In real life, there are many surfaces that are created putting different materials over one another, like paint over wood, metal over glass, a plastic decal over a metal surface, or even dust over a regular surface. Maxwell materials work in that natural, layered way. A layer represents a complete material, that can (and usually will) contain several BSDF, coatings, displacement components, etc.
Each layer should be thought of as an independent material, and the different layers can be stacked vertically from bottom to top, just like with the Photoshop layers palette. The top layers conceal the bottom layers, and this means that the order of the layers in the material arrangement can change the material’s look and behavior completely. This makes it easy to create complex materials. Layers can be renamed by double-clicking on its name or re-arranged their stacking order by dragging and dropping them up or down in the layers list.
Take a Lets look at this an example:
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This is just a simple material composed mixing a diffuse textured component with a reflective mirror-like specular component (to provide that shiny reflection) in the same way it is explained in the Blending BSDFs section. The two BSDFs were renamed to "texture" and "shine" for an easier arrangement.
They are mixed in a 75-25 proportion, regarding the Weights of both BSDFs. And this is just a gold-like material, created with one single BSDF. The BSDF has been renamed to "gold".
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Both materials can be applied together on the same surface, one over another, like different substances can be applied in layers on the same surface.
If we put a layer with the gold material over the textured material, we will get just an uniform golden surface, as the layer above conceals the layers underneath , if there is no opacity value, blending mode or mask applied to the upper layer the Layer opacity is set to 100%, and the Layer is set to normal blending mode and there is no Layer mask applied (just like the image layers layer masks work in Photoshop work).
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Instead, we 
We can use a greyscale map to define the opacity of the upper layer, on the Layer Properties panel. The white areas of the map indicate where that layer will be completely visible (and may conceal the materials underneath), and the black areas of the map indicate where that material won't exist, allowing us to see the material underneath , producing a surface composed by a shiny texture material with certain gold painted details.
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Or course, like in Photoshop, you
You can stack as many Layers on a material as you want, and control the visibility of each layer using their opacity Layer weight values , and masks or their blending mask.
On . In this example, we've added another extra layer on top working as an emitter, using a mask to define where that emitter is visible, adding certain emitting details on the surface.
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Layer Opacity
Like in Photoshop, you can set the Opacity value of a layer to define its visibility, and thus, the visibility of the layers underneath. The following example represents a red glass material with a metal material above it. By setting the Opacity of the metal layer at i.e. 15%, you get a resulting material that is 15% metal and 85% red glass.
An
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An interesting use for the Opacity value is for creating AGS (stands for Architectural Glass Solution). An AGS is a "sort" of glass material, very useful for window panes on architectural projects, where you don't need the time consuming properties of actual dielectric glass (refraction and caustics) but you want transparency and reflection. |
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See a description of how to create and use AGS materials on the Material examples - How To section. |
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Layer Mask
In addition to Opacity values, you can also map the opacity of a layer using a mask map, a greyscale image that indicates where the layer is visible (white areas of the map) and where it is not (black areas). The map is set in the layer Opacity/Mask field. In the red glass example, we can use a map as a mask to indicate where the metal material is visible and where not, creating a red glass material with metal inlays.
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When using a layer mask, the Layer opacity parameter still plays a part in the overall opacity that the Layer will have. For example if you set the Layer opacity to 50%, this will be the maximum opacity of the Layer, even in the areas of the mask that are totally white. |
Another interesting use for layers masks is to create billboards or clipmaps: Having one single layer and mapping its opacity with a mask, will produce a material that is completely invisible on in the black areas of the map, creating a billboard (or climapclipmap). Billborads Billboards are very useful to mimic plants, trees, leaves or people without the need of modeling them.
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A good way to visualize this effect is thinking of projecting light through several photographic slides. It can be useful to create interesting blends between the textures used in the BSDFs of the different layers and also to create plastics that have a thin shiny coating on the surface that still allows the color of the plastic to show through beneath it.
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