Monocular 3D hand reconstruction can facilitate a broad array of applications in human-computer interaction, augmented reality, virtual telepresence, or sign language recognition. Still, the current state-of-the-art methods do not always generalize to samples captured in a non-laboratory environment. Our work is aimed at operating in the wild, and is therefore trained primarily on a large-scale dataset of hand action images gathered from YouTube videos; at the same time our system largely outperforms the current state-of-the-art as evaluated on public benchmarks.
Even though our method can deliver a dense hand mesh, we outperform systems that try to solve a more narrow task of estimating coordinates of a sparse set of keypoints usually corresponding to hand joints and fingertips. The standard approach is to regress heatmaps of 2D landmarks and find a matching 3D pose [62, 40, 37]. More recent works, that also focus on full-blown mesh reconstruction, rely on fitting deformable models to estimated landmarks, effectively exploiting a prior distribution in 3D shape space to perform 3D reconstruction [5, 60, 2, 20]. Such models are either parameterized by angles of rotation and weights of a linear deformation function  or latent features learned from data [17, 27]. We follow the latter approaches, which have proven to be the most promising, but we substantially improve their accuracy and robustness.
Our first key contribution is a weakly-supervised training method for 3D mesh reconstruction. We introduce a fast and accurate method for dataset generation from unlabeled images, that relies on fitting a hand model to 2D keypoint detections while respecting a prior on rotation angles. Our data generation algorithm processes a sequence of YouTube videos, generating a curated dataset of images with 3D hand meshes. We have selected a filtered subset of 50,000 training annotations comprising hundreds of subjects performing a wide variety of tasks.
Beyond the data-driven advancements, we also obtain a substantial improvement by using a spatial mesh convolution method that is based on neighbourhood ordering. We train our end-to-end system from scratch by using an objective function that merely enforces the reconstruction of a hand mesh that is aligned with the image.
Our system attains 50% lower hand pose estimation error compared to the currently best model on the in-the-wild scenario, which is our main focus. It also outperforms previous methods on hand pose estimation in controlled environments, without tuning or overfitting on a particular dataset.
To summarise, our contributions are as follows:
We introduce an automated approach to generate training data from unannotated images for 3D hand reconstruction and pose estimation. The proposed method is scalable and does not require large computational resources.
We release the dataset of 50,000 meshes aligned with frames selected from over a hundred YouTube videos.
We propose a simple loss function for mesh reconstruction that allows training neural networks with no intermediate supervision.
We largely outperform the current state-of-the-art on pose estimation and mesh reconstruction tasks, while using a simple and single-shot encoder-decoder architecture.
We focus our attention on the hand pose estimation task using only RGB images as input. This is significantly more challenging than hand pose estimation when a depth sensor is also available [58, 47, 16, 38, 54]. Hand mesh recovery is a more general task as it aims at reconstructing a relatively dense point cloud and estimates both the pose and subject-specific shape of the target object. We categorize a method as belonging to the latter category if it allows generation of different subject-specific shapes.
Hand Pose Estimation
Multiview bootstrapping  is an iterative improvement technique where the initial annotations of a hand from multiple views, obtained by executing a 2D keypoint detector, are triangulated in 3D and reprojected for training in the next iteration if they meet the quality criteria. This system is part of the OpenPose framework  that we use for weak supervision.
incorporate depth regularization to supervise 3D pose estimation represented by angles of rotation. In contrast, variational approaches learn a shared latent space by an alternated training between variational autoencoders, disentanglement of the latent representation , or the alignment of latent spaces . This allows for sampling outputs in any of the learned modalities. Gao et al.  address the problem of estimating the pose of hands interacting with objects in a variational framework. Generative networks can also be used to transfer synthetic images into a space of real-world images  by modifying a cycle adversarial loss  to include a geometric consistency term that ensures pose-preservation during an image translation. Tekin et al.  estimate hand and object poses with a single-shot network.
Recently, Cai et al.  proposed to build a spatio-temporal graph and use graph convolutions in the spectral domain to estimate a 3D pose.
Hand Mesh Recovery
is a hand model parameterized by angles of rotations specified for each joint in the kinematic tree and blend weights of a linear function that models the shape of the person. Different articulations are obtained by applying linear blend skinning which interpolates rotations matrices assigned to the joints to transform a vertex according to the angles. Additionally, pose-dependent corrective offsets are learned to address the loss of volume caused by the skinning method. Recently, numerous works have been proposed to find a 3D pose by regressing MANO and camera parameters.
