The localization of facial landmark points, such as eyebrows, eyes, nose, mouth and jawline, is one of the core computational components in visual face analysis, and is applied in a gamut of applications: face recognition , face verification , and facial attribute inference , to name a few. Robust and accurate localization entails difficulties due to varying face poses, illumination, resolution variations, and partial occlusions, as depicted in Fig. 1.
Classical face localization schemes such as Active Appearance Models (AAM)  and Active shape models (ASM)  apply generative models aiming to learn a parametric statistical model of the face shape and its gray-level appearance in a training phase. The model is applied in test time to minimize the residual between the training image and the synthesized model. Parametric shape models such as Constrained Local Models (CLM) , utilize Bayesian formulations for shape constrained searches, to estimate the landmarks iteratively. Nonparametric global shape models  apply SVM classification to estimate the landmarks under significant appearance changes and large pose variations.
Regression based approaches [10, 11] learn high dimensional regression models that iteratively estimate landmarks positions using local image features, and showed improved accuracy when applied to in-the-wild face images. Such schemes are initiated using an initial estimate and are in general limited to yaw, pitch and head roll angles of less than .
Following advances in object detection, parts-based models were applied to face localization [12, 13] where the facial landmarks and their geometric relationships are encoded by graphs. Computer vision was revolutionized by Deep Learning-based approaches that were also applied to face localization [14, 15, 16], yielding robust and accurate estimates. Convolutional neural networks (CNNs) extract high level features over the whole face region and are trained to predict all of the keypoints simultaneously, while avoiding local minima. In particular, heatmaps were used in CNN-based landmark localization schemes following the seminal work of Pfister et al. , extended by the iterative formulation by Belagiannis and Zisserman .
In this work we propose a novel Deep Learning-based framework for facial landmark localization that is formulated as a Cascaded CNN (CCNN) consisting of dual cascaded heatmaps and regression subnetworks. A outline of the architecture of the proposed CNN is depicted in Fig. 2, where after computing the feature maps of the entire image, each facial landmark is coarsely localized by a particular heatmap, and all localizations are refined by regression subnetworks. In that we extend prior works [17, 18] where iterative heatmaps computations were used, without the additional refinement subnetwork proposed in our work. The heatmaps are estimated using a Cascaded Heatmap subnetwork (CHCNN) consisting of multiple successive heatmap-based localization subnetworks, that compute a coarse-to-fine estimate of the landmark localization. This localization estimate is refined by applying the Cascaded Regression CNN (CRCNN) subnetwork. The cascaded layers in both the CHCNN and CRCNN are non-weight-sharing, allowing to separately learn a particular range of localizations. The CCNN is experimentally shown to compare favourably with contemporary state-of-the-art face localization schemes. Although this work exemplifies the use of the proposed approach in the localization of facial landmarks, it is of general applicability and can be used for any class of objects, given an appropriate annotated training set.
Thus, we propose the following contributions:
First, we derive a face localizations scheme based on CNN-based heatmaps estimation and refinement by a corresponding regression CNN.
Second, both heatmap estimation and regression are formulated as cascaded subnetworks that allow iterative refinement of the localization accuracy. To the best of our knowledge, this is the first such formulation for the face localization problem.
Last, the proposed CCNN framework is experimentally shown to outperform contemporary state-of-the-art approaches.
This paper is organized as follows: Section 2 provides an overview of the state-of-the-art techniques for facial landmark localization, while Section 3 introduces the proposed CCNN and its CNN architecture. The experimental validation and comparison to state-of-the-art methods is detailed in Section 4. Conclusions are drawn in Section 5
2 Related work
The localization of facial landmarks, being a fundamental computer vision task, was studied in a multitude of works, dating back to the seminal results in Active Appearance Models (AAM)  and Constrained Local Models (CLM)  that paved the way for recent localization schemes. In particular, the proposed scheme relates to the Cascaded Shape Regression (CSR),  and Deep Learning-based [15, 14, 20, 21, 22] models.
