Disparity Map

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Can somebody explain me what exactly does a disparity map return .Cause there isint much given in the documentation and i have a few quedstions related to it.

1. Does it return difference values of pixels wrt both images?
2. How to use diaparity values in the formula for depth estimation i.e. Depth = FocalLength*Baseline/disparity
3. I have read somewhere that disparity map gives a function of depth f(z) . Please explain what it means.If depth is purely an absolute value how can it be generated as a function or is it a function wrt the pixels?

Help required...Please help...Thanks in advance..