3D Hand Pose Estimation: From Current Achievements to Future Goals
In this paper, we strive to answer two questions: What is the current state of 3D hand pose estimation? And, what are the next challenges that need to be tackled? Following the successful Hands In the Million Challenge (HIM2017), we investigate 11 state-of-the-art methods on three tasks: single frame 3D pose estimation, 3D hand tracking, and hand pose estimation during object interaction. We analyze the performance of different CNN structures with regard to hand shape, joint visibility, view point and articulation distributions. Our findings include: (1) isolated 3D hand pose estimation achieves low mean errors (10 mm) in the view point range of [40, 150] degrees, but it is far from being solved for extreme view points; (2) 3D volumetric representations outperform 2D CNNs, better capturing the spatial structure of the depth data; (3) Discriminative methods still generalize poorly to unseen hand shapes; (4) While joint occlusions pose a challenge for most methods, explicit modeling of structure constraints can significantly narrow the gap between errors on visible and occluded joints.
The field of 3D hand pose estimation has advanced rapidly, both in terms of performance [27, 29, 46, 45, 51, 54, 51, 57, 58, 6, 4, 50, 37, 25, 5, 4, 60, 3, 31] and dataset quality [9, 42, 44, 49, 56, 21]. The most successful methods treat the estimation task as a learning problem, using random forests or convolutional neural networks (CNNs). However, a review from 2015  surprisingly concluded that a simple nearest-neighbor baseline outperforms most existing systems. It concluded that most systems do not generalize beyond their training sets , highlighting the need for more (and better) data. Manually labeled datasets such as [30, 40] contain just a few thousand examples, making them unsuitable for large-scale training. Semi-automatic annotation, combining manual annotation with tracking, helps to scale the dataset size [42, 44, 49], but in the case of  the annotation error can be of the same order as the lowest estimation errors. Synthetic data generation solves the scaling issue, but has not yet closed the realism gap, leading to some kinematically implausible poses .
A recent study confirmed that cross-benchmark testing is poor due to different capture set-ups and annotation methods . It showed that training a standard CNN on a million-scale dataset achieves state-of-the-art results. However, the estimation accuracy is not uniform, highlighting the well-known challenges of the task. These include variations in view point and hand shape, self-occlusion, and occlusion caused by objects being handled.
In this paper we conduct a series of analyses, looking at how methods perform in these scenarios. The Hands In the Million (HIM2017) challenge  serves as a test-bed for this analysis. This benchmark dataset includes data from BigHand2.2M  and the First-Person Hand Action dataset (FHAD) , allowing the comparison of different algorithms in a variety of settings. The challenge considers three different tasks: single-frame pose estimation, tracking and hand-object interaction. We aim to answer the question of where we are, as a field, in terms of accuracy. We consider network architectures, preprocessing strategies and data representations. Over the course of the challenge the best average 3D estimation error could be reduced from 20 to less than . This paper analyzes the errors with regard to seen and unseen subjects, joint visibility, and view point distribution.
We conclude with providing a number of insights that can be useful when designing the next generation of methods.
Public benchmarks and challenges in other areas such as ImageNet  for scene classification and object detection, PASCAL  for semantic and object segmentation, and the VOT challenge  for visual object tracking, have been instrumental in driving progress in their respective area. In the area of hand tracking, the review from 2007 by Erol et al.  proposed a taxonomy of the different methods. Generally, machine learning approaches have been found effective for solving single-frame pose estimation, possibly in combination with iterative model optimization to achieve a more precise fit, e.g.. The review by Supancic et al.  compared 13 methods on a new dataset and concluded that deep models seem well-suited to the pose estimation task . It also highlighted the need for large-scale training sets in order to train models that generalize well. In this paper we extend the scope of previous analyses by comparing deep learning methods on a large-scale dataset, carrying out a fine-grained analysis of error sources and different design choices.
