how to extract the Mat of non square object in opencv?

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I am using opencv (both c++ and java) to segement a white paper in the picture showed below, this works , but now I have to split the paper into 8 small section and get the Mat of each section, since each section is not a pure square, it is hard to use submate(startrow, endrow, startcol, endcol).

Is there any way in opencv to extract the Mat for this kind of none square object?