Fixing Strange Black Shading In Blender Cycles: Clearing Custom Split Normals Data

What Causes the Black Shading

The main culprit behind strange black shading issues in Blender Cycles is custom split normals data. This refers to cases where individual faces and vertices have been assigned separate custom normals that deviate from the standard calculated normals. When custom split normals are present, it can disrupt proper shading and lighting calculations in the Cycles render engine.

Here is a more detailed overview of how custom split normals corrupt shading:

Normals define the direction a surface is facing and are used for lighting calculations. Custom normals override the standard ones.
With inconsistent normals, light interacts incorrectly with the model’s surfaces leading to dark/black patches.
The custom normals data often stems from modeling errors, importing issues, or attempts to sharpen edges by manually splitting normals.
Cycles handles complex materials and shaders assuming clean normals data is present.
For physically-based rendering, unified normals are especially important across contiguous surfaces.

In summary, custom split normals introduce bad data into Cycles, confusing the rendering engine, and resulting in incorrect dark shading. Tracking down and removing this errant data is key.

Table of Contents

Locating Affected Geometry

Finding what parts of your model contain custom split normals is an important first step before correcting them. Here are some techniques in Blender to identify issues:

Using Edit Mode and Face Select to Find Issues

Tab into edit mode and select all faces with A key
In the Mesh Display panel enable Custom Split Normals
Faces with custom normals will highlight in blue/purple color
You can then select the affected faces by clicking on them directly

Example Code to Select Problem Faces

Here is some Python code for Blender that selects all faces with custom split normals:

import bpy
import bmesh

# Get the active mesh
obj = bpy.context.edit_object
me = obj.data

# Get a BMesh representation
bm = bmesh.from_edit_mesh(me)  

# Select faces with custom normals
for f in bm.faces:
  if f.smooth == False:
    f.select = True

# Update the selection
bmesh.update_edit_mesh(me, True)

With errant faces selected, you can now move on to stripping the custom data causing issues.

Removing Custom Data

Once you’ve located parts of your model containing custom split normals, you can now remove the bad data entirely. Blender offers some reset tools that can help automate this process.

Reset Tools to Clear Custom Normals

“Clear Custom Split Normals Data” – Object data properties panel
“Recalculate Outside” – Useful after applying fixes manually
“Recalculate Normals” – Modeling toolbar operation

Code Examples to Clean Geometry

In some cases you may need to build your own custom solution. Here is some sample code to strip custom normals from an object’s mesh data:

import bpy
import bmesh
import mathutils

obj = bpy.context.edit_object
me = obj.data

bm = bmesh.from_edit_mesh(me)

for v in bm.verts:
  v.normal = mathutils.Vector((0.0, 0.0, 0.0))

for f in bm.faces:
  f.smooth = True
  
bmesh.update_edit_mesh(me)

This resets all vertex normals to neutral and sets faces to use smoothed shading. Run these tools as needed to eliminate any source of bad custom normals.

Correcting Shading Issues

With the custom split normals data removed, shader issues caused by bad data should disappear. But some additional clean up may be required still:

Recalculating Normals after Cleaning

With blanks normals now, new ones must be recalculated
Apply “Recalculate Outside” or “Recalculate Normals” again
Check for and fix any face normals flipped the wrong way

Lighting Considerations after Fixes

Eliminating custom normals can change lighting and reflection behavior
Materials using specular maps or anisotropic effects will need rebalancing
For physically correct results, lighting should become more uniform over the mesh
Some additional scene and light adjustments may still be required after fixes

Be prepared to redo some lighting and material work after resolving any normals issues. Clean geometry combined with careful tuning will give you the best final renders.

Preventing Future Custom Data

After putting in the effort to fix normals issues, you’ll want to prevent them coming back. Here are tips on stopping custom splits at the source:

Setting Import and Modeling Workflows to Avoid

When importing assets, enable auto-smoothing to keep normals consistent
Avoid applying additional edge split modifiers manually after import
Careful vertex merging to topology keeps surrounding normals in sync
Mark sharp edges only where truly needed for crisp borders

Tips for Stopping Custom Splits from Happening

Model cleanly from start avoiding unnecessary geometry cuts
Use supporting loops and clean topology flow for good base normals
Optimize angles between faces helping smoothed shading work best
Minimize vertex count and redundant edges where possible

Planning mesh density, layout, and geometry prior to extensive editing prevents fighting fires later. Apply discipline early and you can avoid normalization and shading defects before they take hold.