SLAM – Loop Closing with Visually Salient Features

1. SLAM – Loop Closing with Visually Salient Features Paul Newman, Kin Leong Ho Oxford University Robotics Research Group

2. Motivation • Loop Closing – the task of deciding whether a vehicle has returned to a previously visited area • Popular approaches – nearest neighbour statistical gate, joint compatibility test • Closing loop with visually salient features to avoid dependence on global position estimate

3. Visual Saliency • Scale saliency detector [ Kadir/Brady IJCV 2001] • - form p.d.f. of pixel properties within local region at varying scales for each pixel • detection of region at a particular scale where weighted entropy is peaked • selected regions are considered more “interesting” p.d.f of local pixels overscale s around position x entropy

4. Visual Saliency Dissimilarity p.d.f of across scale Weighting Function Saliency metric entropy Weighting function

5. Wide-Baseline Stability • Maximally stable extremal region (MSER) detector [ Matas etal. BMVC 2002] • -pixels taking on values in the range D = {dmin ….dmax}

6. MSER detector Saliency detector

7. Feature Description -Scale invariant feature transform (SIFT) descriptor [David Lowe IJCV 2004] -128 dimensional descriptor

8. MSER Detector SIFT Descriptor Query Image Selected Regions Saliency Detector Similarity Measure Matched Images Laser Scan Database Image Database

9. Demonstration of wide-baseline stability of visually salient features under perspective distortion and variation in illumination conditions

10. Matching Performance Similar posters found in the environment.

12. A Delayed State Formulation Control Past poses Scan matching between Past poses produces observation z with which to update state-vector State vector contains only past vehicle poses. (Atlas IJRR 2004 )

13. Delayed State Formulation II EKF update

14. Closing Small Loops

15. Closing Big Loops

16. Closing the loop

17. Issues -hard decision making -using saliency detector as binary selector -repetitive visual features in urban environment

18. Demonstration

19. Questions Thank you!