Allowing Negative Volume Bodies

[Gregory son of Carl]

What do you think about allowing Part Design Bodies to have negative volumes?

The basic concept is that you can start a body with a subtractive feature (Pocket, Hole, Groove, etc.). A negative volume body would look and behave similarly to a normal 3D shape, however when you fuse it into another body, that volume is removed instead.

Negative volume bodies would give users some more flexibility in how features can be organized on the tree. Subtractive features could be compartmentalized in the same ways that additive features can.

For reference, negative volumes exist in CATIA. Interestingly, the official documentation uses them in the chapter about optimizing model performance. Here's a link if you are curious. Just scroll down the table of contents and select 'Optimizing Part Design Application' to open the chapter I'm talking about. They recommend putting features into separate bodies and then combining them into the final part. The reason that is faster is because the CATIA solver can tell which bodies are unchanged and skip them when updating.

CATIA Traditional Method

CATIA Recommended Method (Negative volume body: Body.2)


Would negative volume bodies be a good fit for FreeCAD? I'm curious to hear other opinions on the matter.

[adrianinsaval]

What's the advantage over modeling both as additives and then using a regular boolean cut? I like the concept too but it seems unnecessary

[Gregory son of Carl]

I'd say there is a marginal improvement to model clarity/readability. If you delete the cut boolean and keep the tool body, there is no evidence left about whether that body was intended as a cut tool or a fuse tool. With a negative body, that information is explicit even without the boolean feature.

[chrisb]

With a negative body, that information is explicit even without the boolean feature.

Not really, if you cut a negative body it would be like fusing the positive.

Besides, due to an error you can already start with a subtractive primitive, loft or sweep, but it still acts like an additive.

[chrisb]

With a negative body, that information is explicit even without the boolean feature.

Not really, if you cut a negative body it would be like fusing the positive.

Nevertheless, from a theoretic point of view this is rather intriguing. You could even go further and start a body with a hole and then add something around it.

Besides, due to an error you can already start with a subtractive primitive, loft or sweep, but it still acts like an additive.

[Gregory son of Carl]

Not really, if you cut a negative body it would be like fusing the positive.

True. I think a negative volume body still feels more intentional, although it will come down to whether or not the designer is applying good modeling practices. Using double negative booleans would not be recommended because its confusing to read.

due to an error you can already start with a subtractive primitive

Just call it a feature

[chrisb]

Just call it a feature

No, sorry, it then behaves like an additive feature.

[ulrich1a]

Would negative volume bodies be a good fit for FreeCAD?

This is already available in FreeCAD in the Part-Workbench. You can select a shape and revert it. It is available in the menu under Part/Reverse shapes.
A reverted shape has than a negative volume and behave different in boolean operations. You can distinguish such parts, if you switch the lightning from two sides to one side. Reverted shapes are than black.

Ulrich

[Gregory son of Carl]

This is already available in FreeCAD in the Part-Workbench

Radical! It's incredible how fully featured the Part workbench already is. I hope it become more compatible the Part Design workbench one day. I prefer to use the Part Design boolean tool because it is clear which one is the target body and which one is the tool body.