing and unselectable body element in browser, but forced display

You’d notice that I put in a component creation action in the timeline just before I create the spline. That’s because the sphere object otherwise is unselectable.  I would have expected that I could select it as shown (FIG 3)

The sphere doesn’t even show it the browser.But it is displayed and in fact prevents me from selecting the faces of the eye socket (unless I left-click-and-hold, which now explicitly also reveals that the sphere object is in fact still there). This disappearance act is somewhat baffling but once I componentize the sphere, then I won’t hide in sculpt mode.

Did I baby step my way into a bug or a feature here?

Problem 2: the pull command doesn’t behave

With the sphere in place, I’d like to pull the vertices of few faces in the eye socket to the sphere, so I prepare the pull commands as follows (FIG4):

The auto select option of the command uses I-don’t-know-which target to create a wacko cubist painting of sorts. So I select the sphere explicitly (that’s why componentization is necessary!). Then my head turns into this medussa (FIG5):

And that’s for trying to align the 9 spline vertices to the sphere. Can you explain what happens?

Problem 3:  How do I make a simple trim according to the sphere?

Related: can I simply cut out the intersection of the sphere with the selected surfaces of the spline? I’m not after an exact projection of the fitter object onto the surface, just something that’ll locally tessellate the involved spline surfaces so as to approximate the surface curve (which may itself not be spline representable).

PS: for other beginners, alt-1, alt-2, and alt-3 are your friends, but the object must be fully selected for some reason – these keystrokes switch 

[Oceanconcepts]

I’m having a little trouble following- but this looks to be a t-spline object. If I understand correctly that you want to subtract the sphere from a sculpt (t-spline) object, that isn’t possible unless you first convert the t-spline to a solid (you can still go back and edit the original t-spline, and edits will update the solid, so long as you are capturing design history). It seems as if you might be getting tripped up by the different workspace environments and the way they relate to the timeline. Entering the sculpt environment kind of forces you to “step outside” the timeline until you finish the feature, and will ghost the visibility of other types of objects. You would need to finish the feature (convert) before you can do any boolean operations.  It is also possible to work in non-history mode (right click on the base component and select “do not capture design history”). That can be easier when getting started with basic tools. 

Matching vertices to a sphere could also be done using the Edit Form tool- there are also options for forcing vertices to snap to a solid using the Object Snap option in the Edit Form tool.  You appear to have selected entire faces rather than vertices, that could give startling results with the Pull tool.  See Paul’s video here:  https://www.youtube.com/watch?v=_F4rg31UjBE

[dunderhead · Accepted solution]

Ron -- your reply was extremely helpful, especially the video link: How to use Object Snap Command, higly recommended, thx.

I should have made it clear that I'm not here interested in the Boolean operations after promoting splines to solids, rather in using solids for directly shaping splines and even cutting them. But you guessed that as well!

So, I learned since yesterday (I think, correct me if I'm wrong) that:

• object snapping, where control points are pulled, is computationally light and stable, that's why you can tick a box in the "modify form" command to use it
• but pull snapping, where surface points are pulled, is computationally involved and potentially explosively unstable; you probably MUST prepare the spline surface first manually by moving control points so that the complex optimization problem involved in surface pull has a solution for which control points are minimally moved
• when using surface snapping in particular the target surface must be simplied too in some cases; if you're using a solid, the algorithm may be confused about what face to use, or start walking around the target in ways that are non-intutive and that do not correspond to a notion of overall least-distance
• so these algorithms are probably not holistic, they grap points and edges in some more or less random order and shove things around; they are not based on minimizing some regional objective function or if they are, this function, may have chaotic solutions?
• that means that the proper laying out of a u-v grid on the surface to be shaped is essential before the pull command (and star-points do what here...hhhm???)
• the problem is so hard that no algorithm exists for automating this (?, research problem?)
• handling of object visibility and opacity is separate skill; it is linked to componentization

So here's my result shown rendered, not pretty (my smiley face turn into a platypus after I was presented with an interesting geometric puzzle)!  Part of the eyeballs now are rather exactly spherical, pulled to the solid sphere I constructed post-spline construction and shifted to the beginning of the timeline:

So if anybody else is interested here are my own answers to my questions:

Problem 1: disappearing and unselectable body element in browser, but forced display

Well, I still don't know whether this is a bug or feature, but Paul explains the solution in the video.

Problem 2: the pull command doesn’t behave

I followed the observations above (valid or not!). In particular, I had to cleave the sphere to get surface pull to work; with only the front half in place I was able to use the pull command after having meticously worked the grid with control point snapping. Control point snapping itself behaved better with the full sphere.

Ron suggested that "You appear to have selected entire faces rather than vertices, that could give startling results with the Pull tool." but initially selecting faces (with vertices and edges) was not the issue: the command only retains the 9 vertices.

Problem 3:  How do I make a simple trim according to the sphere?

The result of the properly prepared pull command gave the spline shown below. Now clearly there's a set of faces whose boundary approximate the intersection of the sphere. But there is no "simple trim" -- this is big mouthfull.

If anybody can point me to a ressource that explains T-splines at my level (≈ amoeba!) I'd be grateful!

[Oceanconcepts]

A couple of thoughts:

If you're coming with poly modeling experience, things are very different. In general, with T-Splines, you wan to use the absolute smallest number of control points that will define the form. More controls make things wonkier, not smoother. Trying to get to a complex and finely detailed shape using t-splines is not really the best use of the tool. 

If I were trying to put a spherical eyeball into a head, in the socket, I would either wait until I converted the form to a solid and add with a Boolean operation in the solid modeling space, or use a t-spline sphere that was a separate body, and combine after conversion. Boolean operations in Fusion are very powerful and flexible. and remember, you can still edit the base t-spline form and have the edits flow through. It's really the combination of t-splines and precise solid modeling that make Fusion such a remarkable tool, in my view. 

You may find that editing t-splines using the control points view makes things clearer. And box mode is your very good friend when it comes to complicated shapes. Editing so you don't have intersecting surfaces in box mode will make successful results far more likely. If the form doesn't make sense in box mode, something is wrong.

[dunderhead]

I'm impressed too with the overall concept behind Fusion 360 and I've made good use of it already. But to use a tool efficently you have too understand its limitations. Here T-splines pose a surprising challenge!

I'm an engineer with some CAD background, btw. The solid modelling part of Fusion 360 were much easier to handle, even with the lack of documentation.

The engineer within me has put me on crazed course to understand star-points, the cubic approximations, the creasing, etc, but it's pretty much banging the head against the wall.... For an illustration see my attempt below at refining the tesselation around a cut-out in a plane. "Repair body" here shows, but just in part, some of the structure of the mathematical object, but not the u-v grid and not all the t- or star- points I think. It also shows, in part, how little I've yet understood!

Perhaps "like working with soap bubbles" might capture the elegance and frustrations molding T-splines?

I chose the protruding part of an eyeball on purpose because an spline appromixation is sort-of simple (half a quadball, 12 faces, makes semi-sphere surface). So my experiment involved understanding how to extrude or molding an existing spline according to an existing shape that's representable with few control points.  

Thanks for input on this, people seem to want to do this sort of thing all the time despite the warnings... This would be a good subject for an FAQ.