Making outlines with Blender
If you want your renderings to look like drawings, the single most important thing is to add outlines. Even on semi-realistic renderings, outlining can improve clarity.
The difference outlines can make. (slightly modified blender example)
Inverted Hull
Hull shading involves rendering a slightly larger version of the object, the hull, inside out, and in a different color (usually black). Because the vertex order is inverted, backface culling causes only the parts of the hull behind the object to be rendered. This results in an outline around the edges of the object.
To do this in Blender, add a new material to the object (in slot 2), and set the color to the outline color (black for the example).
Then in material properties, make sure to enable backface culling for the newly added material.
To add the inverted hull, add the solidify modifier on the object, set the material offset (Materials -> material offset) on the modifier to 1, and check Normals -> Flip on the modifier.
Finally set the offset to 1 (add hull outside of object) and thickness to 0.05m.
You can adjust the thickness parameter on the solidify modifier to change the thickness of the outline.
If you enable viewport shading you can see the effect of it in the viewport and while you are adjusting the thickness.
One limitation of the inverted hull method is that it only draws outlines behind objects, and not on edges in front of the object, meaning that it still requires shading for more complex geometries.
Notice the lack of outlining the edges on the front of the cube.
Edge detection Cel shading
Edge detection based Cel shading outlines all of an objects edges, making the geometry very visible, even without lighting.
Cel shading, applied on a unlight object.
Cel shading and lighting with blender’s Principled BSDF.
Another way to add outlines is to run a sobel filter on the depth and normal maps of the final rendered image, then use that to create the outlines.
To do this in Blender, first, open Scene -> Viewlayers in the sidebar, then enable the Z and Normal passes under Data -> Passes.
This makes the renderer provide the needed data for the outlines.
Then go into the compositing tab and check use nodes.
Create two Filter nodes, and connect the Depth and Normal render layers to the Image input and set the filter type to Sobel.
The result on running a Sobel filter on the depth pass.
Now pass the image output of the filters into the Image input of Blur nodes to smooth out the jagged edges, and increase the size parameter to 5.
Then combine the Image outputs of the Blur nodes with the Image inputs of a Alpha Over Node, and set the Alpha to 0.5.
Finally, pass the Image output of that Alpha Over node into the Alpha input of another Alpha over node, passing the Image output of the Render layers node into the first Image input, and setting the second Image input to the color you want for the outlines. Pass the Image output into the Image input of the composite node and run a render to test it out.
You can adjust the Fac value on the Sobel filters to adjust where outlines are added.
Example files
These are simple scenes that demonstrate both methods: