Interposition
An object that occludes another is closer
Shading
Shape info. Shadows are included here
Size
Usually, the larger object is closer
Linear Perspective
parallel lines converge at a single point
Surface Texture Gradient
more detail for closer objects
Height in the visual field
Higher the object is (vertically), the further it is
Atmospheric effects
further away objects are blurrier
Brightness
further away objects are dimmer