Boukhayma et al.  regress these parameters from an image and heatmaps obtained from OpenPose. The method is evaluated on images in the wild but the error is relatively high as we show in the Evaluation section. The works by Zhang et al.  and Baek et al.  introduce methods that iteratively regress model parameters from the heatmaps. Hasson et al.  predict MANO parameters and reconstruct objects the hands interact with.
Ge et al.  use graph convolutions in the spectral domain to recover a hand mesh. The system includes a heatmap regression module and, in case of real-world images, is supervised with 2D keypoints, depth maps, and meshes approximated with a pretrained model from ground truth heat maps. Kulon et al. 
reconstruct a hand from an image by regressing a latent vector of a pretrained mesh generator and estimating a camera projection.
In this paper, we introduce an architecturally simpler approach that significantly outperforms prior works by computing loss on meshes with points localized in the image coordinate system and leveraging spatial mesh convolutions.
Body Mesh Recovery
Approaches based on regressing MANO parameters described in the previous subsection originate from works on body mesh recovery [3, 28, 22, 48, 42, 50, 41, 39, 21, 55, 25]. Our work is similar to Kolotouros et al , who encode an image and concatenate its features with coordinates of a template mesh in a canonical pose. The resulting graph is passed through a sequence of spectral graph convolutions to recover the body model corresponding to the image. In contrary, we decode a mesh directly from the image encoding and apply spatial convolutions with pooling layers.
Geometric Deep Learning
Our interest lies in applying deep learning to learn invariant shape features in a triangular mesh. To this end, spectral approaches [8, 12, 23, 29]
express convolutions in the frequency domain. Spatial convolutions define local charting[34, 4, 36] that seems more suitable for learning on manifolds or designing meaningful pooling functions. Alternatively, convolution operators can be defined with an attention mechanism to weight the neighbour selection [52, 51].
In terms of applications similar to our method, Ranjan et al.  use fast spectral convolutions  to find a low-dimensional non-linear representation of the human face in an unsupervised manner. Kulon et al.  show that autoencoders can be used to learn a latent representation of 3D hands. Recently, a spiral operator  has been incorporated into a convolutional framework to train a deformable model of the human body  or solve mesh correspondence and classification tasks . In this paper, we apply spiral filters to generate hands directly from an image encoding.
3 YouTube Hands-in-the-Wild Dataset
Despite the wealth of possible applications, there are no datasets for monocular 3D hand reconstruction from RGB images in-the-wild. The only existing collection of images of hands captured in a non-laboratory environment  contains less than 3000 samples with a manual annotation of 2D points.
In order to train a neural network for the 3D reconstruction of hands across different domains, we have built a system for fast and automated dataset generation from YouTube videos, providing us with a diverse set of challenging hand images. Instead of annotating them manually, we use a weakly-supervised approach that first detects keypoints using OpenPose, and then lifts them into 3D shapes by iteratively fitting a 3D deformable model. While the accuracy of the trained model could be bounded by the performance of OpenPose, we show that it results in a state-of-the-art 3D hand reconstruction and pose estimation system when evaluated on external datasets with manual annotations.
3.1 3D Shape Representation
Our approach relies on fitting the MANO model in tandem with a prior on angles of rotation. MANO predicts vertices on the hand surface and joints through a differentiable function that maps shape and pose parameters into an instance of the model represented by a mesh:
where depends on the training procedure and is a set of learned model parameters that we omit in a further discussion. Moreover, we have camera parameters for scaling the model and to translate it. Global orientation is modeled by the first row of .
Rather than model the joint angles as free variables, which can lead to impossible estimates during optimization, we constrain them to lie in the convex hull of some pre-computed cluster centers. For joint we use Euler-angle clusters
obtained via k-means and express any angle as follows:
While constraining the final angle estimate to take plausible values, at the same time this expression allows us to optimize over unconstrained variables . We represent all constrained angles in terms of a parameter matrix , while allowing global orientation to be unrestricted.
This simple approach requires only a small dataset of angles, unlike VAE priors; restricts pose space to plausible poses, unlike PCA priors that are characterized by unrealistic interpolation; and allows fitting unseen poses. However, it does not model pairwise dependencies, which we leave to future work.