2 Evaluation tasks
We evaluate three different tasks on a dataset containing more than a million annotated images based on a standardized evaluation protocol. The benchmark images were sampled from two datasets: BigHand2.2M  and First-Person Hand Action dataset (FHAD) . Images from BigHand2.2M cover a large range of hand view points (including third-and first-person views), articulated poses, and hand shapes. Sequences extracted from FHAD are used to evaluate hand pose estimation in the hand-object interaction scenario. Both datasets contain -depth maps with 21 joint annotations obtained from magnetic sensors and inverse kinematics. The evaluation tasks are 3D single hand pose estimation from (1) individual frames, (2) video sequences, given the pose in the first frame, and (3) frames with object interaction, e.g. with a juice bottle, salt shaker, knife, or milk carton. See Figure 1 for an overview. For each scenario, the goal is to accurately estimate the 3D locations of 21 joints. Bounding boxes are provided as input for tasks (1) and (3). The training data is sampled from the BigHand2.2M dataset. Only the interaction task uses test data from the FHAD dataset. See Table 1 for dataset sizes and the number of total and unseen subjects for each task.
|subjects (unseen)||5||10 (5)||10 (5)||2 (0)|
3 Evaluated methods
We evaluate the top 10 methods among 17 participating methods in total , and use the Holi CNN from  as baseline, which shows state-of-the-art performance  and is easy to implement. Table 2 lists the methods and some of their key properties. We indirectly evaluate DeepPrior  and REN , which are components of rvhand , as well as DeepModel , which is the backbone of LSL . In the following we group methods based on different design choices.
2D CNN vs. 3D CNN. 2D CNNs are commonly used in 3D hand pose estimation [22, 2, 12, 41, 1, 20, 19, 54, 28, 59, 14]. Common pre-processing steps include cropping and resizing the hand volume, normalizing the depth values to [-1, 1]. For example, the input for slivorezzz  is the cropped depth image resized to 8080 pixels, while the input for THU VCLab  is a cropped depth image of size 9696 (see Table 2, third column). Recently, several methods use a 3D CNN [13, 10, 23, 53], where the input can be a 3D voxel grid [23, 53], or a projective D-TSDF volume . Ge et al.  projected the depth image onto three orthogonal planes and trained a 2D CNN for each projection, then fusing the results. They later proposed a 3D CNN  by replacing 2D projections with a 3D volumetric representation (projective D-TSDF volumes ). In the HIM2017  challenge, they propose an end-to-end trained 3D CNN . mks0601  proposes a 3D CNN to estimate the per-voxel likelihood for each hand joint. The network has has a encoder and decoder. NAIST_RV  propose a 3D CNN with a hierarchical branch structure, where the input is a 505050 3D grid.
|Method||Model||Input||Aug. range (s,,t)||3D||De||Hi||St||M||R|
|mks0601 ||3D CNN, per-voxel likelihood for each joint||888888 voxels||[0.8, 1.2] [-40,40] [-8,8]||✓||✓||✗||✗||✗||✓|
|slivorezzz ||RCN+ network  with 17 convolutional layers||8080||[0.7, 1.1] [0, 360] [-8,8]||✗||✓||✗||✗||✓||✓|
|oasis ||3D CNN with 8 conv layers and 3 fully connected layers||3D hand volume||random scaling*||✓||✓||✗||✗||✗||✗|
|THU_VCLab ||Pose-REN : REN  + cascaded + hierarchical.||9696||[0.9,1.1] [-45,45] [-5,5]||✗||✗||✓||✗||✓||✓|
|NAIST_RV ||3D CNN with 5 branches, one for each finger||505050 3D grid||[0.9,1.1] [-90, 90] [-15,15]||✓||✗||✓||✗||✗||✗|
|Vanora ||shallow CNN trained end-to-end||resized 2D||random scaling*||✗||✗||✗||✗||✗||✗|
|strawberryfg ||ResNet-152 + ||224224||None||✗||✗||✗||✓||✗||✓|
|rvhand ||ResNet  + REN  + Deep Prior ||192192||[0.9,1.1] [-90, 90] [-15,15]||✗||✗||✓||✓||✗||✓|
|mmadadi ||Hierarchical tree-like structured CNN ||192192||[random] [-30, 30] [-10,10]||✗||✗||✓||✓||✗||✗|
|LSL ||ScaleNet to estimate hand scale + DeepModel ||128128||[0.85,1.15] [0,360] [-20,20]||✗||✗||✗||✓||✗||✗|
|baseline||CNN with 3 conv. layers (Holi CNN of )||9696|
Detection-based vs. Regression-based. Detection-based methods [23, 22, 11, 41, 59] produce a probability density map for each joint. The method by slivorezzz  is an RCN+ network  (based on Recombinator Networks (RCN) ) with 17 layers and 64 output feature maps for all layers except the last one, which outputs a probability density map for each of the 21 joints. mks0601  use a 3D CNN to estimate per-voxel likelihood for each joint, and a simple CNN to estimate the correct center of mass from the cropped depth map. For training, 3D likelihood volumes were generated by placing normal distributions at the locations of hand joints.
