In this lesson, you will learn how to bypass standard PBR (Physically Based Rendering) lighting to achieve a hand-drawn, illustrative aesthetic using custom lighting models and edge detection.
Breaking the PBR Pipeline
Standard Unreal Engine materials use the Default Lit shading model, which assumes light falls off smoothly according to the inverse-square law. To achieve a "Toon" look, we must intercept the light calculation.
The core of a toon shader is quantizing the NdotL (Normal dot Light) calculation. Instead of a smooth gradient from light to shadow, we force the output into discrete bands. In your Material Graph, you can achieve this by passing the Dot product of your World Normal and Light Vector into a Ceil or Step function, or by using a Posterize node.
// Conceptual HLSL snippet for quantized lighting
float NdotL = dot(WorldNormal, LightDirection);
float QuantizedLighting = floor(NdotL * Levels) / Levels;
By multiplying your base color by this QuantizedLighting value, you create the signature "stepped" shadow look found in cel-shaded animation.
Implementing Edge Detection
Stylized shaders often require clean, bold silhouettes. Since standard engine lighting doesn't generate outlines, we typically use a Post-Process Material.
The most common technique is a Sobel Filter performed on the SceneDepth or Normal buffer. By sampling the surrounding pixels of the current fragment, we compare the depth or normal values. If the difference between neighbors exceeds a certain threshold, we draw a black outline. This creates a high-contrast boundary that reinforces the "drawn" feel of your assets.
To ensure these outlines are artistic rather than purely technical, you can combine this with Inverted Hull meshes—where you duplicate your model, invert the normals, and expand the geometry slightly—to create depth-aware outlines that exist as part of the geometry itself rather than a full-screen pass.
By combining quantized surface shading with screen-space edge detection, you effectively strip away the "realistic" lighting simulation of Unreal Engine and replace it with an artistic, high-contrast visual identity.
Key Takeaway: Achieving a stylized look requires replacing physically accurate light interpolation with quantized bands and using depth-based post-processing to define clear, illustrative silhouettes.