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Please refer to the following references for more information:

• CS231A · Computer Vision: from 3D reconstruction to recognition, Silvio Savarese
• Lecture 6 - Stereo Systems Multi-view geometry, Silvio Savarese
• Computer Vision: Algorithms and Applications, Richard Szeliski
• Learning OpenCV 3 - Computer Vision in C++ with the OpenCV Library, Gary Bradski, Adrian Kaehler
• [...]

Rectification will make the left and right images fronto-parallel:

Relation between depth of a triangulated point and disparity is the following:

From equation:

you will get [Eq. 1].

In OpenCV notation, depth of a triangulated point is calculated with:

The 4x4 disparity-to-depth mapping matrix Q is:

The relation between 3D point in homogeneous coordinate and disparity is:

Remember that 2D image point expressed in pixel can be converted to meter in the normalized camera frame (Z=1) with:

If left principal point c_x and right principal point c'_x are equals, we get the original equation to compute the depth:

Minus sign should come from if you compute d = (u - u') or d = (u' - u).

As everything is in homogeneous coordinate, 3D point is retrieved with: