If you are using normal map for your cel shading material instance, you will see an option “Use Normal Map For Line Detection” in the material instance. By turning on the switch, the crease line drawing material will use normal-map-applied-normal-vectors of the character. It makes the interior lines much more detailed. But, in some cases users may want to turn the option off because the normal-map-based lines can be too much detailed (Figure 4).

Figure 4. (Left) The Use Normal Map for Line Detection Switch, (Right) The On/Off Results