Inferring Unseen Views of People
Chao-Yeh Chen and
The University of Texas at Austin
We pose unseen view synthesis as a probabilistic tensor completion problem. Given images of people organized by their rough viewpoint, we form a 3D appearance tensor indexed by images (pose examples), viewpoints, and image positions. After discovering the low-dimensional latent factors that approximate that tensor, we can impute its missing entries. In this way, we generate novel synthetic views of people—even when they are observed from just one camera viewpoint. We show that the inferred views are both visually and quantitatively accurate. Furthermore, we demonstrate their value for recognizing actions in unseen views and estimating viewpoint in novel images. While existing methods are often forced to choose between data that is either realistic or multi-view, our virtual views offer both, thereby allowing greater robustness to viewpoint in novel images.
Problem: dilemma for human images!
Though we have lots of images for human poses, but they are either:
- Realistic snapshots, but limited views.
- Multi-view imagery, but artificial lab conditions.
Our Idea: infer images of people in
-Infer the pose in missing view with tensor completion.
(1) Representing poses from different views
- Pose = person captured in one instant in time.
- Compute HOG descriptor for poses in all available views.
- Situate them in tensor according to (discretized) viewpoint.
(2) Unseen view inference as tensor completion problem
- Recover the latent factors for the 3D tensor X.
- Use latent factors to impute the pose in unseen views.
- Solve with probabilistic factorization [Xiong et al. 2010].
(3) Learning with unsynchronized single-view images
Infer new views for snapshots observed from just a single viewpoint.
- Beyond synchronized multi-view data, we want to learn from single-view “in the wild” snapshots of people.
- Link snapshots with similar 3D pose, but different viewpoint.
Datasets: IXMAS multi-view images (Weinland et al.) and H3D Flickr images (Bourdev et al.).
Visualizing inferred views
- visualize the inferred views using inverted-HOG (Vondrick et al.).
Visualization of inferred unseen views in IXMAS dataset.
Visualization of inferred unseen views in H3D dataset.
Accuracy of inferred views
- Our inferred views have lowest Summed Square Difference
(SSD) compared to two baselines.
- Memory: memory-based tensor completion.
- Copy: copy observed images from nearby views.
Error in inferred views.
Impact of data sparsity
- Our method’s accuracy is fairly stable up until about 40% (i.e., when 60% of the tensor is unobserved).
Accuracy in unseen views as a function of tensor sparsity.
Application: Recognizing actions in unseen views
- Use our inferred views to train a system to recognize actions from a viewpoint it never observed in the training images.
Action recognition accuracy (mAP) in an unseen viewpoint on IXMAS. Numbers in parenthesis are standard errors.
Cross-view action recognition accuracy on IXMAS.
Application: Viewpoint estimation
- Improve viewpoint estimation by adding the view-specific training instances created by our method with the real images.
Average mAP, compared to view synthesis baselines with HOG features.
Classification accuracy vs. state-of-art with poselet features.
- Novel learning approach for inferring human
appearance in unseen viewpoints.
- Accommodates both synchronized multi-view and unsynchronized single-view images.
- Valuable for viewpoint robustness in human analysis tasks on two challenging datasets.