About Cel Shading

Cartoonists stylize reality, rendering only those details considered important for the overall design of the cartoon. Mickey Mouse has no fur or whiskers. Snoopy doesn't have claws or discernable nostrils. Disney females often lack visible fingernails.

Edges may "ink" your models, but it doesn't "think" about design. A literal-minded "inker," Edges will trace a line along those edges of polygons that qualify as "ink lines" in a model. The four key types of Edges follow these rules:

1. Silhouette Edges - appear at the edge shared by two single-sided polygons when the normal of one of those polygons faces the camera, and the normal of the other polygon faces away from the camera. The edges that qualify as Silhouette Edges thus change with the angle of the model to the camera. An edge must be shared by two polygons to qualify as a Silhouette Edge - Silhouette Edges will never appear on a model that consists of a single polygon. Double-sided surfaces face the camera at all times, so Silhouette Edges will never appear on them.
2. Unshared Edges - appear on edges not shared by any other polygon. Unshared Edges, when checked, will always appear on a model that consists of a single polygon. For non-SubPatch models, selecting an area of geometry and then applying a "Cut and Paste" operation will create an Unshared Edge at the border of the selection in your object. You might use this trick to get Edges to help you describe the panels in a spaceship, for example. (Any overlapping points of a SubPatch model will get "merged" into a shared edge in Layout, so the "Cut and Paste" trick will only help you sharpen edges in your model - it will not give you an "Unshared Edge" in Layout).
3. Sharp Creases - might also be called "UnSmoothed Edges." Whether or not a given edge qualifies as a "Sharp Crease" depends on the Smoothing Angle of the surface to which that edge belongs. Lowering the Smoothing Angle of a surface will increase the number of edges traced; increasing the Smoothing Angle will decrease the number of "Sharp Crease" edges. If the "Smoothing" option is left unchecked in the Surface Editor, most (if not all) of the polygon edges in your model will qualify as "Sharp Creases." Their territory sometimes overlaps with that of Silhouette Edges, as the edges of polygons that turn away from the camera might qualify as a "Sharp Crease." This type of Edge does not care whether the polygons have double-sided surfaces or not, so you can often use them to put a "Silhouette Edge" on a double-sided surface.
4. Surface Borders - this type of Edge will appear at the edge shared by two polygons when said polygons have different surface names. The polygons might have the exact same texture parameters, but "Surface Borders" will not care. If the surface names differ, then the polygons will have an ink line appear at their shared edge (unless the artist leaves "Surface Borders" unchecked).

Fortunately for us, Edges consistently follow the above rules. The modeler, then, can both predict and control where the ink lines will appear.

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