Bend Modifier Zone of Influence

This is about the Zone of Influence, aka zoi, of the Bend Modifier in two of it's type settings: Constraint and In Cage.

If you've ever tried to use a Bend Modifier in the middle of a
complex model, you have probably run into some weirdness.
You can also stack multiple Bend Modifiers and this
can also lead to some of that same behavior.

The problem occurs either when other parts of the same mesh are in the invisible
zoi of the Bend Modifier, or the stack of BMs bends things back into the zoi.
(sorry, I was trying not to use BM but I got tired of typing.) :poop::LOL:

The zoi of the Constraint setting extends up Y+ and to the sides Z+- of the pink cage of the BM object to ♾️.


The In Cage setting the zoi extends only to ♾️ on the Y+ axis of the BM object.

Stacking Bend Modifiers complicates things and trying to
predict the outcome eludes me, so I have to rely on testing.

Here I stacked BMs with the Constraint setting.

Here's an animated illustration of what's actually going on when Bend Modifiers get stacked.
Remember the BM zones of influence extend to ♾️ on Y+ and Z+-.


Each bend has a its own zoi that is affected by the previous bend, even though this has not been made editable yet.

Next showing the proof.
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For the first proof of concept I "bent" the top Bend Modifier
and the mesh distorts as it enters multiple zones of influence.

This time I bent the second Bend Modifier from the top.

You can see the bend above the one I'm manipulating is safe from
the zone of influence of the BM below it, but not the others.

Let's skip to the bottom Bend Modifier.

You can see how all the zones of influence of all the BMs above this one move in unison.

So far my zone of influence theory pans out...

But if I select a group of polygons, in this case the disconnected
sphere, and move them out of the zoi. Why does this happen?

It's not the motion but the fact that the parts of the object
affected don't have to be in the "zone of influence".

I was trying to establish that any part of the object above the base
of the Bend Modifier will be "modified". So in essence the object
itself determines the area of modification.

The zoi is determined by the scale of the BM and or its Cage
and maybe should be called the zone of interference.
Here's an example of how the order in the Object Browser is important.

Also the Cage of the three top Bend Modifiers have all been edited in such a way
that doesn't change there affect on the mesh, just to show what's possible.

The bottom three are rotated 180 degrees so Y is facing down.
Notice the stacking order.



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Paperclip Exercise

For a change I'm adding a practical exercise utilizing the information from the tutorial.

As I've said before every day household objects are a great source of subjects for modeling.
For this exercise the perfect object is a common paperclip. The first question for me was
if it can be done with multiple Bend Modifiers at all. I wasn't 100% sure.

For the exercise make a paperclip with a stack of as few Bend Modifiers as possible.
Extra: Pose the finished model in at least one "bent" paperclip position.

For more experienced modelers you're on your own to get it done.
It's a good bet you have at least one paperclip around or you can find
an image on line like I did or just use the rendered image below.
There is a great video of a German machine online that makes paperclips.
You can also find the $185 Prada paperclip.

For beginners or mid level modelers I'll include some extra help.



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:) Thank you, ZooHead, for a great intro to stacked modifiers.

* On a tangent / note to newbies:
* Bear in mind that there are alternatives to applying the bend modifier:
1 Sweep on a spline
2 Mesh on a spline (which is a Java script)

Screenshot 2022-03-14 at 18.29.35.png
Thanks Helmut, it's good to show alternate methods.

My first try was with a spline/sweep to see if it was faster/easier
and it is a lot more fiddly to make the spline in this case.
Helmut's post brought to mind the problem of choosing the right
method/tool for a particular job when you have more than one option.

I often start with one method/tool and realize It's somehow problematic
and then do it another way. In this case it was intentional for comparison.

I also built a second spline based version from chopped up
spline circles but it still took a while and it was tedious.

There are other consideration for instance are you going to animate it.
If so is it a character, does it have eyes etc.?

Here's a simple animation, easy for the Bend Modifier and a nightmare with Splines.
And on top of that, you can only do Point Animation with splines, which further complicates things.


If it was a character animation I would build multiple models of all types to cover my butt.
Now I keep thinking about eyes and how to deal with them with Bend Modifiers.

With Bones the eyes would be within the hierarchy of the skeleton,
with BMs I made it a separate object with duplicate modifiers.

I was able to add another BM to the paperclip as well as a Twist Modifier.
I animated them and copy and pasted the keyframes to the eye object modifiers.

* You can model a paper clip based on a helix with about 1 1/2 turns. Reduce the sections to 6 and switch linear to Bezier.
* The resulting spline has just 7 control points which can be fiddled in animations. Adding bend and twist should produce acrobatic paperclips.

* As mentioned by ZooHead, each modus operandi has advantages and disadvantages. As a rule of thumb, select a method which provides a maximum of freedom. A bit of brainstorming before tickling mouse and keyboard is always useful.

Screenshot 2022-03-16 at 07.39.48.png
Here's a great opportunity to test how hard it is to animate the straightening of a spline based paperclip.

If by chance anyone is following along and did build the paperclip with Bend Modifiers you can compare
how easy/hard it is to do it to a spline based model. This one has 8 Bezier points.

Refer to post #15 and try to imitate any of the straightening moves without changing the length of the object.
After that, imagine the art director says we have to change it to some other angle.


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It occurred to me that a hybrid model is possible with the Spline/Sweep and Bend Modifiers.

I can use the Spline/Sweep to make a straight cylinder and then use the BMs for the bending.
That way I still have the ease of animating the BMs and the added flexibility of the Splines.
I used a Line Spline with around 75 segments. In this screen shot the spline
has been made editable and unnecessary points have been deleted.

Hybrid Spline Bend.jpg


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