1. A method of detecting objects intruding into a monitoring zone, said method performed by a projective volume monitoring apparatus and comprising: capturing a stereo image from a pair of image sensors; correlating the stereo image to obtain a depth map comprising range pixels represented in three-dimensional Cartesian coordinates; converting the range pixels into spherical coordinates, so that each range pixel is represented as a radial distance along a respective pixel ray and a corresponding pair of solid angle values within the horizontal and vertical fields of view associated with capturing the stereo image; obtaining a set of flagged pixels by flagging those range pixels that fall within a protection boundary defined for the monitoring zone; accumulating the flagged pixels into corresponding cells of a two-dimensional histogram that quantizes the solid angle ranges of the horizontal and vertical fields of view; and clustering cells in the histogram to detect intrusions of objects within the protection boundary that meet a minimum object size threshold.