Defining things and stuff. The literature provides definitions for several aspects of stuff and things, including:

(1) Shape: Things have characteristic shapes (car, cat,phone), whereas stuff is amorphous (无定形的)(sky, grass, water)[21, 59, 28, 51, 55, 39, 17, 14].

(2) Size: Things occur at characteristic sizes with little variance（变化）, whereas stuff regions are highly variable in size [21, 2, 27].

(3) Parts: Thing classes have identifiable parts （可辨认的）[56, 19], whereas stuff classes do not (e.g. a piece of grass is still grass, but a wheel is not a car).

(4) Instances: Stuff classes are typically not countable [2] and have no clearly defined instances [14, 25, 53].

(5) Texture（结构化）: Stuff classes are typically highly textured [21, 27, 51, 14].

Finally, a few classes can be interpreted as both stuff and things, depending on the image conditions (e.g. a large number of people is sometimes considered a crowd).