how i can use pointer return from Cvptr*D

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OpenCV

cvPtr*D() is that you can use these pointer functions to
gain access to a particular point in the matrix and then use pointer arithmetic to move
around in the matrix from there.
what is important thing i must remember when use this pointer ??
o reilly opencv book say :
1\ It is important to remember that the channels are contiguous in a multichannel matrix.
2\ Use the step data member .