Part 2 - Creating Revit Swept Blends along sinuous multi-segment paths

In my previous blog post I described some of my attempts to create a swept blend along a sinuous multi-segment path.

What I ended up with last time was a form created by selecting a series of profiles hosted onto the nodes of a divided path (without selecting the sweep path as part of the form).   In this example I had to indent the ends of the divided path so that the profiles were more or less linear and Revit could create the form.

 

Each of the profiles has an instance height parameter so that it could be easily adjusted to give an undulating form

My next task was to extend the form back the full length of the Divided Path.  I used a similar technique in a previous post where I had hosted points on a spline.  In this case it seemed easier because I could adjust the Divided Path properties by changing its 'Beginning and End Indent' rather than selecting individual points.

Going for broke and setting it right back to zero certainly did not work - it was unable to create the form.  Technically I was only trying to modify the form - so it would be interesting to know if the warning message is telling the truth:  is it actually recreating?  or modifying, in which case the dialog box wording is wrong.

A less dramatic change to the beginning indent from 3700mm to 3000mm was still too much for Revit.

Finally I had to settle for a mere 50mm change to the beginning and 400mm at the end - not enough to make a significant difference

I tried recreating the form with more profiles, spaced more closely - but that did not work either, when I tried reducing the indents

So I came to the conclusion that Revit is just not capable of creating a sinuous swept blend form like this if you do not include the path in the form creation.

Spline Logic

So that took me back to an idea that I had rejected earlier on:  tracing a spline over the chain of arcs/lines in order to achieve a single element path for the swept blend.  And, I needed the spline to follow the arc chain as closely as possible, and preferably to adjust when the underlying arcs are changed.

• Just hovering the cursor over the spline tool was enough to tell me that it would be hopeless, as you cannot snap to the arcs - it would all be guesswork and lots of adjustment later on

 

• The 'Spline by Points' tool is more promising as the spline goes through specific points exactly

• This tool works in two different ways - you can just start drawing the spline and it places points as you click.  But I made the mistake of trying the second method which involves placing points first then selecting them so the command links them into a spline.

• Revit did not create the spline in the order that I placed the points, nor in the order I picked them - it was seemingly random.  Perhaps this explains why it could not create forms earlier, out of my series of profiles - if it uses the same strange logic to decide the order

• With this technique, you could re-host the points into the right order by selecting each point, and using 'Pick New Host' to put it where it should be - a very laborious process.

 

• It is much better to use the spline command by the first method of placing them on the arcs in the order you want - and it hosts the points on the arcs nicely.

• You need to place a reasonable number of points, especially in areas where the arcs switch direction or curve tightly - the reason for this is that the spline will not follow the arcs exactly, so more points will make it more accurate

•  You can slide points along the arc to improve the accuracy - providing you put enough points on each arc

• Once the spline is created, you could host profiles directly onto the spline points.  That would be fine if you just wanted a smooth transition between two profiles at start and end.  If you want an undulating shape, there are two reasons I would not host the profiles on the spline points:

1.  The orientation of the profiles will most likely be different to when they were hosted on divided path nodes.  In this example I got lucky and they were correct:

 

2.  You would need to place every profile.  It is much easier to create a divided path and host one profile onto a node, then repeat it.  In this example I was unlucky and the orientation was different to the original divided path!  This is typical of Revit - the mysteries of why a divided path on a series of arcs vs on a spline will produce nodes that have different xyz orientations is just that:  a mystery.  I will write another blog post on that subject some other time.

• For the sake of this example, let us assume it was the same orientation so we could just go ahead and 'Repeat' the profile, then dissolve ('Remove') the repeater.
• To create the form, you need to select all the profiles AND the spline (not the divided path)

• Now the length of the swept blend should be adjustable easily by changing the divided path indents.  
• The heights of each profile can be changed to give it an undulating shape
• The first and last profile can have very small width and height to taper the shape

• If you hide the profiles (by subcategory) you'll get a cleaner shape
• The profile could be made of a single half ellipse, as in in the next example - to eliminate the horizontal joint lines
• Finally you have a beautiful piece of undulating furniture.  Or a Loch Ness monster, depending on your project.

Anyone who has read this far will agree that this is a tortuous process (pun intended), riddled with workarounds.  The main purpose of this blog post is to demonstrate to the software developers how complicated it is to create some forms in Revit - how many roadblocks there are in the software; how many workarounds you have to learn; and how long it takes.  If anyone has learnt any tricks along the way, that is a bonus.

There are other programs around that could do this in minutes rather than hours (or days) - but they lack the precision of being able to document something that is buildable and follows a more geometric basis, such as a series of arcs rather than splines.  So I would much rather be able to do this in Revit to start with.

Related topics:

• Part 1 - Creating Sinuous Swept Blends
• Tightly Curved swept Blends
• Unable to Create Form Element
• Swept Blend using Adaptive profile
• Hollow Section Sweep with Adaptive Profiles
• Create Form using Repeater
• Got Your Repeaters in a Twist
• Repeaters Index page

• Traditional Revit vs Adaptive/Conceptual Massing Environment

Part 1 - Creating Sinuous Curve Swept Blends in Revit Adaptive/Massing Environment

Loch Ness monster created in Revit

In a recent blog post I was trying to create a swept blend (or loft) along a single spline path
and I encountered a few problems, which were mostly (but not all) solved by including the spline path as part of 'Create Form' process.  That was just a preliminary test for what I ultimately wanted to create:   A swept blend along a sinuous multi-segment path made up of arcs and lines - not a spline (as seen above).
Here follows the next instalment in my trials and tribulations.  I will start simply by comparing the traditional methods and conceptual massing creation of a sweep, before moving on to a swept blend.