Regression-based methods [2, 53, 28, 12, 20, 1, 19] directly map the depth image to the joint locations or the joint angle parameters of a hand model [38, 59]. rvhand  combines ResNet , Region Ensemble Network (REN) , and DeepPrior  to directly estimate the joint locations. LSL  used one network to estimate a global scale factor and a second network  to estimate all joint angles, which were fed to a forward kinematic layer to estimate the hand joints.
Hierarchical models divide the pose estimation problem into sub-tasks [2, 14, 20, 53, 1]. The evaluated methods divide the hand joints either by finger [20, 53], or by joint type [2, 14, 1]. mmadadi  designs a hierarchically structured CNN, dividing the convolution+ReLU+pooling blocks into six branches (one per finger with palm and one for palm orientation), each of which is then followed by a fully connected layer. The final layers of all branches are concatenated into one layer to predict all joints. NAIST_RV  choose a similar hierarchical structure of a 3D CNN, but use five branches, each to predict one finger with the palm. THU VCLab , rvhand  and REN  implicitly apply constraints for both fingers and joint-type (across fingers) in their multiple regions extraction step, each region contains a certain subset of joints. All regions are concatenated in the last fully connected layers to estimate the full hand pose.
Structured methods embed physical hand motion constraints into the model [19, 20, 28, 52, 59]. Structural constraints are included in the CNN model [28, 26, 19] or in the loss function [20, 52]. DeepPrior  learns a prior model and integrates it into the network by introducing a bottleneck in the last layer of the CNN. LSL  uses prior knowledge in DeepModel  by embedding a kinematic model layer into the CNN and estimated model parameters of a pre-defined fixed hand model. mmadadi  includes the structure constraints in its loss function, which incorporates physical constraints about natural hand motion and deformation. strawberryfg  applies a structure-aware regression approach, Compositional Pose Regression , and replaces the original ResNet-50 with ResNet-152. It uses phalanges instead of joints as pose representation and defines a compositional loss function that encodes long-range interaction between the phalanges.
Multi-stage methods propagate results from each stage to enhance the training of the subsequent stages [2, 22]. THU_VCLab  use REN  to predict an initial hand pose. In the following stages, feature regions on the feature maps are extracted under the guidance of the previous estimated hand pose to estimate the current pose. slivorezzz  has five stages: (1) 2D landmark estimation using an RCN+ network , (2) estimation of corresponding depth values by multiplying probability density maps with the input depth image, (3) inverse perspective projection of the depth map to 3D, (4) error compensation for occlusions and depth errors (a 3-layer network of residual block) and, finally (5) error compensation for noise (another 3-layer network of residual block).
Residual networks. ResNet  is adopted by many methods [1, 14, 22, 23, 41, 2]. mks0601  used residual blocks as main building blocks. strawberryfg  implement the Compositional Pose Regression method  by using ResNet-152 as basic network. slivorezzz  use two small residual blocks in its fourth and fifth stage.
The aim of this evaluation is to identify success cases and failure modes. We use both standard error metrics [29, 48] and new proposed metrics to provide further insights. We consider joint visibility, seen vs. unseen subjects, hand view point distribution, articulation distribution, and per-joint accuracy.
|Seen Visible||Unseen Visible||Seen Occ||Unseen Occ|
4.1 Single frame pose estimation
For this task, we evaluated eleven state-of-the-art methods (Table 2) and indirectly evaluated three methods that were used as components, DeepPrior , REN , and DeepModel . Figure 3 shows the error curves. We first analyze errors with regard to different sources, including unseen hand shape, joint occlusion, hand view point, and hand articulation.