CSR schemes localize the landmark points explicitly by iterative regression, where the regression estimates the localization refinement offset using the local image features computed at the estimated landmarks locations. Such schemes are commonly initiated by an initial estimate of the landmarks based on an average face template, and a bounding box of the face detected by a face detector, such as Viola-Jones . Thus, the Supervised Descent Method by Xiong and De-la-Torre 
learned a cascaded linear regression using SIFT features computed at the estimated landmark locations. Other schemes strived for computational efficiency by utilizing Local Binary Features (LBF) that are learnt by binary trees in a training phase. Thus, Ren et al. in  proposed a face alignment technique achieving 3000 fps by learning highly discriminative LBFs for each facial landmark independently, and the learned LBFs are used to jointly learn a linear regression to estimate the facial landmarks’ locations.
Chen et al.  applied random regression forests to landmark localization using Haar-like local image features to achieve computational efficiency. Similarly, a discriminative regression approach was proposed by Asthana et al.  to learn regression functions from the image encodings to the space of shape parameters. A cascade of a mixture of regressors was suggested by Tuzel et al. , where each regressor learns a regression model adapted to a particular subspace of pose and expressions, such as a smiling face turned to the left. Affine invariance was achieved by aligning each face to a canonical shape before applying the regression.
A parts-based approach for a unified approach to face detection, pose estimation, and landmark localization was suggested by Zhu and Ramanan , where the facial features and their geometrical relations are encoded by the vertices of a corresponding graph. The inference is given by a mixture of trees trained using a training set. An iterative coarse-to-fine refinement implemented in space-shape was introduced by Zhu et al. , where the initial coarse solution allows to constrain the search space of the finer shapes. This allows to avoid suboptimal local minima and improves the estimation of large pose variations.
Deep Learning was also applied to face alignment by extending regression-based schemes for face alignment. The Mnemonic Descent Method by Trigeorgis et al.  combines regression as in CSR schemes, with feature learning using Convolutional Neural Networks (CNNs). The image features are learnt by the convolution layers, followed by a cascaded neural network that is jointly trained, yielding an end-to-end trainable scheme.
in a coarse-to-fine scheme, using successive stacked autoencoders. The first subnetwork predicts an initial estimate of the landmarks utilizing a low-resolution input image. The following subnetworks progressively refine the landmarks’ localization using the local features extracted around the current landmarks. A similar CNN-based approach was proposed by Shi et al., where the subnetworks were based on CNNs, and a coarse face shape is initially estimated, while the following layers iteratively refine the face landmarks. CNNs were applied by Zhou et al.  to iteratively refine a subset of facial landmarks estimated by preceding network layers, where each layer predicts the position and rotation angles of each facial feature. Xiao et al. in  introduced a cascaded localization CNN using cascaded regressions that refine the localization progressively. The landmark locations are refined sequentially at each stage, allowing the more reliable landmark points to be refined earlier, where LSTMs are used to identify the reliable landmarks and refine their localization.
A conditional Generative Adversarial Network (GAN) was applied by Chen et al. 
to face localization to induce geometric priors on the face landmarks, by introducing a discriminator that classifies real vs. erroneous (“fake”) localizations. A CNN with multiple losses was derived by Ranjan et al. in for simultaneous face detection, landmarks localization, pose estimation and gender recognition. The proposed method utilizes the lower and intermediate layers of the CNN followed by multiple subnetworks, each with a different loss, corresponding to a particular tasks, such as face detection etc. Multi-task estimation of multiple facial attributes, such as gender, expression, and appearance attributes was also proposed by Zhang et al. , and was shown to improve the estimation robustness and accuracy.
Multi-task CNNs with auxiliary losses were applied by Sina et al.  for training a localization scheme using partially annotated datasets where accurate landmark locations are only provided for a small data subset, but where class labels for additional related cues are available. They propose a sequential multitasking scheme where the class labels are used via auxiliary losses. An unsupervised landmark localization scheme is also proposed, where the model is trained to produce equivalent landmark locations with respect to a set of transformations that are applied to the image.
