Using a stereo camera, depth is determined by the concept of triangulation and stereo matching. By constraining the problem on a 2D plane known as the epipolar plane, we can simplify the formal search to a line search along the epipolar line.
Approach
Identify similar points from feature descriptors.
Match feature correspondence using a matching cost function.
Using epipolar geometry, find and match correspondence in one picture frame to the other.