As has been noted by [14, 26], data augmentation is beneficial, especially when training data is small. Augmentation is used in nine of the evaluated methods, see Table 2. Even though the top six performers all employ data augmentation, it is still difficult to generalize to new unseen hand shapes, see Table 3. There still exists a gap of around between seen and unseen subjects. However, some methods generalize better than others, e.g. slivorezzz performs the best on unseen subjects’ test data, even though it is not the best on seen subjects.
4.1.1 Error for all methods and best performance
In Figure 3 (top-left), we draw the success rate (based on per-frame average joint error ) against a varying threshold. The top performer (mks0601) estimates 70% of frames with a mean error of less than , and 20% of frames smaller than . Across all the evaluated methods, all of them achieved a success rate larger than 60% with average error smaller than 20 mm.
Throughout the challenge, by applying new model types, as well as by experimenting with data augmentation, optimization and initialization methods, the average error was reduced from to . For seen and unseen subjects, the mean error dropped from and to and , respectively. Considering typical finger widths of 10-, these methods are becoming applicable to scenarios like pointing or motion capture, but may still lack sufficient accuracy for fine manipulation that is critical in some UI interactions.
4.1.2 Annotation error
To quantify the annotation error, we selected poses, for which all methods achieved a maximum error  of less than . We denote these as Easy Poses, see Figure 2 for examples. The pose estimation task for these can be considered solved, as shown in Figure 2. The estimates of the top five methods are visually accurate and are close to the ground truth, see Figure 3 (top-left). We approximate the Annotation Error by the error on these poses. The error is with a std dev of . The sources include annotation inaccuracies, e.g. small differences of 6D sensor placement for different subjects, and uncertainty in the wrist joint location.
4.1.3 Analysis by occlusion and unknown subject
Average error for four scenarios: To analyze the results with respect to visibility and hand shape, we partition the joints in the test data into four groups: visible joints of seen hands, occluded joints of seen hands, visible joints of unseen hands, and occluded joints of unseen hands. Unseen hand shape and occlusions are responsible for a large proportion of errors, see Table 3 and Figure 3 (top-middle). The error for unseen subjects is significantly larger than for seen subjects. Moreover, the error for visible joints is smaller than for occluded joints. Based on the first group (visible, seen), we carry out a best-case performance estimate for the current state-of-the-art. For each frame of seen subjects, we first choose the best result from all methods, and calculate the success rate based on the average error for each frame, see Figure 3 (top-middle).
2D vs. 3D CNNs: We compare two hierarchical methods with similar structure but different representation. The bottom-left plot of Figure 3 shows mmadadi , which employs a 2D CNN, and NAIST_RV , using a 3D CNN. mmadadi and NAIST_RV have almost the same structure, but NAIST_RV  uses a 3D CNN while mmadadi  uses a 2D CNN. NAIST_RV  is better than mmadadi  for all four scenarios. Among the top 5 ranked methods, three are 3D CNNs, see Table 2.
Detection-based vs. regression-based methods: We compare the average of the top 3 detection-based methods with the average of the top 3 regression-based methods. In four scenarios, detection-based methods outperform regression-based methods, see the top-right plot of Figure 3. In the challenge, the top three methods are detection-based methods, see Table 2. Note that a similar trend can be seen in the field of full human pose estimation, where only one method in a recent challenge was regression-based .
Hierarchical methods: Hierarchical constraints can help in the case of occlusion. The hierarchical models in rvhand  and THU_VCLab  both have similar performance on visible and occluded joints. rvhand  even have better performance on occluded joints when the error threshold is smaller than , see the bottom-right plot of Figure 3. The underlying REN  module, which includes finger and joint-type constraints seems to be critical. Methods using only per-finger constraints, e.g., mmadadi  and NAIST_RV , generalize less well to occluded joints, see the bottom-left plot of Figure 3.
Structural methods: We compare four structured methods LSL , mmadadi , rvhand , and strawberryfg , see the bottom-middle plot of Figure 3. strawberryfg  and mmadadi  have higher success rates when the error threshold is below , while LSL  and rvhand  perform better for thresholds larger than . This indicates that embedding structural constraints in the loss function is a better choice than into the CNN layers. strawberryfg  performs the best, using constraints on phalanges rather than on joints.
Single- vs. multi-stage methods: Cascaded methods work better than single-stage methods, see the bottom-right plot of Figure 3. Compared to other methods, rvhand  and THU_VCLab  both embed structural constraints, employing REN as their basic structure. THU_VCLab  takes a cascaded approach to iteratively update results from previous stages, outperforming rvhand .