Pfister et al.  introduced the use of heatmaps for landmark localization by CNN-based formulation. It was applied to human pose estimation in videos where the landmark points marks body parts, and optical flow was used to fuse heatmap predictions from neighboring frames. This approach was extended by Belagiannis and Zisserman by deriving a cascaded heatmap estimation subnetwork, consisting of multiple heatmap regression units, where the heatmap is estimated progressively such that each heatmap regression units received as input its predecessor’s output. This school of thought is of particular interest to our work that is also heatmaps-based, but also applies a cascaded regression subnetwork that refines the heatmap estimate.
Bulat and Tzimiropoulos  applied convolutional heatmap regression to 3D face alignment, by estimating the 2D coordinates of the facial landmarks using a set of 2D heatmaps, one per landmark, estimated using a CNN with an regression loss. Another CNN is applied to the estimated heatmaps and the input RGB image to estimate the coordinate. A scheme consisting of two phases was proposed by Shao et al.  where the image features are first estimated by a heatmap, and then refined by a set of shape regression subnetworks each adapted and trained for a particular pose.
Kowalski et al.  proposed a multistage scheme for face alignment. It is based on a cascaded CNN where each stage refines the landmark positions estimated at the previous one. The inputs to each stage are a face image normalized to a canonical pose, the features computed by the previous stage, and a heatmap computed using the results of the previous phase. The heatmap is not estimated by the CNN, and in that, this scheme differs significantly from the proposed scheme, and other schemes that directly estimate the heatmaps as part of the CNN [17, 22, 34].
|Paper||Training Sets||Test Sets||Pts#|
|300-W||300-W private test set|
|MENPO||300-W private test set|
|Chen ||LFPW,||300-W private test set||68|
3 Face Localization using cascaded CNNs
The face localization problem is the localization of a set of landmarks points , such that = in a face image . The number of estimated points relates to the annotation convention used, and in this work we used landmark points following most contemporary works. The general and detailed outlines of the proposed CCNN’s architecture are depicted in Figs. 2 and 3, respectively. It comprises of three subnetworks, where the first is a pseudo-siamese (non-weight-sharing) subnetwork consisting of two subnetworks that compute the corresponding feature maps of the input image and an initial estimate of the heatmaps.
The second subnetwork is the cascaded heatmap subnetwork (CHCNN) that robustly estimates the heatmaps, that encode the landmarks, a single 2D heatmap per facial feature location. The heatmaps are depicted in Fig. 4. The CHCNN consists of cascaded 3D heatmaps estimation units, detailed in Section 3.2, that estimate 3D heatmaps such that . The cascaded formulation implies that each CHCNN subunit is given as input the heatmap estimated by its preceding subunit , alongside the feature map . The heatmap subunits are non-weight-sharing, as each subunit refines a different estimate of the heatmaps. In that, the proposed schemes differs from the heatmaps-based pose estimation of Belagiannis and Zisserman  that applies weight-sharing cascaded units. The output of the CHCNN are the locations of the maxima of denoted , such that .
As the heatmap-based estimates are given on a coarse grid, their locations are refined by applying the Cascaded Regression CNN (CRCNN) detailed in Section 3.2. The CRCNN consists of cascaded regression subunits , where each regression subunit applies a regression loss to refine the corresponding heatmaps-based landmark estimate , and estimate the refinement
is a vectorized replica of thepoints in a set, and Eq. 1 is optimized using an loss.
3.1 Base subnetwork
The Base subnetwork consists of two pseudo-siamese (non-weight-sharing) subnetworks detailed in Table II. The first part of the subnetwork, layers A1-A7 in Table II, computes the feature maps of the input image. The succeeding layers A8-A12 compute an estimate of the heatmaps, one per facial feature. These layers apply filters with wide support to encode the relations between neighboring facial features.
