How to define the parameter 'numDisparities' in StereoSGBM?

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I'm using StereoSGBM to get the absolute location of one object. But I found that different value of numDisparities would change the matrix of disparity. For example, when I set numDisparities=64, the disparity of point A is 15. But when I set num numDisparities=128, the disparity of point A is 30 or something. This will certainly change the result caculated by function cvReprojectImageTo3D. I want to figure out the relation between the parameter 'numDisparities' and the final matrix of disparity.