Adding Creases To Your Forms – Deep Dive

Avatar jakefowler September 16, 2014

9 min read


The September update featured a brand new underlying technology for adding creases to T-Splines models. Although this just sounds like an enhancement to an existing modelling feature, the changes we made were far more significant than that: this was the result of a development effort spanning many months to try to remove some of the biggest confusions and pain points for new (and experienced!) users of the Sculpt workspace, which extend beyond just the Crease tool. This post goes into some detail about what exactly these changes are, the problems we tried to solve, and some ideas of what these changes will hopefully allow you to now accomplish.



Converting old models


Probably the first thing to mention is that these changes required us to update the way T-Splines work, and your existing Sculpt models will need to be upgraded to take advantage of the changes. The next time you make edits to any Sculpt models created prior to the September update, you’ll see this dialog box pop up:



We strongly recommend pressing OK here; after a few seconds the model will be upgraded to the new version. The reason we don’t just do this upgrade automatically is that there is a possibility that models may change shape in regions around existing creases, or where surface boundaries have sharp internal corners; and of course we wouldn’t want any risk of changing your models behind your back! But in many cases, this upgrade won’t affect your shape at all. You can Undo if the upgrade made an undesirable shape change.



Creases and Star points


For those who may not have used it before, the Crease tool is available in the Modify menu in the Sculpt workspace, and is designed to add sharp edges to your otherwise smooth Form bodies:





Probably the most common problem people would encounter when using the Crease tool in previous versions was that other edges on the model would become unexpectedly creased as well. Something like this:




In most cases, this was caused by the ‘unhappy relationship’ between creases and star points (for more information on what star points are, check out this video). In previous versions of Fusion 360, a crease which encountered a star point would fire-out creases from that star point in all directions (technically, star points could only either be ‘all smooth’ or ‘all creased’). For models with multiple star points close together – like a box primitive – the creases that got fired out would then likely encounter other star points on the model, eventually resulting in many star points becoming creased.


With the new creasing method, creases can enter a star point without creasing any other edges. This means that a crease can end at a star point, like this:





or travel right through a star point, like this:






A nice simple example of how this can be used: create a primitive box and crease one or more entire loops of edges (this might be easier by selecting faces as inputs for the crease: see the tip lower down). You should end up with one or more rounded-rectangular faces. By playing with the dimensions of the box (either before or after creasing), it’ll be pretty easy to achieve some recognizable consumer product forms…







Crease Fade-outs and Yellow Regions


With our previous creasing method, creases would always have an additional “fade-out” area, travelling 2 edges beyond the selected edges:





The new creasing method works a little differently: creases will now start and stop immediately at the ends of the edges you select.




When testing this internally, we felt that the two-edge fade-out from the old crease method actually gives your creases a nice smooth visual quality. With the new creases stopping immediately, it’s more difficult to achieve smooth transitions like this. So in cases where it’s technically possible to give your crease a smooth fade-out, we’ll still do that


This all relates to the yellow regions you now see when you open the Crease command:




These yellow regions indicate areas where we can’t give you a two-edge smooth fade-out. If an end of a crease lies within a yellow region, this crease end will terminate directly where the edge selection finishes.





If any of your crease ends lie outside of the yellow regions, we will automatically give you a smooth two-edge fade-out here (even if the fade-outs themselves enter a yellow region):




The areas covered by the yellow regions are determined by the positions of particular special points on the model. To be specific, you will get yellow regions within a two-face radius of any of the following points on your model (these points are highlighted as yellow dots when the Crease command is active):

(So note that, as you add creases to your model, this will usually change where the yellow regions exist on your model!)



