When Cel shading is applied, what artists also wants to do is usually make the shadow boundaries sharp. Such a control can be done very easily by built-in UE4 engine light parameter named Shadow Filter Sharpen. When a directional light actor exists in the scene (our CharCelShadingDirectionalLight is basically also a direction light actor), by controlling the parameter, shadow boundary can be smoothed or sharpened. We observed 1.0 value is usually adequate for cel shading characters’ shadow.
Figure 5. Controlling shadow boundary sharpness