BILT Speaker

BILT Speaker
RevitCat - Revit Consultant

Thursday, 27 September 2018

Creating Lofts in Revit Mass - CME Part 6

Here is part 6 of my series on  comparing the five traditional form creation tools with equivalent techniques in the Revit Conceptual Massing Environment.
Previously we analysed the creation of extrusion forms, Blends, Revolves, Sweeps and Swept Blends in the CME.  Hang on - part 6 of 5?  Oh that's right, there is no way to create 'Loft' forms in the traditional Revit environment:

Part 6:  Lofts

It is possible to create a 'Loft' in the Conceptual Massing Environment - it is a little different to a Swept Blend, and has its own rules (and problems).  A loft is a 3D form made by linking a series of planar profiles to create a surface(s), which can be flat, faceted, curved or double-curved (NURBS).  I consider a loft to have a minimum of 3 profiles, as 2 would just be a 'Blend'.  The profiles are linked along a notional path - the difference between this and a Swept Blend is that the latter uses a specific path element, and a loft does not.  Revit will figure out its own path (or not, as the case may be!).

As soon as the edges are not aligned and you have 3 or more profiles, you will get curved surfaces – Revit creates its own spline edges.

The greater the displacement between profiles, the more dramatic the curves

The profiles do not have to be parallel to each other, but as soon as they are at different angles it becomes less predictable as to whether Revit will create the form as you desired. In the example below, the profiles were hosted on reference planes, which can be rotated, but only before the ‘Create Form’ process

Revit may not create the form at all, and you might get this error message:

If the error mentions 'Self-intersecting geometry', it may be due to Revit getting the profile order wrong such that it tries to create a form that doubles back or crosses over itself - something that Revit does not like one little bit.  When you select the profiles in the first place, it makes no difference which order you select them - you have absolutely no control over the order that Revit uses in the Create Form function.  It is not clear how Revit decides on the order - it may be proximity to each other?  However, I don't think it is that simple.

The second part of the error message talks about the 'Reorder Profiles' button - this is not usually visibly apparent.  There is a blank space for it in the bottom left corner of the dialog box.  Wouldn't it be nice if Revit tried to Create Form again with a different profile order;  better still if it asked you to nominate the order!

About once in every few hundred times you get this dialog box, there will actually be a 'Reorder Profiles' button.  Be very careful about clicking on it.  The first time I tried that, Revit crashed out and closed without saving anything.  Thus it is wise to save all files before attempting this.

If you do click on the button, it won't help!  If it doesn't crash, it will just fail again.  It would seem that one of the programmers was aware of this form non-creation ability of Revit, and they tried to add a solution.  However, it seems like this functionality was removed from Revit before it was released to the unsuspecting public.  Sadly one instance of the many variations to the dialog box still has the button there to tantalise us.

If you would like Autodesk to fix this, please go to the Revit Ideas wishlist and vote for this enhancement request:

Control profile order during form creation

This description of Loft creation shows a very simple example - it can get quite complex.  There are many tips and tricks as to how to make Loft form creation easier and more predictable in Revit.  More to follow later . . . .

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