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Salient Region Segmentation

Saliency prediction is a well studied problem in computer vision. Early saliency models were based on low-level hand-crafted feature derived from insights gained in neuroscience and psychophysics. In the wake of deep learning breakthrough, a new cohort of models were proposed based on neural network architectures, allowing significantly higher gaze prediction than previous shallow models, on all metrics. However, most models treat the saliency prediction as a \textit{regression} problem, and accurate regression of high-dimensional data is known to be a hard problem. Furthermore, it is unclear that intermediate levels of saliency (ie, neither very high, nor very low) are meaningful: Something is either salient, or it is not. Drawing from those two observations, we reformulate the saliency prediction problem as a salient region \textit{segmentation} problem. We demonstrate that the reformulation allows for faster convergence than the classical regression problem, while performance is comparable to state-of-the-art. We also visualise the general features learned by the model, which are showed to be consistent with insights from psychophysics.


I. INTRODUCTION
The human visual system receives about 10 8 to 10 9 bits of information per second [1]. In order to process such a large quantity of information efficiently, the visual system relies on dynamic attention: selectively focusing cognitive resources on parts of the scene. This process has been extensively studied by psychologists, some have proposed computational models to mimic this clever mechanism and predict where a human witness would look in a given. The most widespread model is based on Triesman's feature integration theory [2] and the concept of centre-surround difference: regions which have features different from their surroundings are likely to attract a viewer's attention. Such regions, which attract a viewer's gaze, are called salient, and thus computational models predicting such regions are called saliency models. Features commonly used to explain saliency include, eg, colour, intensity and orientation [3], and additional features (eg, depth, contour) can improve the saliency models' predictiveness but it is not fully understood which features are salient. The fast development and success of deep learning approaches in computer vision, together with the availability of large scale datasets in visual attention has allowed to try and learn salient features from scratch by trying to predict human viewers' gaze [4], [5], [6], [7]. These data-driven approaches have demonstrated the potential to predict viewers' gaze with significantly higher accuracy than previous, hand-crafted saliency models.
One challenge when using a data-driven approach to learn saliency from human gaze, is the difficulty of regressing a complete, high-dimensional saliency map: accurate regression of high-dimensional data is known to be a hard problem. More importantly, although it is clear what is signified by a high or low saliency, it is less clear how meaningful is the accurate saliency scoring of intermediate regions (can something be 'somewhat more salient' than another?). Arguably, the regression problem to be solved is much harder than strictly necessary from the problem definition: estimating accurately the relative saliency level of all pixels is intrinsically ambiguous, whereas we are only really interested in the salient regions of the image. In this work, instead of predicting a pixelwise saliency value, we predict salient regions based on three saliency levels (high, medium and low). This simplification of the problem allows us to adopt an image segmentation approach based on an encoder-decoder network such that the network's output is of the same size as its input, avoiding the need to rescale the output. We demonstrate that although our approach involves a major reformulation and simplification of the problem, we can predict viewers' gaze with an accuracy comparable to state-of-the-art approaches.

II. RELATED WORK
Existing saliency prediction methods can be divided into two groups based on whether the features used in saliency arXiv:1803.05759v1 [cs.CV] 15 Mar 2018 prediction are hand-crafted or learned from the data. We will discuss those two categories in turn.

A. Models based on hand-crafted features
Most hand-crafted feature-based models originate from the Treisman's feature integration theory [2], and make use of hand-crafted features based on what has been shown to attract visual attention by Psychologists. Itti & Koch [8], [3] were the first to propose a computational model for saliency prediction based on the feature integration theory and central surround difference. Their model uses a topological architecture at multiple scale to get a conspicuity map for each feature, and then use a combination scheme to combine conspicuity maps for all the features used in saliency prediction to get the final saliency map. Their model uses three common features: colour, intensity and orientation. Later variants on this model [9] also integrate the depth feature into the saliency prediction model. However, many other features could attract our attention; for detailed survey of hand-crafted feature based saliency prediction model, we refer to [1].