TIP: Reducing the size of yellow regions


If you want to create an old-style crease with a long fade-out, but the yellow regions are spreading further into your model than you would like, you can try subdividing or inserting edges in the regions around the yellow dots (use the ‘Exact’ option if you want to keep your smooth shape intact). Since the size of the yellow regions are based on the number of faces away from these points (not physical distance), adding more faces will reduce the physical size of these yellow regions, and give you a larger area of the model in which you can insert old-style creases:





Creasing Around Corners


The previous creasing method would only let you create each crease along a straight line of edges. If you creased an edge at 90 degrees to another creased edge, they would effectively become separate overlapping crease chains:





The new creasing method allows creases to turn 90 degree corners, and will smoothly connect these edges. This allows you to crease any combination of connected edges, and these will become a single smoothly-connected chain:




This offers much more freedom to explore crease shapes, and reduces some of the limitations that are imposed by the underlying ‘direction’ of the surface.  For example, you can now very easily make cleanly defined features within an existing surface:





TIP: Selecting faces as crease inputs


Since creases can now turn 90 degree corners, we thought it would be handy if you could define a set of faces as input, and generate a crease that runs around the border of those faces. To do this, just select one or more faces, then start the Crease command, and the borders of the face set(s) will becomes creased:





Surface Corners


Another separate, but related, issue that caught many people out was that internal corners at surface boundaries (i.e. corners where you have an L-shaped arrangement of faces) would fire-out creases from all edges connected to that corner (along a distance of 2 edges). You would most typically see this when deleting faces, which could add a lot of undesired creases to your model:




With the changes made for the new creasing method, we’re now able to smooth these internal corners, meaning that you can keep your surface nice and clean without worrying about avoiding this arrangement of faces.





For external corners at surface boundaries (such as the 4 corners of a Plane primitive), it’s possible you’d like these to be sharp or smooth, depending on your design intent.  So for these corners, we give you the option of whether you’d like these to be sharp (as in previous versions) or rounded-off like the internal corners.


To round-off a sharp external corner, open the Uncrease tool and select the vertex. To sharpen a rounded-off corner, open the Crease tool and select the vertex.





‘Maintain Crease Edges’ Command Option


Now that creases and star points play nicely together, creasing becomes useful in a lot more contexts. For example, you could have a Bridge feature with creased connections:





Or an extrusion/impression with a hard edge:





Therefore we’ve added an option called Maintain Crease Edges to some of the existing commands – namely Extrude, Merge Edge, Weld Vertices, Fill Hole and Bridge:




This allows you to choose whether you’d like to keep existing creases in these areas of the model, or if you’d like the command to create a smooth shape transition, as you would have in previous versions:





This also works for surface boundary edges; having this option switched on will give you a creased result:






TIP: Ctrl + Alt / Cmd + Opt to maintain creases in Edit Form extrude


One last trick related to this (and my personal favorite): when performing an Extrude in Edit Form (by holding the Alt key while dragging) at a creased region or boundary edge, if you hold both the Ctrl + Alt (Windows) / Cmd + Opt (Mac) keys while dragging, you’ll get a creased extrude rather than a soft extrude. This allows you to build creases directly into your model at the same time as you are fleshing-out your form. Here’s a pen model created from a cylinder primitive with a single Edit Form command, using a combination of Alt/Opt-drag and Ctrl-Alt/Cmd-Opt-drag to create the smooth and sharp transitions on-the-fly:






To finish up, here’s a look at a few models that came out of playing-around while testing this functionality:




This might not be the most elegant car ever created in Fusion 360, but this automotive form shows how the new creasing capabilities allow fairly complex forms & features to be achieved with only a small number of T-Splines faces.



The classic Muji CD Player is now super-easy to build as a single Form body: something that would have been much more difficult prior to this creasing update. Although a shape like this would typically be built in the Model workspace rather than with the Sculpt tools, building something like this with T-Splines gives you new ways to explore scale and form. Once you finish this Form and convert it to a solid model, the creases will still be present as hard edges on your body, which you can then add precise radii to using the Fillet command.





We believe these new capabilities add up to much more than just a tool enhancement: they offer whole new sets of possibilities for T-Splines forms that simply weren’t possible before. We’ve had a lot of fun working with this in the test builds, and we hope this makes T-Splines modelling more intuitive and more enjoyable for you as well. We’d love to see what you come up with using these new capabilities (don’t forget to post them to the Gallery!), and we’d also love to hear your thoughts, comments and feedback about the changes. Get in touch with us in the comments below, or in our community forums.



Jake Fowler

User Experience Designer

Fusion 360


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