A mass machia is a macchia painting where the pattern establishes the largest shapes and values but where the contours of the big shapes are not well defined, only approximated. This is usually executed "with the largest possible brush" for the statement desired.
As a general rule, especially in all'aperto landscape painting, a deliberate effort is made to suppress details. Instead of painting individual trees, one paints a larger mass and indicates just a tree here and there, a stand of trees as a 3-d shape. Individual shapes can be depicted but they must integrate into the larger shapes in an almost seamless manner.
In landscape painting, where especially in diffuse light, it may be difficult to isolate large shapes, the "black mirror" can be employed to reveal the darkest darks and the lightest lights. Also, artists look for horizontal and diagonal lines as aids in this process.
