Shading

In traditional painting, there are six shaded regions.

Outlines are the one of the most important features in cel painting. And shading is simplified to 2 types: halftone and shadow. Hence the boundary between the light and shadow is another feature of cel painting. Nowadays, highlight is always used in cel animation through adding special effects.

In digital(modern) paintings, different techniques for shading are used together for artists to create diverse art styles.

For non-photorealistic rendering, the performance of lighting is the most important feature, and it is also inseparable from the topology of the 3D mesh.

Another thing worth noting is that the 2D animation will show omitted topology details (such as the highlights on the hair) through light and shadow, but the highlights rendered by toon shading renderer will expose the real structure of the 3D topology. When applied to real-time toon shading, artists often fix the highlight on the material by texturing. This method solves the problem to some extent, but it seems strange when the light changes in the scene.

Painting media

Some details should be reduced or altered depending on strokes and techniques.

Watercolor

Watercolor gradients are mostly used in the creation of shadows.

Gouache（opaque watercolor）

Gouache is similar to watercolor in that it can be re-wetted and dries to a matte finish, and the paint can become infused into its paper support. It is similar to acrylic or oil paints in that it is normally used in an opaque painting style and it can form a superficial layer.

Opaque watercolors look thick(strong coverage), wheras watercolors are more transparent(fluidity).

Watercolor is more suitable for illustrations or still scene(background). Opaque watercolor is more suitable for animated character and front scene. Opaque watercolor and watercolor are usually mixed, where watercolor used as background and opaque watercolor for the details.

Alcohol-based markers

Alcohol markers are different because the ink is permanent. However, you can still layer the colors to create striking overlays. For example, gray overlays will dull colors down while yellow will brighten them up.

https://www.bilibili.com/video/BV1mJ411k7qT?from=search&seid=12035595433229581903

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To me, markers style reminds me of childhood, and traditional watercolors look more romantic.

Crayon

https://blendernpr.org/crayon-charcoal-bge-test-train-face-boy/

Cel shading(toon shading)

Cel shading, as a 3D animation and rendering techniques, have regularly been used by Ghibli since the production of Princess Mononoke (1997) – the first feature to benefit from the distribution deal – to achieve complex effects that would be impossible using traditional pen-and-ink methods. Those CG shots were seamlessly blended with the hand-drawn images in production.

Toonz, Toon Shaders and Studio Ghibli

Some paintings of other styles that usually appear in the form of still frames can also be made into animation, but the cost would be high, such as this fully painted feature film which took six years to produce: Loving Vincent.

Motion design and frame rate

I think one of the problems when using non-photorealistic rendering is that the animation looks too fluid and realistic than it should be, especailly for the use in limited animation. Fully animated films are animated at 24 frames per second, wheras limited animation uses 8-12 drawings per second, thereby limiting the fluidity of the animation.

For those who are used to watching limited 2D animation, watching the 3D animation of toon shading will give them some strange feelings, because the movement of the characters is too realistic, leading to the movement of non-photorealistic shadow look too realistic . The human eye is usually very sensitive, such a realistic movement reveals the topology used in 3D animation.