Blender offers a choice of specular and diffuse shaders that can be combined to imitate a wide range of materials. The Toon Shaders allow artists to create images that resemble comic books and animated films. The term 'Cel Shading' is sometimes used to describe this style of shading. (I think this term comes from the celluloid frames that go to make up an animated film.) Cel Shaded images tend to have flat areas of colour, hard edges, and dramatic light and shade.
This is a style I tried to achieve in a series of illustrations called The Ifs. My Ifs pictures were designed for black and white printing, and I felt that toon shading would be ideal because I could get a high contrast, almost film-noir, look. I also chose to use ray-traced shadows because of the hard edges that they can give. Toon shading is, of course, also suitable for colour work.
A toon-shaded character
Lets begin by looking at the toon shader options in Blender. The image below shows the Shaders panel in the Material Buttons (F5).
The Shaders panel
The Diffuse and Specular shaders
Blender uses two shaders to render each material. They can be selected from the two drop-down menus on the left of the panel. The top menu is used to select which Diffuse Shader is used to simulate diffuse reflection, and the bottom menu is used to select which Specular Shader is used to simulate specular reflection, or highlights. In the image above, both the diffuse and specular shaders are set to Toon.
The Ref and Spec sliders
We can mix the diffuse and specular shaders to simulate all kinds of materials by using the Ref and Spec sliders. The Ref slider controls how much diffuse light is being reflected by the material, and the Spec slider controls the amount of specularity. A mat material will have a low specular setting, and a glossy material will have a relatively high specular setting. A degree of experimentation is required to find the appropriate Ref and Spec settings for each material. For toon shading, I tend to leave the Ref slider where it is and just alter the Spec.
The Ref and Spec sliders are common to all Blender's shaders. Unique to the Toon shaders are the Size and Smooth sliders. The Size slider controls how much of the material is covered by each shader. Set a high value and more of the surface of an object will be covered. The Smooth sliders (discussed below) also affect the size of the shading - the higher the Smooth setting, the larger the area the shader covers.
The effect of altering the Ref Size slider from 1.0, to 1.25, to 1.5
The images above show the effect of the Ref Size slider. Shadows are not enabled in these renders, and the shadowing is created solely by changing the size of the area that the diffuse shader effects. The specular shader settings have not been altered. The first image looks quite cool, but the disadvantage of creating shadows this way is that you have no control over the shadow colour - the shadow will always be black, and that can look ugly on light-coloured materials.
In my pictures I made the decision to let the diffuse shader shade the entire material, and I used ray tracing to create the shadows. The images below show my jeans material with the Ref Size set to the maximum value of 3.14, with and without shadows.
The Ref Size slider set to maximum, with, and without, shadows
Experiment with combinations of Size and shadows. Enable the Bias button if you want to prevent materials casting shadows onto themselves (like mine have).
The Size controls are very useful for simplifying a scene's lighting. By setting a high Ref value it is possible to simulate light coming from behind an object eliminating the need for complicated lighting. When I am using the toon shaders, I only use two lights to light a scene. I use one Sun lamp to provide the lighting, and another Sun lamp, with the options Ray Shadow and Only Shadow selected, to cast the shadows. Both lights share the same position. By using separate lamps for light and shadows, I can have better control over the tonal range in a scene. I set the amount of overall light and then mix in the shadow lamp until the shadows are the right darkness.
Lighting effect created with the Size sliders
The lighting in the image above looks quite complicated, but there is only one lamp casting light.
The Smooth sliders control how smooth the edge of each shader is (higher values give smoother shading than low values). The Smooth sliders produce useful results throughout the entire range. The hard edges in my Ifs pictures were achieved with a value as low as 0.01. Compare the results of altering the smoothness of the Specular Shader in the images below.
The effect of changing the smoothness of the Specular Shader from 0.1, to 0.33, to 1.0
I used medium to high Smooth settings to blend the edges of both the Ref and Spec shaders in my Book of Monsters pictures. I decided to use the toon shaders again because they simplify lighting, and they give a less realistic look than the other shaders, which I like. (The image below also has some post-process glow added.)
Ghost material with Ref and Spec Smooth sliders set to 0.5
I just love the toon shaders!
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