The base CNNs and corresponding feature maps
are trained using different losses and backpropagation paths as depicted in Fig.3. is connected to the CRCNN and is thus adapted to the regression task, while is connected to the CHCNN and its feature map is adapted to the heatmaps estimation task. computes the initial estimate of the heatmaps and is trained using a loss, while is trained using the backpropagation of the CRCNN subnetwork.
|Input :||256 x 256 x 3||-||-||-|
|A1-Conv||256 x 256 x 3||3 x 3||1 x 1||2 x 2|
|256 x 256 x 64||-||-||-|
|A2-Conv||256 x 256 x 64||3 x 3||1 x 1||2 x 2|
|A2-ReLu||256 x 256 x 64||-||-||-|
|A2-Pool||256 x 256 x 64||2 x 2||2 x 2||0 x 0|
|A3-Conv||128 x 128 x 64||3 x 3||1 x 1||2 x 2|
|A3-ReLu||128 x 128 x 64||-||-||-|
|A4-Conv||128 x 128 x 64||3 x 3||1 x 1||2 x 2|
|A4-ReLu||128 x 128 x 128||-||-||-|
|A4-Pool||128 x 128 x 128||2 x 2||2 x 2||0 x 0|
|A5-Conv||64 x 64 x 128||3 x 3||1 x 1||2 x 2|
|A5-ReLu||64 x 64 x 128||-||-||-|
|A6-Conv||64 x 64 x 128||3 x 3||1 x 1||2 x 2|
|A6-ReLu||64 x 64 x 128||-||-||-|
|A7-Conv||64 x 64 x 128||1 x 1||1 x 1||-|
|Output :||64 x 64 x 128||-||-||-|
|A8-Conv||64 x 64 x 128||9 x 9||1 x 1||8 x 8|
|A8-ReLu||64 x 64 x 128||-||-||-|
|A9-Conv||64 x 64 x 128||9 x 9||1 x 1||8 x 8|
|A9-ReLu||64 x 64 x 128||-||-||-|
|A10-Conv||64 x 64 x 128||1 x 1||1 x 1||0 x 0|
|A10-ReLu||64 x 64 x 256||-||-||-|
|A11-Conv||64 x 64 x 256||1 x 1||1 x 1||0 x 0|
|A11-ReLu||64 x 64 x 256||-||-||-|
|A11-Dropout0.5||64 x 64 x 256||-||-||-|
|A12-Conv||64 x 64 x 256||1 x 1||1 x 1||0 x 0|
|A12-ReLu||64 x 64 x 68||-||-||-|
|Output :||64 x 64 x 68||-||-||-|
3.2 Cascaded heatmap estimation CNN
The heatmap images encode the positions of the set of landmarks points , by relating a single heatmap per landmark to the location of the maximum of , where the heatmaps are compute in a coarse resolution of of the input image resolution. The heatmaps are computed using the CHCNN subnetwork consisting of Heatmap Estimation Subunits (HMSU) detailed in Table III.
The cascaded architecture of the CHCNN implies that each heatmap subunit estimates a heatmap and receives as input the heatmap estimated by the previous subunit, and a feature map estimated by the Base subnetwork . The different inputs are concatenated as channels such that the input is given by .
The HMSU architecture comprises of wide filters, and corresponding to layers B1 and B2, respectively, in Table III. These layers encode the geometric relationships between relatively distant landmarks. Each heatmap is trained with respect to a loss, and in the training phase, the locations of the facial landmarks are labeled by narrow Gaussians centered at the landmark location, to improve the training convergence.
