Stereo depth calculation:
Simple case, aligned cameras
DISPARITY= (XL - XR)
Z
Similar triangles:
Z = (f/XL) X
Z=  (f/XR) (X-d)
Solve for X:
(f/XL) X = (f/XR) (X-d)
X = (XL d) / (XL - XR)
XL
XR
    f
Solve for Z:
d*f
X
(XL - XR)
(0,0)                  (d,0)