grid in ORBextractor

[jingpang]

@raulmur Hi, in ComputeKeyPoints() in ORBextractor.cc, computation of the levelCols/levelRows can be slightly modified (usage of imageRatio).

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    const int levelRows = imageRatio*levelCols;

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change to

    const int levelRows = levelCols / imageRatio;

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This modification makes levelCols/levelRows = imageRatio = image.cols / image.rows . The keypoints may distribute better when cols>>rows, i think.

( https://github.com/raulmur/ORB_SLAM/blob/master/src/ORBextractor.cc#L533 )

[AlejandroSilvestri]

I believe you are right, ORB-SLAM2 no longer uses this method.