Traditional Sweep

In the traditional family editor, Revit can easily create a sweep along a sinuous multi-segment path, unless the radius of a curve on the path is too small and causes impossible geometry.  This example below is a simple sweep of one profile along several linked arcs and lines.  It creates a pretty good looking form if the arcs/lines are at perfect tangents to each other (in both the path and profile) - it does not show any join lines.

Traditional family editor sweep

CME Sweep

Try creating it in the Adaptive/Conceptual Massing Environment (CME) and you don't get such a good result - and there is less likelihood of Revit actually creating the form at all.  The example below is created using the same geometry as the one above, with some provisos:

• The sweep path of perfect tangent lines and arcs had to be traced as reference lines with the pick line tool as you cannot copy sketch lines from the traditional family editor into the Adaptive/Massing environment (Grrrr - why not?).
• The profile had to be recreated as an adaptive component profile as you cannot use regular Revit profiles in CME.  Refer to Swept blend using profiles in Adaptive Components for how to create and host the profiles.

CME Sweep using one profile and reference lines/arcs as path

Notice the nasty join lines at every change of line/arc in the path;  likewise in the profile, which again has perfect tangent transitions.

CME Swept Blend

If you try something similar as a swept blend you run into roadblocks very quickly:

• Host another profile (similar or in this case same shape, lower height) at the other end of the path
• In this example the easiest way to do that is to place a point on the reference line/arc;  then host the adaptive profile onto the point

• Select the two profiles and the chain of lines
• Create Form
• Computer says 'No'.  Refer to Unable to Create Form for more variations on this dialog box

CME Swept blend on multiple arc/line path - failure

• If you try moving the host points further around the arcs on the path so that the path is less sinuous it still fails

• Try putting the host points and profiles on almost straight section but hosted on different lines - still no go

Another swept blend failure

•  It is not until you put the two profiles onto the same line or arc segment that it actually succeeds in creating a form (if you select only that segment and two profiles)

This technique is getting painful and is obviously not going to work - Revit cannot create a swept blend on multiple segment paths!  What a major limitation on a sophisticated 3D modelling program.

Another approach is required:

How to convert the arcs and lines into one path?  Well you could try tracing a spline over the top - but that has several problems:

• A spline is not easy to set out on a drawing for construction;  it cannot be dimensioned
• A spline will not follow the original path accurately - it will bulge out when there should be a smooth transition from arc to straight line
• The spline will not update if the reference line path is changed

Divide Path

Another option is to use an inbuilt Revit technique:  'Divide Path' - this creates a single element that sits on top of multiple lines/arcs selected in a chain.

The divided path has an added benefit of automatically generating nodes on which to host the profiles.  The next roadblock is that when you select the profiles and then the divided path, the 'Create Form' icon disappears - it cannot use a divided path as a sweep path.  Aaaaargh!

You could start placing profiles along the path, and try to link them as a form - by selecting them all and 'Create form' (without defining a path).  Sadly it does not work - it is unable to create form.  Even if it could, if they are too far apart it might take shortcuts between each profile (ie. it does not respect the order they are on the path).
If you select only some of the profiles, and create a form, you can see that it will not follow the path at all closely, especially if they are not close together.

 If you create separate forms for each pair (or trio) of profiles you get pretty awful results:

Even if it could get the order right it also cannot do all the profiles in one go as the curves are too tight for it.

So, is the answer to increase the number of nodes on the divided path and add more profiles?
It gets a bit tedious placing adaptive profiles, and sometimes they flip for no apparent reason.

So maybe we can use another Revit tool to place multiple profiles:  Repeaters

Repeaters

If you place just one profile on a divided path node, you can use the 'Repeat' command to array the profile onto every node (they still sometime flip!).
If you select all the profiles, Revit should have a better chance of more closely following the path as it joins them all, right?
The next problem is that you cannot create forms directly from a repeater - you have to first dissolve the repeater using the new (in v2015 r2) 'Remove Repeater' command.

This command was wrongly named in my opinion, as it dissolves the repeater back to individual profiles rather than removing it altogether.

If you select all the profiles, Revit still cannot create a form as the path is too sinuous for it, and it still gets the order of profiles wrong even though they are closer to each other

Unable to create Form

The next trick I tried was to push the profiles back along the divided path so that each was closer together, and did not try to go around such a sinuous curve.  This is simple enough on a Divided Path by changing its 'Beginning and End Indent' properties

 

 I then selected all the profiles and 'Create Form'.  Finally some success:

By hiding the profiles, it showed that the form does not have vertical joints - this is because we did not actually define the path by selecting arcs in the path.  that probably means the resultant form is not following exactly along the arcs, but it is close.  We still get joints running along the top because the profile is made up of arcs and lines.  That would not have happened in the traditional Revit family environment - Harrumph!

But this is still effectively a sweep, not a swept blend.  That part is easy to solve:  each profile has a height instance property (because I made them that way - it could be a type property).  So the heights of each one can be adjusted, and the form follows along.

 

 The profile heights could have been adjusted before creating the form

Once you hide the profiles, it is starting to look like the Loch Ness monster that I was trying to create

The next problem to solve is how to get the form to spread along the whole length of the path . . . .

That is an issue to be solved in the next post Part 2 - sadly it is not as simple as you might guess (if you have followed the process so far)