Visual SLAM Challenges

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Hello,

Could anyone help me in choosing a subject for my master thesis in Visual Slam..what are the current challenges in visual slam? i read about several methods (slam based on filters,slam based on sfm and bundle adjustment, biologically inspired slam...) and i realised that there is indeed a big advancement in this field so i wanna ask if there still some challenges in visual slam that could present a problem for my thesis.. Any suggestion will be highly appreciated

Thank you