As mentioned in the introduction, material can be thought of as being bound by physical laws. There are things it can do and things it cannot. It can split apart and heal again, and change color and shape, but it cannot occupy the same space as another sheet of material or rotate around two of its axes. We will be dealing with these properties throughout the book, but it is a good idea to begin with a quick look at the things material can and can't do.
The third dimension is fundamental when it comes to material. This is what gives the user the illusion that they are interacting with something more tangible than a rectangle of light. The illusion is generated by the widening and softening of shadows beneath material that is closer to the user. Material exists in virtual space, but a space that, nevertheless, represents the real dimensions of a phone or tablet. The x axis can be thought of as existing between the top and bottom of the screen, the y axis between the right and left edges, and the z axis confined to the space between the back of the handset and the glass of the screen. It is for this reason that material should not rotate around the x or y axes, as this would break the illusion of a space inside the phone.
The basic laws of the physics of material are outlined, as follows, in the form of a list:
All material is 1 dp thick (along the z axis).
Material is solid, only one sheet can exist in one place at a time and material cannot pass through other material. For example, if a card needs to move past another, it must move over it.
Elevation, or position along the z axis, is portrayed by shadow, with higher objects having wider, softer shadows.
The z axis should be used to prompt interaction. For example, an action button rising up toward the user to demonstrate that it can be used to perform some action.
Material does not fold or bend.
Material cannot appear to rise higher than the screen surface.
Material can grow and shrink along both x and y axes.
Material can move along any axis.
Material can be spontaneously created and destroyed, but this must not be without movement. The arrivals and departures of material components must be animated. For example, a card growing from the point that it was summoned from or sliding off the screen when dismissed.
A sheet of material can split apart anywhere along the x or y axes, and join together again with its original partner or with other material.
This covers the basic rules of material behavior but we have said nothing of its content. If material can be thought of as smart paper, then its content can only be described as smart ink. The rules governing how ink behaves are a little simpler:
Material content can be text, imagery, or any other form of visual digital content
Content can be of any shape or color and behaves independently from its container material
It cannot be displayed beyond the edges of its material container
It adds nothing to the thickness (z axis) of the material it is displayed on