3.2 Parametric Model Fitting
Our supervision comes in the form of 2D landmarks extracted by OpenPose. We define a fitting procedure that tries to find a matching 3D pose from the MANO mesh through a sparse matrix that regresses from model vertices to joint and fingertip positions, delivering the hand pose :
We fit the model to 2D annotations by minimizing the following objective:
consisting of a 2D reprojection term , a bone length preservation cost , and a regularization term .
In particular, the joint error term minimizes the distance between 2D joints
where are 2D detector predictions and is the intrinsic camera projection to 2D, initialized as in the approach of Pavlakos et al. . is an experimentally chosen mask that amplifies the influence of fingertips by and wrist by and reduces influence of the metacarpophalangeal (MCP) joints (base of each finger) that are often inaccurately annotated by .
The bone loss ensures that the length of each edge in the hand skeleton tree is preserved, i.e.
The regularization term
penalizes deviations from the mean pose to ensure realistic deformations. The hyperparametersand were chosen experimentally.
We use the Adam optimizer with different learning rates for camera, pose, and shape parameters (, , respectively) and small learning rate decay (multiplicative factor of ) after each 500 iterations. We start with 1,500 iterations optimizing over camera parameters and global orientation where the joints set is reduced to a wrist and MCP joints excluding thumb. Afterwards, we perform 2,500 iterations over all parameters. We fit 4,000 samples per batch on GeForce RTX 2080 Ti which takes on average 10 min.
3.3 Automated Data Collection
The data collection system iterates a list of YouTube links, downloads a video, extracts frames, runs OpenPose, fits MANO to each frame, and selects a small subset of filtered samples. The depth of the projected mesh is proportional to the ratio of standard deviations betweencoordinates of the projected mesh and its world position.
Filtering of plausible samples is performed by thresholding total OpenPose confidence score, per-joint confidence score, and the mean squared error between projected MANO joints and OpenPose predictions normalized by the distance from the camera.
To create our YouTube dataset, we process 102 videos for the training set and randomly select at most 500 samples per video that meet the threshold conditions. Most of the samples cover sign language conversations performed by people of wide variety of nationality and ethnicity. Some videos include, for example, a hundred people signing a common phrase to a webcam all over the world. The validation and test sets cover 7 videos with an empty intersection of subjects with the training set. We selected test videos to be diverse and challenging including conversations captured outdoor, dance poses, American, Australian, and Taiwanese sign languages (Figure 6, left half). Additionally, we run our system on the COCO dataset  and extract 7,048 hand images for training. The combined training set contains 54,173 samples and validation and test sets count 1,525 images each.
4 Hand Reconstruction Network
We propose a simple encoder-decoder system, as demonstrated in (Figure 1), that directly reconstructs the mesh in image coordinates. We use a spatial convolutional mesh decoder , which we experimentally have shown to be superior with respect to alternative decoding strategies. In the following subsections we explain the spatial operator of choice and the upsampling method which constitute the building blocks of our decoder.
4.1 Spiral Operator
In our decoder we use the spiral patch operator for constructing spatial neighborhoods. Lim et al.  define a spiral selection of neighbours for a center vertex by imposing the order on elements of the -disk obtained in the following way:
where is the set of all vertices adjacent to any vertex in the set . The spiral patch operator  is then the ordered sequence
As orientation, we follow the clockwise direction from each vertex and randomly initialize the first neighbour of . An example spatial configuration is demonstrated in Figure 2. The spiral convolution of features with the kernel can then be defined by
where the spiral length is fixed for each vertex in the same layer of a neural network.
We create a hierarchy of meshes with the number of vertices reduced by the factor of 2 at each stage of the decoder. We contract vertex pairs based on quadric error metrics  that allow the coarse vertices to be a subset of the denser mesh. We project each collapsed node into the closest triangle in the downsampled mesh  and use the barycentric coordinates of the projected vertex to define interpolation weights for the upsampling matrix.
More specifically, we project a vertex discarded during contraction into the closest triangle of the downsampled mesh obtaining . We compute its barycentric coordinates such that and . The upsampling matrix , for which and hold, is formed by setting , , , and for .
The resulting topology hierarchy obtained using the sampling strategy is shown in Figure 4. The number of vertices at each layer .