B. Models based on machine learning method
Although psychological research has provided elements of answer, what visual features are salient is still an open question, hence there is an interest to use machine learning to learn salient features from data. Kienzle et al. [10] first tried to learn features from eye fixation data, and found confirmation that visual patterns similar to central surround difference attract viewers' attention. Since 2014, the development of deep learning approaches and their successes in many computer vision problems, along with the availability of large scale datasets in visual attention, lead to the rise of deep learning approaches in saliency prediction.
Vig et al. [11] were the first to use a deep neural network for saliency prediction, although many other approaches followed. They use an optimisation algorithm to search the best features in a deep model created for face recognition. After that, most deep models for saliency prediction use transfer learning method and fine-tuning on a deep network pre-trained for image classification with some modification, removing the fully connected layer and building a regressor above the convolutional part of the network. The final step is to rescale the network's output to the input image's size. The main difference between those deep models is the loss function used for training: [12] directly use Euclidean loss function to train their model; [13] use the Kullback-Leibler divergence as the loss function, as it is one of the evaluation metric for saliency model; [6] compares different loss functions and found that the Bhattacharyya distance is the best loss function; and [5] use the maximum likelihood method to train their model.
The approach proposed in this article is also based on deep learning, but in contrast to those who treat saliency prediction as a regression problem, we reformulate the problem as a segmentation problem, and we use the encoder-decoder architecture to achieve pixel-wise prediction rather than the coarse prediction provided by other deep networks.

III. SALIENT REGION SEGMENTATION
Before introducing our method, we first introduce three terms that will be used in the later parts (example in Figure 1).
Fixation map (FM): A fixation map is a binary map, which records the human eye fixation locations using eye tracker when generating the dataset for saliency prediction.
Saliency map (SM): A saliency map is derived from fixation maps by convolving a Gaussian filter with the fixation map. It is the ground truth for all of the saliency prediction model at the moment.
Salient region map (SRM): A salient region map is derived by quantizing a saliency map into several saliency levels. It is the ground truth in our method. See Figure 1.
All existing saliency prediction models work by trying to predict saliency levels at every pixel. However, estimating intermediate saliency values accurately increases significantly the complexity of the learning problem, with little benefit. Therefore, we propose to reformulate the problem as a segmentation problem to separate regions of high saliency from regions of low saliency, rather than regressing a pixel-wise saliency map.

A. Salient Region Thresholding
The saliency map is a continuous map while the salient region map is a discrete map. The salient region map is derived directly from the saliency map by assigning each pixel a saliency level according to their saliency value. The higher the saliency value of a pixel, the higher its saliency level: where R(x) is the saliency level of salient region map at pixel x , S(x) is the saliency value of saliency map at pixel x, and K is the number of saliency levels. The resulting salient region map will be used as ground truth in the training stage.

B. Encoder-Decoder Segmentation
The classical Convolutional Neural Network is an encoder plus a classifier or a regressor. The size of such a network's output is very small compared to the input image due to the successive pooling layers, therefore preventing pixel-wise classification in this architecture. Recently, the encoder-decoder architecture has achieved great performance in semantic segmentation task; and therefore this is the architecture we adopt for salient region segmentation in this paper. There are mainly two different encoder-decoder architectures: the FCN (Fully Convolutional Networks) [14] and SegNet [15]. The main difference between these two approaches is the upsampling process used in the decoder. In FCN, they learn an upsampling filter to do deconvolution against the corresponding pooling layer such that the output size is equal to the input size. In contrast, SegNet uses indices in the pooling stage, and do unpooling to get a sparse feature map at first in the decoder (this method is also used in CNN feature visualisation of [16]), then learn a filter to get a dense feature map. As the SegNet architecture has a higher classification accuracy than FCN for semantic segmentation, we adopt this architecture for salient region segmentation.