|Input :||64 x 64 x 136||-||-||-|
|B1-Conv||64 x 64 x 136||7 x 7||1 x 1||6 x 6|
|B1-ReLu||64 x 64 x 64||-||-||-|
|B2-Conv||64 x 64 x 64||13 x 13||1 x 1||12 x 12|
|B2-ReLu||64 x 64 x 64||-||-||-|
|B3-Conv||64 x 64 x 64||1 x 1||1 x 1||0 x 0|
|B3-ReLu||64 x 64 x 128||-||-||-|
|B4-Conv||64 x 64 x 128||1 x 1||1 x 1||0 x 0|
|B4-ReLu||64 x 64 x 68||-||-||-|
|Output :||64 x 64 x 68||-||-||-|
3.3 Cascaded regression CNN
The Cascaded regression CNN (CRCNN) is applied to refine the robust, but coarse landmark estimate computed by the CHCNN subnetwork. Similar to the CHCNN subnetwork detailed in Section 3.2, the CRCNN comprises of subunits detailed in Table IV. Each subunit is made of two succeeding subnetworks: the first computes a feature map of the regression CNN using layers C1-C3 in Table IV, while the second subnetwork, layers C4-C6 in Table IV, estimates the residual localization error. The input to each regression subunit is given by
that is a concatenation of both feature maps and computed by the Base CNNs, the corresponding heatmap estimate and a baseline heatmap estimate . The output of the regression subunit is the refinement term as in Eq. 1, that is the refinement of the heatmap-based localization. It is trained using a regression loss, and the final localization output is given by the output of the last unit .
|64 x 64 x 332||-||-||-|
|C1-Conv||64 x 64 x 332||7 x 7||2 x 2||5 x 5|
|C1-Pool||32 x 32 x 64||2 x 2||1 x 1||1 x 1|
|C2-Conv||32 x 32 x 64||5 x 5||2 x 2||3 x 3|
|C2-Pool||16 x 16 x 128||2 x 2||1 x 1||1 x 1|
|C3-Conv||16 x 16 x 128||3 x 3||2 x 2||1 x 1|
|C3-Pool||8 x 8 x 256||2 x 2||1 x 1||1 x 1|
|Output :||8 x 8 x 256||-||-||-|
|Input :||8 x 8 x 512||-||-||-|
|C4-Conv||8 x 8 x 512||3 x 3||2 x 2||1 x 1|
|C4-Pool||4 x 4 x 512||2 x 2||1 x 1||1 x 1|
|C5-Conv||4 x 4 x 512||3 x 3||2 x 2||1 x 1|
|C5-Pool||2 x 2 x 1024||2 x 2||1 x 1||1 x 1|
|C6-Conv||2 x 2 x 1024||1 x 1||1 x 1||0 x 0|
|Output :||1 x 1 x 136||-||-||-|
The heatmap-based representation of the facial landmarks is essentially a general-purpose metric-space representation of a set of points. The use of smoothing filters applied to such representation relates to applying kernels to characterize a data point based on the geometry of the points in its vicinity [43, 44]
, where the use of filters of varying support allows approximate diffusion-like analysis at different scales. Moreover, applying multiple convolution layers and nonlinear activation functions to the heatmaps allows to utilize convolution kernels that might differ significantly from classical pre-designed kernels, such as Diffusion Kernels, as the filters in CNN-based schemes are optimally learnt given an appropriate loss.
In the proposed scheme the heatmap is used as a state variable that is initiated by the Base subnetwork (Section 3.1) and iteratively refined by using two complementary losses: the heatmap-based (Section 3.2) that induces the graph structure of the detected landmarks, and the coordinates-based representation, refined by pointwise regression (Section 3.3).
Such approaches might pave the way for other localization problems such as sensor localization  where the initial estimate of the heatmap is given by a graph algorithm, rather than image domain convolutions, but the succeeding CNN architecture would be similar to the CHCNN and CRCNN subnetworks, and we reserve such extensions to future work.
4 Experimental Results
The proposed CCNN scheme was experimentally evaluated using multiple contemporary image datasets used in state-of-the-art schemes, that differ with respect to the appearance and acquisition conditions of the facial images. We used the LFPW , M2VTS , Helen , AFW , i-bug , COFW , 300-W  and the MENPO challenge dataset .
In order to adhere to the state-of-the-art 300-W competition guidelines [19, 28] landmarks were used in all of our experiments, where the input RGB images were resized to dimensions, and the pixel values were normalized to . The heatmaps were computed at a spatial resolution, where the landmark’s labeling was applied using a symmetric Gaussian, with
. The convolution layers of the CCNN were implemented with a succeeding batch normalization layer, and the training images were augmented by color changes, small angles rotation, scaling, and translations. The learning rate was changed manually and gradually, starting withfor the initial epochs, followed by for the next five epochs, and was then fixed at for the remainder of the training, where the CCNN was trained for 2500 epochs.
