Deep Incremental Scene Understanding. Federico Tombari & Christian Rupprecht Technical University of Munich, Germany

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1 Deep Incremental Scene Understanding Federico Tombari & Christian Rupprecht Technical University of Munich, Germany

2 C. Couprie et al. "Toward Real-time Indoor Semantic Segmentation Using Depth Information" JMLR, 2014 S. Izadi et al., KinectFusion: Real-time 3D Reconstruction and Interaction Using a Moving Depth Camera, UIST 2011 Scene Understanding and SLAM Scene understanding with deep learning (typically frame-wise) [Couprie14] SLAM from RGB-D data allowing real-time scene reconstruction [Izadi11] Can we fuse the two, while still being real-time?

3 Beyond SLAM: fusing reconstruction with scene understanding Fusing multiple viewpoints over time improves semantic perception and object pose estimation SLAM++ [Salas-Moreno13] Incremental Scene Understanding on Dense SLAM [Li16] C. Li et al., Incremental scene understanding on dense SLAM, IROS 2016 R. Salas-Moreno et al., SLAM++: Simultaneous Localisation and Mapping at the Level of Objects, CVPR 2013

4 Incremental 3D Segmentation Real-time segmentation of SLAM reconstruction [Tateno15], yielding constant complexity wrt. the size of the reconstruction K. Tateno, F. Tombari, N. Navab, Real-Time and Scalable Incremental Segmentation on Dense SLAM, IROS 15

5 Real-time also on Google Tango..

6 What if a depth sensor is not available? Is semantic mapping/incremental scene understanding still possible from a single RGB camera?

7 Monocular SLAM state of the art FEATURE-BASED DIRECT ORB-SLAM [Mur-Artal14] LSD-SLAM [Engel14] Not dense on texture-less regions MAIN LIMITATIONS No pure rotational motions No absolute scale J. Engel et al., LSD-SLAM: Large-Scale Direct Monocular SLAM ECCV 2014 R. Mur-Artal et al., ORB-SLAM: A Versatile and Accurate Monocular SLAM System IEEE Trans. Robotics 2015

8 Depth prediction with CNNs Goal: Use a CNN to predict a dense depth map from a single RGB image RGB Image Depth Ground Truth (Kinect) Depth Prediction An alternative to monocular SLAM?

9 FC ResNet with UpProjections [Laina16] CNN Architecture ResNet-50 avg Memory FC limitations pool Restriction of full connections: high dimensional outputs can produce billions of parameters Residual blocks I. Laina, C. Rupprecht, V. Belagiannis, F. Tombari, N. Navab: Deeper Depth Prediction using fully Convolutional Residual Networks, 3DV 2016

10 FC ResNet with UpProjections [Laina16] CNN Architecture avg FC pool difficult convergence blurry predictions need for bigger datasets vs ground truth prediction Residual blocks

11 FC ResNet with UpProjections [Laina16] CNN Architecture Residual blocks fully convolutional ResNet with progressive up-sampling

12 FC ResNet with UpProjections [Laina16] CNN Architecture Residual blocks

13 FC ResNet with UpProjections [Laina16] CNN Architecture Residual blocks

14 FC ResNet with UpProjections [Laina16] CNN Architecture Residual blocks

15 Multi-task FC ResNet RGB Input Depth GT (Kinect) Depth Prediction 4-class Sem. Seg. 40-class (RGB-Only) 40-class (RGB + Depth Pred.)

16 Monocular SLAM and CNN depth prediction are complementary Monocular SLAM Accurate on depth borders but sparse CNN-SLAM [Tateno17] takes the best of both world by fusing monocular SLAM with depth prediction in real time CNN Depth Prediction Dense but imprecise along depth borders 1. can learn the absolute scale 2. dense maps 3. can deal with pure rotational motion K. Tateno, F. Tombari, I. Laina, N. Navab: CNN-SLAM: Real-time dense monocular SLAM with learned depth prediction" CVPR, 2017

17 CNN-SLAM framework Every Key-frame Input RGB Image CNN Semantic Segmentation CNN Depth Prediction Key-frame Initialization Pose Graph Optimization Global Map and Semantic Label Fusion Camera Pose Estimation Frame-wise Depth Refinement Camera pose estimated via direct method at each new frame Set of key-frames, each associated to a depth map Each key-frame depth map D ki is Every input frame 1. initialized via Fully Convolutional ResNet [Laina16] 2. refined with depth values D t estimated via short-baseline stereo matching [Engel14], weighted by the associated uncertainty U ki, U t : D ki u = U t u D ki u + U ki u D t u U ki u + U t u