4.1.4 Analysis by number of occluded joints
Most frames contain joint occlusions, see Figure 4 (top plot). We detect occlusion of a joint by thresholding the distance between the joint’s depth annotation value and its re-projected depth value. A visible joint lies within a certain range behind the point cloud. As shown in Figure 4 (bottom plot), the average error decreases nearly monotonously with the increasing number of visible joints.
4.1.5 Analysis based on view point
The view point is defined as the angle between the palm and camera directions. The test data covers a wide range of view points for Single frame pose estimation task, see Figure 5 (top). View points in the [70, 120] range have a low mean error of . View points in the [0, 10] range have a significantly larger error due to the amount of self occlusion. View points in the [10, 30] range have an average error 15-. View point ranges of [30,70] and [120, 180] have a similar error of 10-. Third-view and egocentric view are typically defined by the hand facing toward or away from the camera, respectively. However, as shown in Figure 5, there is no clear separation by view point, suggesting uniform treatment of both cases. Note that slivorezzz  outperforms others with a margin of 2- on extreme view points, [150,180] due to their depth prediction stage.
4.1.6 Analysis based on articulation
We evaluate the effect of hand articulation, measured as the average of 15 finger flexion angles, on estimation accuracy, see Figure 6. To reduce the influence from other factors such as view point, we select frames with view point angles within the range of [70, 120]. We evaluate the top-five performers, see Figure 6 (bottom plot). For articulation angles smaller than 30 degrees, the performance is , when the average articulation angle get larger, the performance dropped to 9-10 mm (in the range of [35, 70]). When the average articulation angles is larger than 70, almost making a fist, the mean error increases to over .
4.1.7 Analysis by joint type
Based on the visibility of each joint and whether the subject is in the training data or not, we cluster joints into four groups: visible joints of seen subjects, occluded joints of seen subjects, visible joints of unseen subjects and occluded joints of unseen subjects. We report the top five performers, see Figure 9. For the easiest case (the visible joints of seen subjects), all 21 joints have a similar average error of 6-. For seen subjects, along the kinematic hand structure from the wrist to finger tips, occluded joints have increasingly larger errors, reaching in the finger tips. Visible joints of unseen subjects have consistently larger errors (10-) than that of seen subjects. Occluded joints of unseen subjects have the largest errors, with a relatively smaller error of the palm, and larger errors of finger tips (24-). We draw two conclusions: (1) all the top performers have difficulty in generalizing to new hand shapes, (2) occlusions have more effect on finger tips than other joints. An interesting observation is that middle and ring fingers tend to have smaller error in MCP and PIP joints than other fingers. One reason may be that the motion of these fingers is more restricted. The thumb’s MCP joint has a larger error than for other fingers, because it tends to have more discrepancy among different subjects.
|slivorezzz_track ||scanning window + post-processing + pose estimator ||10.5|
|NAIST_RV_track ||hand detector  + hand verifier + pose estimator ||12.6|
|THU_VCLab_track ||Estimation  with the aid of tracking + re-initialization ||13.7|
4.2 Hand pose tracking
In this task we evaluate three state-of-the-art methods, see Table 4 and Figure 7. Discriminative methods [22, 53, 2] break tracking into two sub-tasks: detection and hand pose estimation, sometimes merging the sub-tasks . Based on the detection methods, 3D hand pose estimation can be grouped into pure tracking [29, 22], tracking-by-detection , and combination of tracking and re-initialization , see Table 4.
Pure tracking: slivorezzz_track estimate the bounding box location by scanning windows based-on the previous frame’s result, including a motion estimate. Hand pose within the bounding box is estimated using slivorezzz .
Tracking-by-detection: NAIST_RV_track is a tracking-by-detection method with three components: hand detector, hand verifier, and pose estimator. The hand detector is developed from U-net  to predict a binary hand-mask, which, after verification, is passed to the pose estimator NAIST_RV . If verification fails, the previous frame’s result is chosen.
Hybrid tracking and detection: THU_VCLab_track  make use of the previous tracking result and the current frame’s scanning window. The hand pose of previous frame is used as a guide to predict the hand pose in the current frame. The previous frame’s bounding box is used for the current frame. During fast hand motion, Faster R-CNN  is used for re-initialization.
