This paper proposes a novel method which captures the discriminative aspects of an indoor scene to correctly predict its semantic category (e.g., bedroom, kitchen etc.). This categorization can greatly assist in context aware object and action recognition, object localization, robotic navigation and manipulation [49, 48]. However, owing to the large variabilities between images of the same class and the confusing similarities between images of different classes, the automatic categorization of indoor scenes is a very challenging problem [34, 49]. Consider, for example, the images shown in Fig. 1. The images of the top row (Fig. 1 a) belong to the same class ‘bookstore’ and exhibit a large data variability in the form of object occlusions, cluttered regions, pose changes and varying appearances. The images in the bottom row (Fig. 1 b) are of three different classes and have large visual similarities. A high performance classification system should therefore be able to cope with the inherently challenging nature of indoor scenes.
To deal with the challenges of indoor scenes, previous works [6, 18, 28, 34, 24] propose to encode either local or global spatial and appearance information. In this paper, we argue that neither of those two representations provide the best answer to effectively handle indoor scenes. The global representations are unable to model the subtle details, and the low-level local representations cannot capture object-to-object relations and the global structures [34, 20, 40]. We therefore devise mid-level representations that carry the necessary intermediate level of detail. These mid-level representations neither ignore the local cues nor lose the important scene structure and object category relationships.
Our proposed mid-level representations are derived from densely and uniformly extracted image patches. In order to extract a rich feature representation from these patches, we use deep Convolutional Neural Networks (CNNs). CNNs provide excellent generic mid-level feature representations and have recently shown great promise for large-scale classification and detection tasks [30, 3, 36, 10]. They however tend to preserve the global spatial structure of the images , which is not desirable when there are large intra-class variations e.g., in the case of indoor scene categorization (Fig. 1). We therefore propose a method to discount this global spatial structure, while simultaneously retaining the intermediate scene structure which is necessary to model the mid-level scene elements. For this purpose, we encode the extracted mid-level representations in terms of their association with codebooks111A codebook is a collection of distinctive mid-level patches. of Scene Representative Patches (SRPs). This enhances the robustness of the convolutional feature representations, while keeping intact their discriminative power.
It is interesting to note that some previous works hint towards the incorporation of ‘wide context’ [20, 16, 50] for scene categorization. Such high-level context-aware reasoning has been shown to improve the classification performance. However in this work, we show that for the case of highly variant indoor-scenes, mid-level context relationships prove to be the most decisive factor in classification. The intermediate level of the scene details help in learning the subtle differences in the scene composition and its constituent objects. In contrast, global structure patterns can confuse the learning/classification algorithm due to the high inter-class similarities (Sec. IV-C).
As opposed to existing feature encoding schemes, we propose to form multiple codebooks of SRPs. We demonstrate that forming multiple smaller codebooks (instead of one large codebook) proves to be more efficient and produces a better performance (Sec. IV-D). Another key aspect of our feature encoding approach is the combination of supervised and unsupervised SRPs in our codebooks. The unsupervised SRPs are collected from the training data itself, while the supervised SRPs are extracted from a newly introduced dataset of ‘Object Categories in Indoor Scenes’ (OCIS). The supervised SRPs provide semantically meaningful information, while the unsupervised SRPs relate more to the discriminative aspects of the different scenes that are present in the target dataset. The efficacy of the proposed approach is demonstrated through extensive experiments on five challenging scene classification datasets. Our experimental results show that the proposed approach consistently achieves state of the art performance.
The major contributions of this paper are: 1). We propose a new mid-level feature representation for indoor scene categorization using large-scale deep neural nets (Sec. III), 2) Our feature description incorporates not only the discriminative patches of the target dataset but also the general object categories that are semantically meaningful (Sec. III-C), 3). We collect the first large-scale dataset of object categories that are commonly present in indoor scenes. This dataset contains more than 1300 indoor object classes (Sec. IV-A), 4). To improve the efficiency and performance of our approach, we propose to generate multiple smaller codebooks and a feasible feature encoding (Sec. III-C), and 5). We introduce a novel method to encode feature associations using max-margin hyper-planes (Sec. III-D).
Ii Related Work
Based upon the level of image description, existing scene classification techniques can be categorized into three types: 1). those which capture low level appearance cues, 2). those which capture the high level spatial structure of the scene and 3). those which capture mid-level relationships. The techniques which capture low-level appearance cues [6, 18] perform poorly on the majority of indoor scene types since they fail to incorporate the high level spatial information. The techniques which model the human perceptible global spatial envelope  also fail to cope with the high variability of indoor scenes. The main reason for the low performance of these approaches is their neglect of the fine-grained objects, which are important for the task of scene classification.