18 Key-frame depth refinement Key-frame depth refinement allows estimating fine structures on previously blurred surfaces Gradual fusion of CNN-predicted depth with monocular SLAM: elements near intensity gradients will be more and more refined by the frame-wise depth estimates elements within low-textured regions will gradually hold the predicted depth value from the CNN Refining depth in Key-frame RGB image in Key-frame RGB image in current frame

19 Qualitative results SLAM on pure rotational motion

20 Qualitative results Absolute scale estimation

21 First demonstration of fully monocular real-time semantic mapping

22 Many prediction tasks are ambiguous Many prediction tasks contain uncertainty. In some cases, uncertainty is inherent in the task itself [Rupprecht17]. What will the other driver do? What is the label for this image? C. Rupprecht, I. Laina, R. DiPietro, M. Baust, F. Tombari, N. Navab, G. D. Hager: Learning in an Uncertain World: Representing Ambiguity Through Multiple Hypotheses" arxiv: , 2017

23 Simple example: next frame prediction single prediction a square is bouncing around the frame it randomly switches color between black and white the CNN predicts the next frame in the sequence the mean of black and white is gray, which is also the background the frame is constant gray

24 Approximations with the mean p(x) x Learning the mean can lead to very unlikely solutions

25 Approximate with multiple hypotheses a simple meta-loss transforms any model into a multiple hypothesis predictor (MHP)

26 Simple example: next frame prediction prediction 1 prediction 2 now we transformed the same network into a multiple hypothesis model with two predictions it is able to separate black and white blocks for the future frame

27 Image Classification

28 Human Pose Estimation the variance of prediction can help detecting ambiguities the predictions for the location of the hands varies much more than for the shoulders

29 Future Frame Prediction with more predictions future frames become sharper the model does not need to blend together all possible outcomes

30 hypotheses Multiple Hypothesis for Depth Prediction input ground truth hypotheses mean variance

31 Multiple Hypothesis Prediction for CNN-SLAM the variance can be used to estimate confidences confidences will be used as initialization for the refinement of the keyframe with MHP depth prediction the overall accuracy increases original CNN-SLAM CNN-SLAM with MHP correct pixels: 10.6% correct pixels: 36.0%

32 Conclusion We presented a framework for real-time scene understanding fusing semantic segmentation and SLAM reconstruction Depth prediction complements monocular SLAM in low texture regions and global scale Multiple hypotheses allow for improved 3D reconstruction Combine deep learning with 3D computer vision to leverage the best of both worlds Slide 32

33 Credits (alphabetical) Dr. Max Baust Dr. Vasilis Belagiannis Robert DiPietro Prof. Greg Hager Iro Laina Prof. Nassir Navab Keisuke Tateno We gratefully acknowledge the donation from Nvidia of two GPUs that helped the development of the presented research activities.

34 References [Couprie14] C. Couprie, C. Farabet, L. Najman, Y. LeCun: "Toward Real-time Indoor Semantic Segmentation Using Depth Information" JMLR, 2014 [Engel14] J. Engel et al., LSD-SLAM: Large-Scale Direct Monocular SLAM ECCV 2014 [Izadi11] S. Izadi et al., KinectFusion: Real-time 3D Reconstruction and Interaction Using a Moving Depth Camera, UIST 2011 [Laina16] I. Laina, C. Rupprecht, V. Belagiannis, F. Tombari, N. Navab: Deeper Depth Prediction using fully Convolutional Residual Networks, 3DV 2016 [Li16] C. Li et al., Incremental scene understanding on dense SLAM, IROS 2016 [Mur-Artal15] R. Mur-Artal et al., ORB-SLAM: A Versatile and Accurate Monocular SLAM System IEEE Trans. Robotics 2015 [Rupprecht17] C. Rupprecht, I. Laina, R. DiPietro, M. Baust, F. Tombari, N. Navab, G. D. Hager: Learning in an Uncertain World: Representing Ambiguity Through Multiple Hypotheses" arxiv: , 2017 [Salas-Moreno13] R. Salas-Moreno et al., SLAM++: Simultaneous Localisation and Mapping at the Level of Objects, CVPR 2013 [Tateno15] K. Tateno, F. Tombari, N. Navab, Real-Time and Scalable Incremental Segmentation on Dense SLAM, IROS 15 [Tateno17] K. Tateno, F. Tombari, I. Laina, N. Navab: CNN-SLAM: Real-time dense monocular SLAM with learned depth prediction" CVPR, 2017