Normal Edit modifier creates flipped normal #51645

Ubuntu 17.04

Blender Version
Broken: Blender 2.78 - build bot Hash: 9f044cb422

Short description of error
Normal Edit modifier creates flipped normal resulting holes in mesh with single side rendering (visible when backafe culling is eneabled).

Exact steps for others to reproduce the error

Create Monkey
Set Auto Smooth
Add Custom Normals Data
Turn on Backface Culling

Add Normal Edit Modifier

Changed status to: 'Open'

Added subscriber: @SirPigeonz

Added subscriber: @FloridaJo

Actually you have assumed the normals are flipped, but they are not. Turn on normals display and you will see. (maybe change title to "Normal Edit Modifier interferes with backface culling"
Seems to be though something flakey with the backface culling method or the 'set split normals' modifier is interfering with the polygon point order.
Backface culling should only cull faces with points in the polygon that are in counterclockwise order.

I don't know if this is normal or not though. I wouldn't think so.

Added subscriber: @mont29

Changed status from 'Open' to: 'Archived'

I see no bug here, obviously if you put custom normals too far away from 'normal' values, you'll get plenty of artifacts, among which reversed face normals… That’s why the modifier has a Max Angle setting, set it to something slightly lower than 90° should fix most of those, by preventing custom normals to “cross” their faces’ planes.

@mont29
Are you sure?
If I use data transfer modifier with Suzane and UV Sphere, normals look the same as in Normal Edit modifier but faces don't freak out and flip to other side. I get exactly same effect without this artifact. Why one works and other is broken?
Also I had to use 33 degrees which is not slightly.

Changed status from 'Archived' to: 'Open'

I was waiting for reply but it probably got lost. Opening bug again.
It should work as in post above. Looks like a valid bug.

Changed status from 'Open' to: 'Archived'

No it should not, just draw straight lines between object center and vertices, you'll see that on a mesh with quite a bit of concave areas like our monkey, you'll generate over 90° away normals compared to auto-generated ones in some places.

Data transfer uses much, much more complex and sophisticated algorithm than mere 'spherical' normals, and even there it can fail in some cases.

Hmm, ok I understand.
Maybe at least error message in modifier to indicate that some normals are now rotated beyond 90 degrees?
If it can't be fixed it would be good to at least check for errors that this algorithm may produce without user knowledge.

Once again, there is no error at all here! There is no magic barrier preventing a vertex normal to go “opposite” to some face normal around it.

Use the tools wisely according to what they do and on what geometry you apply them on, a simple 'spherical' normal setting will most likely not work great with a complex, heavily convex mesh like the monkey head. At least not without some control (like normal angle limitation, and/or reduced influence…).

Exclamation marks are not needed, nor sarcasm.
I understand that this is normal behavior of this algorithm in some cases, but that doesn't change that it can produce problematic results without user knowing it.
Why those artifacts are created is secondary in this situation.

I'm not reporting bugs to argue, or make anybody offended. I just want to help improve Blender.
In this situation my intention was only to give constructive feedback from my perspective how this tool performs. I didn't had ill intentions at all.

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Normal Edit modifier creates flipped normal #51645

Add Normal Edit Modifier