Considering the need to extract global features as well as the characteristics of the constituent objects, Quattoni et al.  and Pandey et al.  represented a scene as a combination of root nodes (which capture the global characteristics of the scene) and a set of regions of interest (which capture the local object characteristics). However, the manual or automatic identification of these regions of interest makes their approach indirect and thus complicates the scene classification task. Another example of indirect approach to scene recognition is the one proposed by Gupta et al. , where the grouping, segmentation and labeling outcomes are combined to recognize scenes. Learned mid-level patches are employed for scene categorization by Juneja et al. , Doersch et al.  and Sun et al. . However these works involve a lot of effort in learning the distinctive primitives which includes a discriminative patch ranking and selection. In contrast, our mid-level representation does not require any learning. Instead, we uniformly extract the mid-level patches densely from the images and show that these perform best when combined with supervised object representations.
Deep Convolutional Neural Networks have recently shown great promise in large-scale visual recognition and classification [31, 3, 36, 15, 53]. Although CNN features have demonstrated their discriminative power for images with one or multiple instances of the same object, they do preserve the spatial structure of the image, which is not desirable when dealing with the variability of indoor scenes 
. CNN architectures involve max-pooling operations to deal with the local spatial variability in the form of rotation and translation. However, these operations are not sufficient to cope with the large-scale deformations of objects and parts that are commonly present in indoor scenes [10, 53]. In this work, we propose a novel representation which is robust to variations in the spatial structure of indoor scenes. It represents an image in terms of the association of its mid-level patches with the codebooks of the SRPs.
Iii The Proposed Method
The block diagram of our proposed pipeline called ‘Deep Un-structured Convolutional Activations (DUCA)’ is shown in Fig 2. Our proposed method first densely and uniformly extracts mid-level patches (Sec III-A), represents them by their convolutional activations (Sec III-B) and then encodes them in terms of their association with the codebooks of SRPs (Sec III-D), which are generated in supervised and unsupervised manners (Sec III-C). The detailed description of each component of the proposed pipeline is presented next.
Iii-a Dense Patch Extraction
To deal with the high variability of indoor scenes, we propose to extract mid-level instead of global  or local [6, 18] feature representations. Mid-level representations do not ignore object level relationships and the discriminative appearance based local cues (unlike the high level global descriptors), and do not ignore the holistic shape and scene structure information (unlike the low level local descriptors). For each image, we extract dense mid-level patches using a sliding window of pixels with a fixed step size of . In order to extract a reasonable number of patches, the smaller dimension of the image is re-scaled to an appropriate length (700 pixels in our case). Note that the idea of dense patch extraction is analogous to dense key-point extraction , which has shown very promising performance over well-designed key-point extraction methods in a number of tasks (e.g., action recognition ).
Before the dense patch extraction, we augment the images of the dataset with their flipped, cropped and rotated versions to the enhance generalization of our feature representation. First, five cropped images (four from the corners and one from the center) of size are extracted from the original image. Each original image is also subjected to CW and CCW rotations of radians and the resulting images are included in the augmented set. The horizontally flipped versions of all these eight images (1 original + 5 cropped + 2 rotated) are also included. The proposed data augmentation results in a reasonable performance boost (see Sec. IV-D).
Iii-B Convolutional Feature Representations
We need to map the raw image patches to a discriminative feature space where scene categories are easily separable. For this purpose, instead of using shallow or local feature representations, we use the convolutional activations from a trained deep CNN architecture. Learned representations based on CNNs have significantly outperformed hand-crafted representations in nearly all major computer vision tasks[3, 11]. Our CNN architecture is similar to the ‘AlexNet’  (trained on ILSVRC 2012) and consists of 5 convolutional and 3 fully-connected layers. The main difference compared to AlexNet is the dense connections between each pair of consecutive layers in the 8-layered network (in our case). The densely and uniformly extracted patches from the images, are fed to the network’s input layer after mean normalization. The processed output from the network is taken from an intermediate fully connected layer ( layer). The resulting feature representation of each mid-level patch has a dimension of 4096.
Although, CNN activations capture rich discriminative information, they are inherently highly structured. The main reason is the sequence of operations involved in the hierarchical layers of CNN which preserve the global spatial structure of the image. This constraining structure is a limitation when dealing with highly variable indoor scene images. To address this, we propose to encode our patches (represented by their convolutional activations) to an alternate feature space which turns out to be even more discriminative (Sec. III-D). Specifically, an image is encoded in terms of the association of its extracted patches with the codebooks of the Scene Representative Patches (SRPs).