C. Median Frequency Balancing
For training our network, we use a softmax with cross entropy loss function. However, the dataset is highly unbalanced: low saliency regions occupies a much larger proportion of the whole map than high saliency regions do (see Figure  1). Therefore, we use Eigen and Fergus's median frequency balancing method [17] to weight the loss caused by each class: where E(x) is the loss caused by pixel x during training, K is the number of saliency levels, W i is the weight of i th saliency level determined by median frequency balancing method, p i is the prediction probability that the saliency level of pixel x is i, and g i is the ground truth that the saliency level of pixel x is i.
The whole model architecture is in Figure 2.

D. Region Restriction
To make the salient region more selective, we use the output of a binary (salient/non-salient) model to restrict the regions of the output with more quantization levels, such that the pixels that are non-salient in the binary model's output are inhibited in the quantized model's output (example in Figure 3).

E. Training The Model
We trained two models, one model with three saliency levels and one model with two saliency levels. We use the second model's output to restrict the first model's output. The parameters of the encoder part are initialized from the VGG-16 network [18]. The filters in the decoder are initialized by MSRA method in Caffe [19]. During the training stage, all layers are learnt with an initial learning rate of 0.01, we decrease the learning rate using the step learning policy with a step size of 500. All training is based on the SALICON dataset [20]. We only use the training dataset (10K images) in SALICON to train our model, and we use the first 500 images in the validation dataset of SALICON as validation. It takes almost 22 hours to train each model on a K40 GPU.

IV. EVALUATION
In this section, we first describe the datasets and the evaluation metrics for saliency prediction models, followed by a comparison and discussion of the quantitative and qualitative aspects of the result.

A. Datasets
SALICON [20]: This is the largest dataset publicly available with fixation map and saliency map in visual attention domain. It consists of 20,000 images taken from the MSCOCO dataset, 10,000 in the training dataset, 5,000 in the validation dataset, and 5,000 in the testing dataset. The eye fixation data is recorded by mouse-click instead of eye-tracking system. We use the training dataset for training the model, and the first 500 data in the validation dataset for validation and all the validation data for testing the classification accuracy.
MIT1003 [21]: This dataset contains 1,003 images with fixation and saliency map. The fixation map comes from 15 viewers when free viewing the original image for 3 seconds recorded by the eye-tracking system. We use this dataset to evaluate the loss in several saliency evaluation metrics compared to the traditional pixel-wise saliency value prediction methods.
MIT300 [22]: This dataset contains 300 images and it is the MIT saliency benchmark. The fixation map (not publicly available) of this dataset recorded the fixation locations of 39 people. We use this dataset to compare the performance of our model with other state-of-the-art saliency prediction models over several evaluation metrics.

