Existance of Hull() operation

[BarbourSmith]

Does Replicad have a function similar to the OpenSCAD hull() operation?

https://spolearninglab.com/curriculum/lessonPlans/hacking/resources/software/3d/openscad/openscad_hull.html

I find that I use it quite a bit and I'm wondering if Replicad has something similar, or if there is some way that I could implement similar behavior.

[sgenoud]

As far as I can tell, hull operations are more of a mesh / CSG operation than a BRep one. Problems solved by hulls in openscad should be easier to solve with a sweep or a loft in replicad (or any BRep based tool).

The hull creates a convex shape around a set of points (and join them with lines). In a BRep environment we tend to have more complex structures than straight lines and it makes the whole endeavour more complicated.

This is as much as I understand. I have not used openscad myself that much, and therefore don't know directly how hulls are used. But I am open to see if I can do something if it is useful in practice!

[lf94]

Stitching together a curved surface is probably the closest thing. iirc open cascade can do this

[BarbourSmith]

That makes a lot of sense that it's more of a mesh based thing.

Here's a basic use case that I'm not sure how to accomplish with loft.

Let's say I have two extruded circles like this:

But I want them to be melded like this:

How would I do that?

[lf94]

you would sketch using arcs and lines

[sgenoud]

This is a quick implementation of what it could look like!

[avibryant]

This is a good short article on use cases of hull in OpenSCAD: https://hackaday.com/2018/02/13/openscad-tieing-it-together-with-hull/

[sgenoud]

Ok, this is definitely an interesting API to expose somehow - the workflow of having a bunch of shapes and asking to wrap them feels very useful.

[sgenoud]

It looks like cad query has such a function for 2D, but it only supports lines and arcs. I might give it a look (and see if it could be extended to cubic bézier as well).

[lf94]

as shown , the implementation with sketching is like 1000x simpler than the hull.py... i dont see why to bother.

[sgenoud]

Have you looked at the hackaday post? It feels to me that, while not as necessary as in the case of openscad a helper like hull can make some tasks easier to implement - it removes some geometry computations that the programmer would need to do.

[sgenoud]

Note that build123d has a more straightforward (but approximative) approach

[lf94]

yes completely agree about the usage enabling less computations for a set of problems. i read and am familiar with hull. still clearly there are severe limitations to hull.py vs openscad's hull, which works on any arbitrary surface. we are entering brep limitations now. better to think of a diiferent workflow or operation to achieve similar developer performance.

[BarbourSmith]

For me the big selling point of hull over other options like using arcs and lines is that hull is easy to do with a flow interface, while arcs and lines work well with a graphical interface or a code interface.

[sgenoud]

It feels like there is also something to say about imperative vs declarative APIs. hull here feels like a declarative operation - you have a bunch of shape and you want to wrap them.

I guess that flow interfaces work better with declarative APIs (?). Similarly, do you miss a constraints API (a very declarative one)?

[BarbourSmith]

That's a great point about declarative vs imperative. The flow interface does seem like it works better to represent a declarative API. To some extent I think that's a bandwidth thing because the flow interface is somewhat limiting. I'm imagining that there will always be cases where the user will need to write Replicad code directly to accomplish certain things, but the more that they can do without needing to write code directly, the easier it is to pick up and get started.

[BarbourSmith]

What was the final verdict on this? I still feel like not having a hull option is really holding me back from making things the way that I'd like to

Does loft accept 3d shapes? That would do it for sure

[BarbourSmith]

Thank you! Let me know if I can help (like pay money or test something or anything that helps!)

[sgenoud]

Not sure how I want to include this in replicad itself, but I have implemented a hull like operation (that I called shrinkWrap, because the hull is for points).

I have it as loose code for now, but you should be able to play with it (and find some bugs).

[sgenoud]

Ok, I packaged it here https://github.com/sgenoud/replicad-shrink-wrap, you can easily add it to a project in the studio (or import it via npm otherwise):

import shrinkWrap from "https://cdn.jsdelivr.net/npm/replicad-shrink-wrap/dist/studio/replicad-shrink-wrap.js";
/** @typedef { typeof import("replicad") } replicadLib */
/** @type {replicadLib} */
const { drawRoundedRectangle, drawCircle, draw } = replicad;

export default function main() {
  const circle = drawCircle(7).translate(8, 18);
  const rect = drawRoundedRectangle(20, 14, 5);

  return [
    { shape: rect, color: "silver" },
    { shape: circle, color: "silver" },
    shrinkWrap(rect.fuse(circle), 50),
  ];
}

[BarbourSmith]

This looks AMAZING 😍!!!!

[BarbourSmith]

From some quick playing around it works great...also the updates to the workbench look phenomenal too!

Thank you!