Iii-C Scene Representative Patches (SRPs)
An indoor scene is a collection of several distinct objects and concepts. We are interested in extracting a set of image patches of these objects and concepts, which we call ‘Scene Representative Patches’ (SRPs). The SRPs can then be used as elements of a codebook to characterize any instance of an indoor scene. Examples of these patches for a bedroom scene include a bed, wardrobe, sofa or a table. Designing a comprehensive codebook of these patches is a very challenging task. There can be two possible solutions: first, automatically learn to discover a number of discriminative patches from the training data and second, manually prepare an exhaustive vocabulary of all objects which can be present in indoor scenes. These solutions are quite demanding. First, because of the possibility of a very large number of objects, and second, this may require automatic object detection, localization or distinctive patch selection, which in itself is very challenging and computationally expensive.
In this work, we propose a novel approach to compile a comprehensive set of SRPs. Our proposed approach avoids the drawbacks of the above mentioned strategies and successfully combines their strengths i.e., it is computationally very efficient while being highly discriminative and semantically meaningful. Our set of SRPs has two main components, compiled in a supervised and an unsupervised manner. These components are described next.
Iii-C1 Supervised SRPs
A codebook of supervised SRPs is generated from images of well known object categories expected to be present in a particular indoor scene (e.g., a microwave in a kitchen, a chair in a classroom). The codebook contains human-understandable elements which carry well-defined semantic meanings (similar to attributes  or object banks ). In this regard, we introduce the first large-scale database of objects categories in indoor scenes (Sec. IV-A). The introduced database includes an extensive set of indoor objects (more than ). The codebook of supervised SRPs is generated from images of the database by extracting dense mid-level patches after re-sizing the smallest dimension of each image to pixels. The number of SRPs in the compiled codebook is equal to the object categories in the OCIS database. For this purpose, in the feature space, each SRP is a max-pooled version of convolutional activations (Sec III-B) of all the mid-level patches extracted from that object category. The supervised codebook is then used in Sec. III-D to characterize a given scene image in terms of its constituent objects.
Iii-C2 Unsupervised SRPs
The codebook of unsupervised SRPs is generated from the patches extracted from the training data. First, we densely and uniformly extract patches from training images by following the procedure described in Sec. III-A. The SRPs can then be generated from these patches using any unsupervised clustering technique. However, in our case, we randomly sample the patches as our unsupervised SRPs. This is because, we are dealing with a very large number of extracted patches and an unsupervised clustering can be computationally prohibitive. We demonstrate in our experiments (Sec. IV-C, IV-D) that random sampling does not cause any noticeable performance degradation, while achieving significant computational advantages.
Ideally, the codebook of SRPs should be all-inclusive and cover all discriminative aspects of indoor scenes. One might therefore expect a large number of SRPs in order to cover all the possible aspects of various scene categories. While this is indeed the case, feature encoding from a single large codebook would be computationally burdensome (Sec. IV-D
). We therefore propose to generate multiple codebooks of relatively smaller sizes. The association vectors from each of these codebooks can then be concatenated to generate a high dimensional feature vector. This guarantees the incorporation of a large number of SRPs at a low computational cost. To this end, we generate three unsupervised codebooks, each withSRPs. The codebook size was selected empirically on a small validation set.
The SRPs in the supervised codebook are semantically meaningful, however, they do not include all possible aspects of the different scene categories. The unsupervised codebook compensates this shortcoming and complements the supervised codebook. The combinations of both supervised and unsupervised codebooks, results therefore in an improved discrimination and accuracy (see Sec. IV-D).
Iii-D Feature Encoding from SRPs
Given an RGB image , our task is to find its feature representation in terms of the previously generated codebooks of SRPs (Sec. III-C1 and III-C2). For this purpose, we first densely extract patches from the image using the procedure explained in Sec. III-A. Next, the patches are represented by their convolutional activations as discussed in Sec. III-B. The patches are then encoded in terms of their association with the SRPs of the codebooks. The following two strategies are devised for this purpose.
Iii-D1 Sparse Linear Coding
Let be a codebook of SRPs, a mid-level patch is sparsely reconstructed from the SRPs of the codebook using:
is the regularization constant. The sparse coefficient vector is then used as the final feature representation of the patch.
