Allow users to opt out of merging extrudes

[franknoirot]

Right now we coerce all negative Extrude operations off sketches on face to be a cut operation. Conversely, all positive Extrudes off faces are coerced within the engine to be unioned. I think this is limiting users and burying implicit modeling behavior in what should be explicit operations. There is currently no way to create an Extrude off of another surface and produce a second body.

I propose that we need a method keyword argument to allow users to perform the following types of extrude:

1. create: create a new body
2. add: add to the existing body
3. remove: remove from the existing body
4. natural: our current behavior, which performs an add with a positive distance and a cut with a negative

The critical part is that we leave no modeling operations implicit, that is, not written in the canonical KCL.

Current behavior example

The following KCL

sketch001 = startSketchOn(XY)
profile001 = startProfile(sketch001, at = [0, 0])
  |> angledLine(angle = 0, length = 138.07, tag = $rectangleSegmentA001)
  |> angledLine(angle = segAng(rectangleSegmentA001) + 90, length = 101.38)
  |> angledLine(angle = segAng(rectangleSegmentA001), length = -segLen(rectangleSegmentA001))
  |> line(endAbsolute = [profileStartX(%), profileStartY(%)])
  |> close()
extrude001 = extrude(profile001, length = 100)
sketch002 = startSketchOn(-YZ)
sketch003 = startSketchOn(extrude001, face = rectangleSegmentA001)
profile002 = circle(sketch003, center = [59.58, 51.89], radius = 21.43)
extrude002 = extrude(profile002, length = 40) 

// This will error, since the user is (unknowingly) merging two already-merged parts
part001 = union([extrude001, extrude002])

Results in 1 cut body. Nowhere in the KCL is a cut operation mentioned.

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Milestone for tracking: https://github.com/KittyCAD/modeling-app/milestone/16

[franknoirot]

I brought this up in today's cross-team meeting and we discussed ways of doing this without breaking existing models, and some good points came up:

1. This should not be done until CSG with bodies that have surface tangency is made not a problem within the engine, otherwise we're setting up users for frustration.
2. A method argument, which when not included does the current "natural" behavior of unioning positive sweeps and cutting negative ones, could take Add and Remove options if we supported enum argument values, or perhaps we could have those as standalone boolean keyword arguments?

[franknoirot]

@adamchalmers I converted this to a discussion so that we can thread the discussion. Once we feel good about the approach we can lock this discussion

[pierremtb]

IMO we had good insights on our current default behavior (merge on extrude on face and subtract on negative extrude on face), very alinged with typical CAD software behvior, just need to make sure we have a way to opt out as needed.

[franknoirot]

@Irev-Dev raised a good point today about a bit of trickiness with the artifact graph for bodies that are created with the method=create (name TBD) argument value. We need to ensure that selections and other behavior in the app still work, by ensuring that the dependency between the two bodies is still captured. I figure you and @jtran could reach a consensus here on the approach for that?

[pierremtb]

I believe @mlfarrell mentioned in the call today that we should log an issue engine side to expose the option to the api so we can call it

[franknoirot]

Mike just helped me understand something I didn't about Extrude being special among our Sweep-family operations over in https://github.com/KittyCAD/engine/issues/3578#issuecomment-3025796786:

For the purposes of this issue, only Extrude will have be made able to opt out of the auto-merge behavior, and in the future (maybe when the engine team feel good about CSG perf for it) we'll have to make the other Sweep-family operations able to opt into it.

Currently Sweeps, Revolves, and Lofts from sketches-on-face do not automatically merge with their parent body. They already have the "Create new body" behavior right now.

[franknoirot]

@benjamaan476's taken over the issue over there

[pierremtb]

Ooops, generalized too quick my bad. Thanks for checking @franknoirot!

[pierremtb]

This mini milestone closed with #7747

Only 'Merge' and 'New' are supported right now