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Simple Deform modifier in bend mode doesn't work on thin geometry
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Description

System Information
Operating system: Windows 10
Graphics card: GTX1080Ti

Blender Version
Broken: 2.8 latest master
Worked: Probably never

Short description of error
Simple Deform modifier in Bend mode doesn't work on thin geometry, such as plane. It's a very common workflow to bend a simple rectangular plane.

Exact steps for others to reproduce the error
1, In a new blender file, create a plane
2, Enter edit mode of the plane, and subdivide the plane a few times to get enough topological density for bending
3, Exit edit mode and apply Simple Deform modifier on the plane
4, Switch Simple Deform mode to Bend
5, Notice that despite Deform angle being set to 45°, the modifier has no effect on X axis
6, Switch to Y axis and notice the modifier has also no effect
7, Switch to Z axis and notice the modifier has effect, although Z axis is the least likely axis user will want to bend a plane along.

Result: Simple Deform modifier set to Bend mode works only on Z axis if the geometry is thin (planar)

Expected: Simple Deform modifier in Bend mode works on any kind of geometry.

Details

Type
Bug

Event Timeline

Havent looked at the code further, but the local Z axis plays an important role [no matter which deform axis you actually pick]

You can get deformations on thin planes if you place your geometry in a way you actually get non-zero values for your verts [in local Z]:

I would guess this is not considered a bug [even though it might seem a little unintuitive...]

I would definitely consider it a bug. I mean, if the geometry is not thin, then it works even without user having to go through the hoops of orienting the topology to cater to the modifier (or destructively applying rotation transform). Here's an example with a box:


X, Y and Z axes respectively, all of them work. Do you see any logical reason why the topology being thin should have any effect on this?

Still havent looked at the code, but if geometry is not thin, its verts most likely are non-zero in local Z. (this seems to be the important part, as mentioned above...)
This might be for historical reasons [iirc in 2.79 you couldnt even pick an axis...], but like I said, would need to check code first...

Still havent looked at the code, but if geometry is not thin, its verts most likely are non-zero in local Z. (this seems to be the important part, as mentioned above...)
This might be for historical reasons [iirc in 2.79 you couldnt even pick an axis...], but like I said, would need to check code first...

I am maybe starting to understand it. Basically it'd need two axes. One to define which axis to bend along and one to define which axis to bend in. So for example "I want to bend a plane in X axis along Y axis". That's indeed probably not a bug but very confusing behavior non the less.

While the behavior might be unintuitive, that is how the code is supposed to work. You will either have to move the origin of the object or define an other bending point with the modifier.

As stated in T61523, https://blender.community/c/rightclickselect/ is the place to discuss how to make this modifier more intuitive.