True stereopsis is perceived due to the following -
Stereopsis is a term that is most often used to refer to the perception of depth and 3-dimensional structure obtained on the basis of visual information deriving from two eyes by individuals with normally developed binocular vision. It is used for binasal disparity Ref khurana 6/e p456
Overlay of contours
Motion parallax
Bi- nasal disparity
Linear perspective
Ocular motility and squint