Detection accuracy: We first evaluate the detection accuracy by evaluating the bounding box overlap, i.e., IoU (Intersection over Union) of the detection and ground truth bounding boxes, see Figure 7 (middle). Overall, slivorezzz_track is better than THU_VCLab_track, which is better than NAIST_RV_track. Pure detection methods have a larger number of false negatives, especially when multiple hands appear in the scene, see Figure 7 (left). There are 72 and 174 missed detections (IoU of zero), for NAIST_RV_track and THU_VCLab_track, respectively. By tracking and re-initialization, THU_VCLab_track achieves better detection accuracy overall. slivorezzz_track using motion estimation and single-frame hand pose estimator has the lowest error.
Tracking accuracy is shown in Figure 7 (right). Even through THU_VCLab performs better than NAIST_RV in the Single frame pose estimation task, NAIST_RV_track performs better on the tracking tasks due to per-frame hand detection.
|NAIST_RV_obj ||Hand-object segmentation CNN + pose estimation ||25.0|
|THU_VCLab_obj ||Hand-object segmentation (intuitive) + pose estimation ||29.2|
|rvhand_obj ||Hand-object segmentation CNN + pose estimation ||31.3|
|slivorezzz_obj ||Two RCNs: Feature maps of first are used in the second RCN’s stage 2.||32.4|
|baseline_obj ||Same as baseline ||46.1|
4.3 Hand object interaction
For this task, we evaluate five state-of-the-art methods, see Table 5 and Figure 8. Compared to the other two tasks there is significantly more occlusion, see Figure 4 (top). The approach presented in, baseline_obj , applies the network directly to this task, resulting in large errors of . Methods explicitly handling occlusion achieve lower errors in the range of 25-: (1) NAIST_RV_obj  and rvhand_obj  segment the hand area from the object using a network. (2) THU_VCLab_obj  removes the object region from cropped hand images with image processing operations . (3) slivorezzz_obj  modify their original network to infer the depth values of 2D keypoint locations. We provide a Best Performance for this task as the Baseline trained on extra data from FHAD  data set. The training data is from the same two subjects. See the black curve in Figure 8 (left).
The current state-of-the-art methods have difficulty generalizing to the hand-object interaction scenario. But NAIST_RV_obj  and rvhand_obj  have similar performance for visible joints and occluded joints, indicating CNN based segmentation can preserve better structure than image processing operations.
5 Discussions and conclusions
The analysis of 11 state-of-the-art methods (one baseline Holi CNN from  and the top 10 methods among 17 participating methods in total) from the HIM2017  challenge provides insights into the current state of 3D hand pose estimation and suggestions for the next generation methods.
(1) 3D volumetric representation used with a 3D CNN shows high performance, possibly by better capturing the spatial structure of the input depth data.
(2) Detection-based methods tend to outperform regression-based methods, however, regression-based methods can achieve good performance using explicit spatial constraints. Making use of richer spatial models, e.g. bone structure , helps further. Regression-based methods perform better in extreme view point cases , where severe occlusion occurs.
(3) While joint occlusions pose a challenge for most methods, explicit modeling of structure constraints and spatial relation between joints can significantly narrow the gap between errors on visible and occluded joints .
(4) Discriminative methods still generalize poorly to unseen hand shapes. Data augmentation and scale estimation can only solve proportional hand shape change, but struggles with per-phalange variations. Integrating hand models with better generative capability may be a promising direction.
(5) Isolated 3D hand pose estimation achieves low mean errors () in the view point range of [40, 150] degrees. However, it is far from being solved for extreme view points, e.g. view point range of [0,10], where the hand is facing away from the camera. Multi-stage methods  tend to perform better in these cases.
(6) In hand tracking, current discriminative methods divide the problem into two sub-tasks: detection and pose estimation, without using the hand shape provided in the first frame. Hybrid methods may work better by using the provided hand shape.
(7) The current methods perform well on single hand pose estimation when trained on a million-scale dataset, but have difficulty in generalizing to hand-object interaction. Two directions seem promising, (a) designing better hand segmentation methods (e.g. along with modeling object) and (b) training the model with large datasets containing hand-object interaction.
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