B. Metrics
The ground truth for our model is the salient region map, not the saliency map. Therefore, we evaluate our model using the classification accuracy and the evaluation metrics based on the fixation map.   Classification accuracy: This is the evaluation used in semantic segmentation tasks, we test the overall classification accuracy (the accuracy for all saliency levels), per class accuracy (the accuracy for each saliency level).
AUC-Judd: This metric is proposed by Judd in [23]. The model prediction is treated as a binary classifier to separate positive from negative samples at various thresholds, and the ROC curve is calculated from true positive (TP) and false positive (FP) rates. The final score is the area under the ROC curve, where larger AUC signify better detection.
sAUC: This metric is introduced in [24] at 2008. It is the same as AUC-Borji but removed the central-bias (when photographing the image, people like to place the interesting objects in the central part of the image).
NSS [23]: This metric is the normalised scanpath saliency between the model's prediction and the fixation map. It is measured as the mean value of the normalised model prediction at fixation locations. Table I records the proposed model's classification accuracy on the SALICON dataset, overall and for each saliency level, showing that classification accuracy is lower for higher saliency levels. In contrast to classical semantic segmentation, an object can belong to multiple classes in salient region segmentation: the high saliency level region is generally surrounded by a low saliency level region. It makes it difficult to distinguish them with higher accuracy, as they easily overlap (in particular, the low saliency region covers the high saliency region).   2) Saliency evaluation metrics: Due to the quantization step, it is expected that even a true salient region map would yield a somewhat lower gaze prediction performance on most metrics compared to the original (continuous) saliency map. This loss is measured one the MIT1003 dataset and recorded in Table II. In this table, we can also see that the estimated loss is larger for AUC metric than for NSS, supporting our hypothesis that quantized regions encode most saliency information. We evaluated our model on the SALICON validation dataset (Table III) and the MIT300 official test set (Table IV)   These results show that although our proposed model does not quite reach the performance of the latest deep architectures such as SalGAN and PDP, it nonetheless achieves high performance. Indeed, when considering the official MIT300 benchmark (Table IV), it is interesting to note that, despite working on quantized regions, our model's NSS score clearly outperforms some previous methods such as Deep Gaze i, Deep Gaze ii. Moreover, the results on the SALICON dataset (Table III) show that the application of the region restriction mechanism described in section III-D appears to simultaneously reduce the AUC scores while at the same time increasing the NSS score by a significant margin. This result outline that AUC is in line with recent arguments by [26] and the NSS score is a better evaluation metric for saliency datasets based on fixation patterns. The reason for the large NSS loss is due to our models output has a high standard deviation, which lead to a pixel value decrease after normalisation when compute the NSS score. See Figure 4. It would appear at first glance that the NSS score for our model prediction should be higher than the salient region map, as it covers more eye fixation locationsthis is not the case. The NSS score for the saliency map is 4.2324, for salient region map is 3.9484, and for our prediction is 3.1471. As the standard deviation for the saliency map is 0.0329, for salient region map is 0.0777, but for our model prediction is 0.1923, after normalisation, the max value for the saliency map is 11.6710, for salient region map is 12.7703, but for our model prediction is 4.9508, which will lead to a large decline in the NSS computation.

D. Reformulation Gain
One important advantage of reformulating the problem as semantic segmentation is that it is a simpler problem to learn. To demonstrate this we compare the training convergence of the our segmentation model with a regression model using the same architecture, same dataset and same learning strategy, but treating saliency map as the ground truth and using a common Euclidean loss. The results are illustrated in Figure 5.
In Figures 5a and 5b, we can see that the proposed method has a much faster convergence speed: it only requires 2,000 iterations to converge (only two epochs). In contrast, the regression-based model is slow to converge and oscillates easily. And in Figures 5c and 5d, we can see that using the deconvolution method in the decoder performs better for both segmentation and regression.

E. General Features Learned by The Network
In recent years, several model for saliency prediction were proposed using deep learning method, but none of them visualised what is learned by those models. Therefore, we adopt the popular deep neural network visualisation technique in [16] to analyse our model. There are three main processes in the visualisation: upsampling, deconvolution, and non-linearity (usually a ReLU function)-the reverse of the forward pass when input an image into the deep neural network. However, in this paper we are more interested in visualising to what patterns the deep neurons are attuned to rather than features for individual images. This required a modification of the upsampling process. The classic upsampling method in feature visualisation is unpooling, using the pooling indices in the forward pass to do unpooling. Because pooling indices only exist when processing an actual image through the network, these indices are not available when visualising a neuron's receptive field in abstraction from any input. Hence, in order to visualise general individual neuron's receptive fields, we set the pooled feature map as a sparse matrix (with only one non-zero value) and do upsampling by repeating this sparse matrix. Here, we show the visualisation results of our model: the features learned by the last layer of the encoder part in our network, there are 512 neurons. Here, we show the general feature for the first 64 neurons. From the Figure 6, we can see that the features learned by our network are very similar to central-surround patterns, which is consistent with the research in Psychology [27].

V. CONCLUSION
In this work, we propose to reformulate the saliency prediction problem as an image segmentation problem, according to different saliency levels rather than the traditional pixwise saliency value prediction. We adopt the encoder-decoder architecture in semantic segmentation to do salient region segmentation. Our results show that even if we treat it as a segmentation problem, it can still challenge the state-ofthe-art performance, and the proposed model trains faster and more reliably than regression-based models. Finally, we also demonstrated that the learnt deep features are consistent with the centre-surround hypothesis.