Using Autosmooth And Split Normals For Realistic Shading In Blender

Why You Need Custom Normals for Realistic Shading

When modeling assets in Blender, achieving a realistic and appealing shading is essential for high quality renders. However, Blender’s default shading can often appear too harsh and angular on curved surfaces, with jagged, uneven transitions between faces.

These artifacts are caused by discontinuities in the face normals between adjacent faces. Normals define the direction a face is pointing, which determines how lighting interacts with the surface. The default blender normals are generated per face, meaning each face is smoothly shaded, but the transitions between faces can be very sharp.

Using techniques like AutoSmooth and Split Normals allows custom control over the face normals to create smooth, gradual transitions between faces on curved surfaces. This avoids the unnatural harsh edges and machbanding artifacts that can otherwise appear.

Avoiding Harsh Shading Edges and Artifacts

On curved surfaces in Blender, the default face normals can cause visible transitions between faces as each face points in a slightly different direction. This leads to harsh shading edges and machbanding – a visual artifact that produces exaggerated dark/light contrast at the edges.

By controlling the normals with AutoSmooth and Split Normals, the normals of adjacent faces are smoothed together or split apart to match the appropriate shading for the geometry. This eliminates the sudden normal discontinuities that cause ugly shading artifacts.

Smooth vs Flat Shading Explained

Smooth and flat shading refer to different approaches for calculating normals and lighting on the surface:

• Smooth shading: Each vertex has a averaged normal from surrounding faces. Lighting is interpolated across vertices, creating smooth transitions.
• Flat shading: Each face has its own distinct normal. Lighting is consistent within a face, but transitions between faces can be very sharp.

AutoSmooth and Split Normals give explicit control to switch between smooth and flat shading on chosen surface areas as needed for a realistic look.

When to Use AutoSmooth and Split Normals

Custom normals with AutoSmooth and Split Normals are most useful on curved organic surfaces – vehicles, characters, terrain, etc to prevent shading artifacts:

• AutoSmooth: Smoothing out transitions on gently curved surfaces
• Split Normals: Maintaining crisp edges bordering smooth surfaces

Simple mechanical models with strictly flat surfaces generally don’t need custom normals. But anything with smooth contours will benefit.

Enabling and Configuring AutoSmooth

AutoSmooth is used to generate custom vertex normals that average between the normals of adjacent faces. This smooths out shading between faces to avoid sudden changes at edges. Enabling it is done in the Object Data Properties.

AutoSmooth Angle Threshold

The key setting for controlling AutoSmooth behavior is the Angle threshold value. Any face angle below this threshold will have its normals smoothed between faces, while angles above will remain flat shaded with the default face normals.

Typical values range from 30 to 80 degrees. Lower values smooth more aggressively; higher values preserve more shading detail between faces.

Controlling Shade Smoothness

In addition to the angle threshold, a few other options affect AutoSmooth blending and smoothness:

• Edge sharpness – Mark specific edges as sharp to exclude them from smoothing
• Edge normals – Define custom split normals manually on edges
• Vertex normals – Manipulate vertex normals directly in the mesh data

Used selectively, these provide absolute control over shading behavior on the model.

Example Code

import bpy

# Access mesh object
obj = bpy.context.object 

# Enable Auto Smooth
obj.data.use_auto_smooth = True  

# Set angle threshold
obj.data.auto_smooth_angle = 80

# Mark specific edges as sharp
edge = obj.data.edges[0]
edge.use_edge_sharp = True

Using Split Normals for Sharp Edges

While AutoSmooth blends normals across edges, Split Normals goes the other direction – forcing adjacent faces to have distinct normals to maintain a sharp boundary.

Maintaining Crisp Edges with Smooth Faces

Many models require both smooth shading on curved surfaces combined with crisp, well-defined edges at key borders and seams – for example the windshield glass on a car model.

Using the Split Normals modifier, these edges can be given separate custom normals allowing an abrupt normal transition between faces while keeping the smooth gradient within the face itself.

Pinpointing Areas Needing Split Normals

Identifying the edges needing split normals is easy – just look for harsh shading borders between faces on otherwise smoothly curved surfaces. The Split Normals modifier can be added specifically in these areas.

For characters, common candidates are borders around ears, noses, elbows and knees that form silhouettes or highly visible seam lines.

Example Code

import bpy

# Select object and enter edit mode
obj = bpy.context.object
bpy.ops.object.mode_set(mode='EDIT')

# Select edges needing split normals 
edges = [obj.data.edges[0], obj.data.edges[5]]

# Mark edges as sharp 
for e in edges:
  e.use_edge_sharp=True

# Back to object mode and add Split Normals Modifier
bpy.ops.object.mode_set(mode='OBJECT')  
mod = obj.modifiers.new("EdgeSplit", 'SPLIT_NORMALS') 
mod.use_edge_angle = False
mod.use_edge_sharp = True

Best Practices for Using Custom Normals

When working with AutoSmooth, Split Normals and custom normals in general, certain guidelines and workflow tips will ensure you use them effectively.

Performance Considerations

Extensive use of vertex normals from AutoSmooth or Split Normals across the entire mesh can become very resource intensive – so apply selectively only where needed.

If performance is critical, doing custom normals in Blender then baking down to face normals prior to export can optimize performance while keeping benefits in renders.

Workflow Tips

• Start with default shading, then enable AutoSmooth and Split Normals to fix artifacts
• Use Edge Info addon to display angles and mark sharp edges
• Visualize normals with overlays during work
• Tweak angle thresholds gradually to balance smoothness vs edges

Common Mistakes to Avoid

• Excessive smoothing that removes definition on key edges
• Inconsistent shading from missing areas needing custom normals
• Flipped faces or normals causing shading to face the wrong direction

Paying attention to the angle thresholds, sharp edges, and normal orientations as you work can prevent these issues.