To convert 2D into 3D for two calibrated cameras you would use these formulas: z = focal*baseline/disparity x = z*u/focal y = z*v/focal
where focal - the focal length of you camera in pixels u = column-Cx, Cx~image_width/2 but calibration will give you more precise value v = -row+Cy, Cy~image_height/2 Baseline - horizontal distance between cameras disparity - difference in horizontal position of the ball in two images
Strictly speaking you need to do rectification only for working with dense stereo. For sparse stereo you only need calibration.