Iii-D2 Proposed Classifier Similarity Metric Coding
We propose a new soft encoding method which uses the maximum margin hyper-planes to measure feature associations. Given a codebook of SRPs, we train
linear binary one-vs-all SVMs. An SVM finds the maximum margin hyperplane which optimally discriminates an SRP from all others. Letbe the learnt weight matrix of all learnt SVMs. A patch can then be encoded in terms of the trained SVMs using: . Since we have multiple codebooks ( in total), the patch is separately encoded from all of them. The final representation of is then achieved by concatenating the encoded feature representation from all codebooks into a single feature vector .
The encoded feature representations from all mid-level patches of the image are finally pooled to produce the overall feature representation of the image. Two commonly used pooling strategies (mean pooling and max pooling) are explored in our experiments (see Sec. IV-D). Finally, in order to perform classification, we use one-vs-one linear SVMs.
Where is the normal vector to the learned max-margin hyper-plane, is the regularization parameter and is the binary class label of the feature vector .
Iv Experiments and Evaluation
We evaluate our approach on three indoor scene classification datasets. These include MIT-67 dataset, 15 Category Scene data set and NYU indoor scene dataset. Confusing inter-class similarities and high within-class variabilities make these datasets very challenging. Specifically, MIT-67 is the largest dataset of indoor scene images containing 67 classes. The images of many of these classes are very similar looking e.g., inside-subway and inside-bus (see Fig 6 for example confusing and challenging images). Moreover, we also report results on two event and object classification datasets (Graz-02 dataset and 8-Sports event dataset) to demonstrate that the proposed technique is applicable to other related tasks. A detailed description of each of these datasets, followed by our experimental setups and the corresponding results are presented in Sec. IV-B and IV-C. First, we provide a description of our introduced OCIS dataset below.
|MIT-67 Indoor Scenes Dataset|
|Method||Accuracy (%)||Method||Accuracy (%)|
|ROI + GIST [CVPR’09] ||OTC [ECCV’14] |
|MM-Scene [NIPS’10] ||Discriminative Patches [ECCV’12] |
|SPM [CVPR’06] ||ISPR [CVPR’14]|
|Object Bank [NIPS’10] ||D-Parts [ICCV’13] |
|RBoW [CVPR’12] ||VC + VQ [CVPR’13] |
|Weakly Supervised DPM [ICCV’11] ||IFV [CVPR’13]|
|SPMSM [ECCV’12] ||MLRep [NIPS’13] |
|LPR-LIN [ECCV’12] ||CNN-MOP [ECCV’14]|
|BoP [CVPR’13] ||CNNaug-SVM [CVPRw’14] |
|Hybrid Parts + GIST + SP [ECCV’12] ||Proposed DUCA||71.8|
Iv-a A Dataset of Object Categories in Indoor Scenes
There is an exhaustive list of scene elements (including objects, structures and materials) that can be present in indoor scenes. Any information about these scene elements can prove crucial for the scene categorization task (and even beyond - e.g., for the semantic labeling or attribute identification). However, to the best of our knowledge, there is no publicly available dataset of these indoor scene elements. In this paper, we introduce the first large-scale OCIS (Object Categories in Indoor Scenes) database. The database contains a total of images spanning more than frequently occurring indoor object categories. The number of images in each category is about . The database can potentially be used for fine-grained scene categorization, high-level scene understanding and attribute based reasoning. In order to collect the data, a comprehensive list of indoor objects was manually chosen from the labelings provided with the MIT-67  dataset. This taxonomy includes a diverse set of objects classes ranging from a ‘house’ to a ‘handkerchief’. A word cloud of the top 300 most frequently occurring classes is shown in Fig. 4. The images for each class are then collected using an online image search (Google API). Each image contains one or more instances of a specific object category. In order to illustrate the diverse intra-class variability of this database, we show some example images in Fig. 5. Our in-house annotated database will be made freely available to the research community.
For the benchmark evaluation, we represent the images of the database by their convolutional features and feed them to a linear classifier (SVM). A train-test split of - is defined for each class. The classification results in terms of the Cumulative Match Curve (CMC) are shown in Fig. 3. The rank-1 and rank-20 identification rates turn out to be only and respectively. These modest classification rates suggest that indoor object categorization is a very challenging task.
Iv-B Evaluated Datasets
The performance of our proposed method is evaluated on MIT-67 dataset, 15 Category Scene data set, NYU Indoor Scene dataset, Graz-02 dataset and 8-Sports event dataset. Below, we present a brief description of each of these datasets followed by an analysis on the achieved performance.
Iv-B1 MIT-67 Dataset
It contains 15620 images of 67 indoor categories. For performance evaluation and comparison, we followed the standard evaluation protocol in  in which a subset of data is used (100 images per class) and a train-test split is defined to be for each class.