The localization accuracy per single face image was quantified by the Normalized Localization Error (NLE) between the localized and ground-truth landmarks
where and are the estimated and ground-truth coordinates, of a particular facial landmark, respectively. The normalization factor is either the inter-ocular distance (the distance between the outer corners of the eyes) [25, 40, 13], or the inter-pupil distance (the distance between the eye centers) .
The localization accuracy of a set of images was quantified by the average localization error and the failure rate, where we consider a normalized point-to-point localization error greater than 0.08 as a failure . We also report the area under the cumulative error distribution curve (AUC) [19, 28], that is given by the area under the cumulative distribution summed up to a threshold . The proposed CCNN scheme was implemented in Matlab and the MatConvNet-1.0-beta23 deep learning framework  using a Titan X (Pascal) GPU.
Where possible, we quote the results reported in previous contemporary works, as most of them were derived using the 300-W competition datasets, where both the dataset and evaluation protocol are clearly defined. In general, we prefer such a comparative approach to implementing or training other schemes, as often, it is difficult to achieve the reported results, even when using the same code and training set.
4.1 300-W results
We evaluated the proposed CCNN approach using the 300-W competition dataset  that is a state-of-the-art face localization dataset of near frontal face images. It comprises of images taken from the LFPW, Helen, AFW, i-bug, and “300W private test set”111The “300W private test set” dataset was originally a private and proprietary dataset used for the evaluation of the 300W challenge submissions. datasets. Each image in these datasets was re-annotated in a consistent manner with landmarks and a bounding box per image was estimated by a face detector.
The CCNN was trained using the 300-W training set and the frontal face images of the Menpo dataset  that were annotated by 68 landmark points, same as in the 300-W dataset. The profile faces in the Menpo dataset were annotated by 39 landmark points that do not correspond to the 68 landmarks annotations, and thus could not be used in this work. The overall training set consisted of images.
The validation set was a subset of images randomly drawn from the training set. The face images were extracted using the bounding boxes given in the 300-W challenge, where the shorter dimension was extended to achieve rectangular image dimensions, and the images were resized to a dimension of pixels.
4.1.1 300-W public testset
We compared the CCNN to contemporary state-of-the-art approaches using the Public and Private 300-W test-sets. The Public test-set was split into three test datasets following the split used in contemporary works [16, 49]. First, the Common subset consisting of the test-sets of the LFPW and Helen datasets (554 images overall). Second, the Challenging subset made of the i-bug dataset (135 images overall), and last, the 300-W public test-set (Full Set). The localization results of the other schemes in Tables V-VII are quoted as were reported by their respective authors.
The results are reported in Table V, and it follows that the proposed CCNN scheme compared favorably with all other scheme, outperforming other schemes in three out of the six test configurations. In particular, the proposed scheme outperforms all previous approaches when applied to the Challenging set that is the more difficult to localize.
|Method||Common set||Challenging set||Full Set|
|Shao et al.||4.45||8.03||5.15|
|Chen et al.||3.73||7.12||4.47|
We also depict in Fig. 5 the AUC accuracy measure of the CCNN when applied to the Helen and LFPW testsets.
4.1.2 300-W private testset
We studied the localization of the 300W Private test set, LFPW and Helen datasets in Table VI where the proposed scheme prevailed in four out of the six test configurations.
|Method||LFPW||Helen||300-W Private Set|
The AUC measure and the localization failure rate are studied in Table VII, where we compared against contemporary schemes using the 300-W public and private test sets. It follows that the proposed CCNN scheme outperforms all other schemes in all test setups.
|Test Set||Method||AUC||Failure (%)|
We also used the 300-W private testset and the corresponding split of 300
Indoor and 300 Outdoor images, respectively, as these
results were reported by the prospective authors as part of the 300-W
challenge results . Figures 6-8 depict the AUC accuracy measure vs.
