Importing Obj Files Into Blender? Clear Custom Split Normals To Fix Dark Spots

What Causes Dark Spots on Imported OBJ Files

When importing .OBJ 3D model files into Blender, dark spots may appear on the surface of the mesh, giving it an irregular shading. This is often caused by custom split normals on the model. Custom split normals are used in some modeling programs to sharpen edges and details. However, they can cause issues when importing models into other software like Blender.

Here’s a more technical explanation of what causes these dark spots:

In 3D modeling, vertex normals define the direction a face is pointing, indicating how light should glance off the surface.
Smooth, properly calculated normals result in smooth, even shading across a model.
Custom split normals break up smooth vertex normals into multiple custom normals on the same vertex, allowing sharp transitions between faces.
When models using custom split normals are imported into Blender, the irregular normals can cause dark spots and uneven shading.

The dark spots are especially noticeable in curved surfaces. Without correcting the normals, attempts to smooth shade the model will be unsuccessful. Thankfully the fix is relatively simple – we just need to recalculate the normals upon importing the OBJ file.

Table of Contents

Identifying Custom Split Normals as the Culprit

When you first import your OBJ model and see dark patches, how can you confirm that custom split normals are the cause? There are a few ways to verify this:

Select the mesh and switch to Edit Mode. Closely inspect the faces and vertices where the shading looks irregular.
Enable “Face Normals” display in the Mesh Display panel. You will see irregular and jagged normals displayed on the faces if custom split normals are present.
Select individual vertices along the dark edges and look at the “Vertex Normals” display in the sidebar. If there are multiple different colored normals on the same vertex, custom split normals are the culprit.

You can also select different areas of the mesh and compare how many vertex normals versus face normals there are. For a healthy, properly shaded model there should be about a 1:1 ratio. If you have a much higher number of vertex normals, you are almost certainly dealing with custom split normals causing those dark spots.

Disabling Custom Split Normals When Importing

Once we’ve identified custom split normals as the cause of dark shading artifacts on the imported OBJ model, how do we avoid the issue? The key is to disable importing of custom normals during the import process.

Here is how to disable custom split normals on OBJ import in Blender:

Open File > Import > Wavefront (.obj)
In the import dialog, locate the “Import Normals” dropdown and select “OFF” instead of the default “On”
Import the OBJ file with this setting disabled

This prevents the custom vertex normals from being imported with the geometry. The vertex and face counts will still be correct, but Lighting will be calculated based on default normals instead.

Disabling import normals forces Blender to recalculate them from scratch later, avoiding any issues from custom normals. We just need to perform that normal recalculation as the next step.

Selecting Your Model and Recalculating the Normals

With our OBJ model imported without custom normals, it may initially look quite dark or have some strange shading artifacts. We now need to tell Blender to recalculate clean vertex normals across the entire mesh.

Here is the process for recalculating normals on an imported OBJ model in Blender 2.8 or later:

In Object Mode, select your imported OBJ model
In the modifier properties panel, click “Add Modifier”
Select “Normal Edit” and left click to confirm.
In the new modifier, locate “Mode” and select “Recalculate”
Click “Apply” in the modifier options.

This goes through and recalculates proper face and vertex normals for the entire model based on the geometry and shape. It will override any traces of the old custom split normals that were causing issues.

You can confirm it worked by checking before and after images in Material Preview mode, where any shading issues are most visible. The surface should now have clean, even shading free of any dark spots or irregularities!

Confirming the Dark Spots are Gone

With recalculated normals and default Lighting, the surface of your model should now render smoothly inside Blender. But let’s visually confirm that those pesky dark spots are gone for good.

Here are some tips for verifying your imported OBJ file no longer has dark shading artifacts after recalculating normals:

Switch to Material Preview mode for best results seeing shading issues.
Slowly orbit around the model, zooming into curved ares that had issues.
Confirm all areas now have smooth, even shading without dark patches.
Test with a subdivision surface modifier enabled to better see any potential shading problems.
Validate in both MatCap and Studio Lighting preview modes.

You can go a step further by rendering out the model or exporting it to tools like Keyshot. But the Material Preview mode inside Blender will generally show any remaining dark spots quite clearly if they still exist.

If any areas still seem to have dark shading issues, you may need to select those elements separately and recalculate normals just for that section. But typically recalculating normals for the whole OBJ solves custom split normal importing issues.

Alternative Ways to Fix Dark Spots

While disabling custom normals on import and recalculating is usually the best approach, there are some other ways to potentially fix dark spots on an imported OBJ file:

Manually editing normals: Go into Edit Mode and adjust specific normals using Ctrl+N.
Smoothing groups: Sometimes defining smoothing groups in the OBJ export can resolve shading issues.
Shade Smooth operation: Select faces in Edit Mode and use Shift+N to force smooth shading.
Mesh Cleanup: Use modifiers like meshesCleanup and meshToCage to tidy topology.

I’d recommend trying the previously outlined steps first. But in rare cases where recalculating normals fails to resolve dark shading spots, test these backup methods.

The most reliable approach is still to properly prepare and export models from the original software without custom split normals enabled. But understanding these normals fixes allows you to clean up models from any source.

Common Questions and Issues

Here are some frequently asked questions and troubleshooting tips related to fixing dark spots from custom split normals on imported OBJ files:

What if Recalculate Normals fails? – Try applying modifiers and clearing custom data, then recalculate again.
Can I prevent this issues when exporting OBJs? – Yes, disable custom normals in original app before exporting.
Why are there still tiny dark faces? Back facing polygons can cause this – recalculate normals with “Inside” enabled.
What if I can’t access the original modeling file? – Importing without normals and recalculating is your best option.

Hopefully those help explain some common sticking points and questions around resolving dark shading spots on OBJ imports. Don’t hesitate to ask other specific questions in the comments!