Surfaces can be transparent, semi-transparent, or opaque. 
Transparent surfaces should be used sparingly, as it becomes to difficult to know where these surfaces begin and end. These surfaces may sometime need background treatment when there is text.

University of Michigan - Ann Arbor

Material surfaces have a default state of opaque white, with 1dp thickness, and casts a shadow. Every attribute is an alteration of this state.

Attributes

Material Design Documentation

Material surfaces can only act in specific ways. 

- Rigid surfaces maintain their size always. 
- Stretchable surfaces may change size up to a certain point, then act as rigid surfaces.
- Pannable surfaces maintain their size but can display more content with scrolling up to a certain point. 
- Composite surfaces are surfaces that are separated into spaces that can present different types of behavior.

Behavior

Surfaces can either be fixed or movable. 

Movement may be restrained to one axis, one axis at a time, or allow for movement along both axes at the same time. 

Movement can be independent or dependent on the movement of other surfaces. 


Surface Positioning and Movement

Surface Opacity

Scrims are temporary treatments used to attract user focus to certain parts of the screen by making specific elements more or less prominent. They can be done at any elevation or to multiple surfaces on an interface. 

Some examples include:
- Darkening or lightening the surface and its content
- Reducing the opacity of the surface and its content

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