NLE, that was normalized using the inter-ocular normalization. We show the
results for the split of indoor, outdoor and combined (indoor+outdoor) test images in Figs. 6, 7 and 8, respectively. The results of the schemes we
compare against are quoted from the 300-W challenge results222Available at:
https://ibug.doc.ic.ac.uk/media/uploads/competitions/300w_results.zip.. It follows that for all three test subsets, the proposed CCNN scheme outperforms the contemporary schemes significantly.
Figure 9 shows some of the estimated landmarks in images taken from the 300-W indoor and outdoor test sets. In particular, we show face images with significant yaw angles and facial expressions. These images exemplify the effectiveness of the proposed CCNN framework.
4.2 COFW dataset results
The Caltech Occluded Faces in the Wild (COFW) dataset  is a challenging dataset consisting of faces depicting a wide range of occlusion patterns, and was annotated by Ghiasi and Fowlkes  with landmark points. The common train/test split is to use 500 images for training and the other 507 images for testing. Following previous works, we applied the same CCNN model as in Section 4.1 to the COFW testset (507 images) and compared the resulting accuracy with several state-of-the-art localization schemes. The experimental setup follows the work of Ghiasi and Fowlkes , where the results of prior schemes were also made public333Available at: https://github.com/golnazghiasi/cofw68-benchmark. In this setup the CFSS  and TCDCN  schemes were trained using the Helen68, LFPW68 and AFW68 datasets. The RCPR-occ scheme  was trained using the same training sets as the CCNN model, while the HPM and SAPM schemes, were trained using Helen68 and LFPW68 datasets, respectively. The comparative results are depicted in Fig. 10 and it follows that the CCNN scheme outperforms the other contemporary schemes. For instance, for a localization accuracy of 0.05 the CCNN outperforms the CFSS, coming in second best, by close to 15%.
4.3 Ablation study
We studied the effectivity of the proposed CCNN cascaded architecture by varying the number of cascades used in the CHCNN (heatmaps) and CRCNN (regression) subnetworks. For that we trained the CCNN using HCNN and CRCNN cascades with the same training sets and setup as in Section 4.1. The resulting CNNs were applied to the same test sets as in Sections 4.1 and 4.2. The results are depicted in Fig. 11, where we report the localization accuracy at the output of the CRCNN subnetwork. It follows that the more cascades are used the better the accuracy, and the most significant improvement is achieved for using more than a single cascade. Moreover, it seems that adding another cascade might improve the overall accuracy by .
In this work, we introduced a Deep Learning-based cascaded formulation of the coarse-to-fine localization of facial landmarks. The proposed cascaded CNN (CCNN) applied two dual cascaded subnetworks: the first (CHCNN) estimates a coarse but robust heatmap corresponding to the facial landmarks, while the second is a cascaded regression subnetwork (CRCNN) that refines the accuracy of CHCNN landmarks localization, via regression. The two cascaded subnetworks are aligned such that the output of each CHCNN unit is used as an input to the corresponding CRCNN unit, allowing the iterative refinement of the localization accuracy. The CCNN is an end-to-end solution to the localization problem that is fully data-driven and trainable, and extends previous results on heatmaps-based localization . The proposed scheme is experimentally shown to be robust to large variations in head pose and its initialization. Moreover, it compares favorably with contemporary face localization schemes when evaluated using state-of-the-art face alignment datasets.
This work exemplifies the applicability of heatmaps-based landmarks localization. In particular, the proposed CCNN scheme does not utilize any particular appearance attribute of faces and can thus be applied, to the localization of other classes of objects of interest. In future, we aim to extend the proposed localization framework to the localization of sensor networks where the image domain CNN is reformulated as graph domain CNNs.
This work has been partially supported by COST Action 1206 “De-identification for privacy protection in multimedia content” and we gratefully acknowledge the support of NVIDIA® Corporation for providing the Titan X Pascal GPU for this research work.
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