Given an image crop centered around the hand, we embed it into a latent vector with 64 parameters. The decoder, , takes the embedding as input and produces a mesh . We use a standard ResNet-50 network as the encoder, . The architecture of the spiral decoder is detailed in Figure 3
. If the spiral sequence is shorter than required due to erroneous triangulation or mesh border, we pad it with a node initially centered at 0. We only considerfor
The loss function consists of the L1 vertex reconstruction term and edge length preservation component
for ground truth meshes and set of edges . Hyperparameters are set to and . There is no explicit pose estimation loss because we found it to have no effect. The joint coordinates are obtained during evaluation from a mesh as in Equation 3.
The network is trained with the Adam optimizer with learning rate
for 150 epochs. Learning rate decay with factor
occurs after 90-th and 120-th epoch. The images are normalized with the mean and standard deviation computed from the ImageNet training set and output meshes are normalized based on statistics computed from the subset of our training data. We augment the data with random image crops and transformations. The data augmentation is necessary for generalization to real world examples where the input images cannot be cropped based on the ground truth annotations. We train the network for 2 days on a single GeForce RTX 2080 Ti with a batch size 32 and crop size of.
Mesh reconstruction methods have started gaining popularity only recently and therefore there are no well-established benchmarks for hand recovery. To show the robustness of our method, we evaluate it on popular hand pose estimation datasets. Moreover, we show the hand reconstruction performance on the FreiHAND dataset and conduct a self-comparison study of different mesh decoders on our YouTube dataset. Lastly, we show that both major contributions, neural network architecture and data annotation system, result in the state-of-the-art performance in the absence of one or other.
The MPII+NZSL (MPII) dataset  contains in the wild images collected by manually annotating two publicly available datasets with 2D landmarks: the MPII Human Pose dataset  showing every-day human activities and a set of images from the New Zealand Sign Language Exercises . It includes blurry, low-resolution, occluded, and interacting with objects hand images what makes the dataset particularly challenging. Training and test sets count 1,912 and 846 samples.
The Rendered Hand Pose Dataset (RHD) consists of 41,258 training and 2,728 testing samples of rendered characters . It has been commonly used for RGB-based hand pose estimation due to its large size and challenging viewpoints.
FreiHAND is a recently released dataset with 130,240 training images . It is the only dataset that contains 3D mesh annotations with backgrounds artificially blended in place of the green screen. The test set consisting of 3,960 samples was collected without the green screen in a controlled outdoor and office environment. It contains difficult poses with object interactions and varying lighting. The test set annotations are not available and evaluation is performed by anonymously submitting predictions to the online competition.
Monocular Total Capture (MTC) is a recent human motion dataset containing body, hands, and face annotations of 40 subjects . The dataset was collected in a laboratory environment posing a risk of domain overfitting; however, due to its diverse and mostly accurate annotations it can be used to learn a robust prior on 3D hand shapes. We select a subset of 30,000 images for training filtered to reduce similar and mean poses.
Stereo Hand Pose Tracking Benchmark (STB) contains 15,000 training images of a single subject performing random poses and counting gestures with 5 different backgrounds and a test set of 3,000 images with the same background . The dataset has been solved in recent publications  with a high error tolerance due to inaccurate ground truth annotations.
Finally, our YouTube dataset is described in Section 3.3.
For each of the datasets, beside FreiHAND that provides vertex annotations, we fit the MANO model following the optimization approach described in Section 3.2. In case of 3D annotations, we only change the intrinsic camera to the identity function in Equation 5 and then we project the output mesh based on dataset camera parameters. The depth is reconstructed as before.
To evaluate hand pose estimation and mesh reconstruction performance, we measure the average euclidean distance (Pose/Mesh Error), the percentage of correct points (2D/3D PCK) for different thresholds, and the Area Under Curve (AUC) for PCK. For 3D benchmarks, we measure the error after rigid alignment. Before computing a 2D error, we orthographically project the estimated 3D pose. For self-comparison study, we report the mean absolute error (MAE) in addition to Mesh Error.
5.3 Hand Pose Estimation
Figure 5.2 shows evaluation results on the MPII dataset. Our system significantly outperforms other approaches, halving the Pose Error of the leading MANO-based method and improving AUC by 0.21 points.
Figure 5.2 shows 3D pose estimation results on RHD. This is a very popular benchmark on which we also outcompete existing methods by a significant margin.
