My history as an artist/illustrator contains heavy useage of the airbrush for creating different visual effects. The airbrush mostly does one thing ...and it does it extremenly well - it creates gradients. This quick tutorial will cover using the Gradient Texture to transition from one material to another. And, it will give me a chance to mention a few settings I personally like when using the toon material.
Noise is also used to transition from one material to another but it creates a look, in this case, completely different than the smooth transition a gradient will.
Here is the scene - one part smoothly transitions from toon to a BAM (Brazil Advanced Material) and for contrast the other elements transition abruptly.
I personally like the illustration quality of the toon material and in Brazil it works very well with the other materials in the scene. Very robust.
This approach to toon is to keep the coloration flat but emphasize it's qualities by varying the outline and then blend into a more realistic material.
Ink Parameters - Ink width
In the Ink width I have placed a Shadow Light Material - this texture reacts to light intensity. Very powerful!
Lit areas are one color and shadow areas another...and a smooth transition between the two. In this case the lit areas will make the line width thin by having the min: setting at a low amount and the shadow areas, thick. These can played with to get the exact look you are after. The Shadow Light Material is such a useful texture - it can add a render hit in some places though.
A gradient texture to transition the line color from red to black has also been added....transitions rule!:-)
The Custom Curve window is used to bring the transition closer together - the handles on the end-points have been dragged out to accomplish this. Two other pieces of geometry have this material assigned in order to show the effect of the line coloration and line thickness modifications.
An Advanced Blend Material is used to blend between this toon material and a BAM using another Gradient Texture. Once assigned to the object everything is automatically mapped to the 'surfaces' and then uses this projection. Not what we are after - an interesting effect as the gradients are mapped per surface but, not the look which works well here.
In the Texture Mapping Properties a Planar projection is assigned and visability is turned on. In general, the planar projection is the only one which works with a Gradient Texture. The projection will need to be rotated and scaled 'to-taste'.
Once the proper projection is used, scaled and rotated the result is what was envisioned. A gradation from one material(Toon) to another(BAM) going from top to bottom of the object. The Custom Curve will most likely be needed to modify the transition point and amount.
The Gradient Texture is one of those textures which can be used anywhere to bring visual interest to a material and object. To emphasise two extremes, to transition between two similar materials......and everywhere in between!