Normal Edit modifier incorrectly causes faces to become 'back faces' #55470

System Information
Operating system and graphics card
N/A

Blender Version
Broken: 2.79b f4dc9f9
Worked: 2.73

Short description of error
In previous versions of Blender (tested 2.73) when adding a normal edit modifier, this would affect vertex normals but not face normals. In 2.79 the face normals are affected and faces are being culled.
I need the old behaviour for a non-realistic cell-shaded look. Lighting should be affected by the vertex normals, but obviously faces should not turn invisible just because the normals have been twisted. The erroneous behaviour persists when exporting to FBX and bringing the model into Unity.

Exact steps for others to reproduce the error
Based on a (as simple as possible) attached .blend file with minimum amount of steps

Open attached .Blend file.
Inspect the Icosphere and notice that a number of its faces are being culled.
Move the 'PointNormalsAt' transform around to see both lighting and face culling change.
Select the Icosphere and turn the Edit Normals modifier off to see the full icosphere with faces outward as they should be.

normaleditbug.blend

Added subscriber: @WispBart

Added subscribers: @mont29, @lichtwerk

changed between 2.77a and 2.78
e3841c91f9, f08018f928, #42526

Imho current behaviour seems actually more correct (for most scenarios... but as you say: there might be other cases where poly flipping is undesired...)

Also not sure what NPR look exactly you are after, there might be solutions on the Unity side of things?

Turn off culling in Unity shader? (for npr this might be enough?)
somthing like this ?

@mont29: or should we make this [polygons_check_flip] an option in the modifier?

This issue was referenced by 0eb3246713

Changed status from 'Open' to: 'Resolved'

Am not happy with re-introducing such buggy behavior, but on the other hand there is not other way I can think of to get same effect, and NPR often relies on buggy/incorrect behaviors anyway, so...

Thanks for the fix!

The look we're going for is flat-shaded characters with hard shadow lines (see Guilty Gear xrd for an example). Turning off backfaces could cause problems down the road, since these are logically just front facing tri's. We just want more control over how much light they are receiving.

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Normal Edit modifier incorrectly causes faces to become 'back faces' #55470