The proposed system also achieves a superior performance on the FreiHAND dataset as proved in Table 1. Notably, most of the competing methods in this benchmark rely on MANO parameters regression.
|Ours||MANO Fit ||MANO CNN ||Boukh. et al. ||Hasson et al. ||Mean Shape |
|Pose Error [mm]||0.84||1.37||1.1||3.5||1.33||1.71|
|Mesh Error [mm]||0.86||1.37||1.09||1.32||1.33||1.64|
5.4 Mesh Reconstruction
We evaluate the quality of mesh reconstructions on the FreiHAND dataset (Table 1
). In addition to the standard metrics, we report the F-score at a given threshold(24].
Qualitative results on the in the wild datasets can be found in Figure 6.
Table 2 and Table 3 show the pose estimation and mesh reconstruction error on different types of mesh decoders. To perform broad hyperparameter search that ensures fair comparison of the baseline methods, we use a reduced training dataset, corresponding to row 6 in Table 5, to speed-up optimization.
Spiral refers to the proposed approach, Spectral follows the CoMA decoder  implemented as in Kulon et al.  but with no camera estimation branch. Spiral GMM is the spiral implementation of  which incorporates decoder pretraining that is fixed afterwards and a camera estimation branch. Spiral GMM, tuned is a fine-tuned version obtained by resuming training of all parameters of the Spiral GMM. Finally, MANO is similar to Boukhayma et al.  but with the loss function on mesh vertices instead of joints. We observe that our edge length preservation loss term stabilizes MANO shape parameters regression that in prior works was addressed by imposing a large L2 penalty  to mitigate divergence.
We find that a spatial method outperforms a spectral approach. Moreover, end-to-end training with loss on meshes in the image coordinate system is better than pretraining in the canonical frame and estimating camera parameters.
|Decoder Type||RHD [mm]||MPII [px]||YouTube [mm]|
|Spiral GMM, tune||11.121||10.117||11.799|
|Decoder Type||MAE [mm]||
|Spiral GMM, tune||8.380||18.057|
5.6 Comparison With Iterative Fitting
|Model||MPII [px]||MPII Filtered [px]|
Our system is supervised with meshes iteratively aligned with keypoint predictions. Naturally the question arises whether the network performance is bounded by the performance of iterative fitting. We compare both approaches on the only existing manually annotated in the wild dataset. Additionally, we select 390 out of 846 samples that were filtered according to sanity checks described in Section 3.3.
Based on Table 4 we observe that our system performs slightly worse to iterative fitting on a filtered dataset. This is expected as these are samples on which the keypoint estimator has very high confidence. Importantly, the network performs much better when the whole dataset is considered.
The network is also characterized by the strikingly faster inference time. Specifically, the reported timing (Section 3.2) for iterative fitting of 4000 samples / 10 minutes is with a batch size of 4000. A single sample takes 110 seconds (GPU) or 70 seconds (CPU) for fitting to 2D annotations, exluding keypoint predictions. By contrast, our network inference time is 60 FPS (GeForce RTX 2080 Ti).
5.7 Dataset and Method Ablation
In Table 5, we evaluate the system trained on different combinations of datasets. By comparing rows 1-3, we observe that training a mesh reconstruction system on our dataset leads to better results than training on other outdoor datasets. Rows 5-8, show that the best results are obtained when our dataset is included during training, even when adding it to the union of all other datasets. We also observe state-of-the-art performance on RHD and MPII without recent datasets such as YouTube, FreiHAND, MTC proving the efficiency of the proposed neural network (Row 4). Similarly, Table 2 shows that the MANO-based method trained with the novel dataset and mesh loss obtains better performance than prior MANO-based implementations indicating the importance of the contributed data collection system.
We have shown that a simple encoder-decoder architecture can obtain superior performance on mesh reconstruction and pose estimation tasks given an appropriately defined objective function. Furthermore, we have proposed an approach for automated data collection that allows adapting the system to non-laboratory domains and additionally improves results on common benchmarks. We believe that these findings justify looking further into mesh generative models for human modeling, as well as other weakly- and self-supervised methods that alleviate the need of acquiring 3D ground truth for humans in unconstrained environments.
M.B. was partially supported by the ERC Consolidator Grant No. 724228 (LEMAN). S.Z. was partially funded by the EPSRC Fellowship DEFORM (EP/S010203/1) and an Amazon AWS Machine Learning Research Award. D.K. was funded by a PhD scholarship.
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