This short tutorial shows how it is possible to create a halftone shading effect using an OSL shader connected to the Toon shader. Thanks to Zap Andersson for this shader and for the help with this tutorial.
The Halftone OSL shader is available here.A scene file can be downloaded here.
Start off by downloading the Halftone shader from the above link.
Assign a toon shader to your model.
Create an OSL shader and import/compile the Halftone OSL shader within it. Once it has compiled you should see the Halftone OSL attributes as values and sliders in the OSL shader.
Connect the OSL shader to the base_tonemap attribute of the toon shader.
You can control the halftone shading effect by connecting a black and white ramp (U/V) to the input value of the Halftone OSL shader.
without ramp
ramp_rgb -> input_value of Halftone
That's it! Have fun experimenting with different scale values and ramps to create different effects.