Iv-B2 15 Category Scene Dataset
It contains images of 15 urban and natural scene categories. The number of images in each category ranges from 200-400. For our experiments, we use the same evaluation setup as in , where 100 images per class are used for training and the rest for testing.
Iv-B3 NYU v1 Indoor Scene Dataset
It consists of 7 indoor scene categories with a total of 2347 images. Following the standard experimental protocol , we used a train/test split for evaluation. Care has been taken while splitting the data to ensure that a minimal or no overlap of the consecutive frames exists between the training and testing sets.
Iv-B4 Inria Graz-02 Dataset
It consists of 1096 images belonging to 3 classes (bikes, cars and people) in the presence of heavy clutter, occlusions and pose variations. For performance evaluation, we used the protocol defined in 
. Specifically, for each class, the first 150 odd images are used for training and the 150 even images are used for testing.
Iv-B5 UIUC 8-Sports Event Dataset
It contains 1574 images of 8 sports categories. Following the protocol defined in , we used 70 randomly sampled images for training and 60 for testing.
|15 Category Scene Dataset|
|GIST-color [IJCV’01] ||ISPR [CVPR’14] |
|RBoW [CVPR’12] ||VC + VQ [CVPR’13] |
|Classemes [ECCV’10] ||LMLF [CVPR’10] |
|Object Bank [NIPS’10] ||LPR-RBF [ECCV’12] |
|SPM [CVPR’06] ||Hybrid Parts + GIST + SP [ECCV’12] |
|SPMSM [ECCV’12] ||CENTRIST+LCC+Boosting [CVPR’11] |
|LCSR [CVPR’12] ||RSP [ECCV’12] |
|SP-pLSA [PAMI’08] ||IFV |
|CENTRIST [PAMI’11] ||LScSPM [CVPR’10] |
|OTC [ECCV’14] ||Proposed DUCA||94.5|
|UIUC 8-Sports Dataset|
|GIST-color [IJCV’01] |
|MM-Scene [NIPS’10] |
|Graphical Model [ICCV’07] |
|Object Bank [NIPS’10] |
|Object Attributes [ECCV’12] |
|CENTRIST [PAMI’11] |
|RSP [ECCV’12] |
|SPM [CVPR’06] |
|SPMSM [ECCV’12] |
|Classemes [ECCV’10] |
|HIK [ICCV’09] |
|LScSPM [CVPR’10] |
|LPR-RBF [ECCV’12] |
|Hybrid Parts + GIST + SP [ECCV’12] |
|LCSR [CVPR’12] |
|VC + VQ [CVPR’13] |
|ISPR [CVPR’14] |
Iv-C Experimental Results
The quantitative results of the proposed method for the task of indoor scene categorization are presented in Tables I, II and IV. The proposed method achieves the highest classification rate on all three datasets. Compared with the existing state of the art, a relative performance increment of 4.1%, 5.4% and 1.3% is achieved for MIT-67, Scene-15 and NYU datasets respectively. Amongst the compared methods, the mid-level feature representation based methods [4, 40, 41] perform better than the others. Our proposed mid-level features based method not only outperforms their accuracy but is also computationally efficient (e.g.,  takes weeks to train several part detectors). Furthermore, once compared with existing methods, our proposed method uses a lower dimensional feature representation for classification (e.g., the Juneja et al.  Improved Fisher Vector (IFV) has dimensionality K; the Gong et al.  MOP representation has K).
In addition to indoor scene classification, we also evaluate our approach on other scene classification tasks where large variations and deformations are present. To this end, we report the classification results on the UIUC 8-Sports dataset and the Graz-02 dataset (see Tables III and V). It is interesting to note that the Graz-02 dataset contains heavy clutter, pose and scale variations (e.g., for some ‘car’ images only 5% of the pixels are covered by the car in a scene). Our approach achieved high accuracies of and respectively on the UIUC 8-Sports and Graz-02 datasets. These performances are and higher than the previous best methods on UIUC 8-Sports and Graz-02 datasets respectively.
|NYU Indoor Scenes Dataset|
|BoW-SIFT [ICCVw’11] |
|RGB-LLC [TC’13] |
|RGB-LLC-RPSL [TC’13] |
|OLB [SCIA’05] ||70.7||81.0||76.5||76.1|
|VQ [ICCV’07] ||80.2||85.2||89.5||85.0|
|ERC-F [PAMI’08] ||79.9||-||84.4||82.1|
|TSD-IB [BMVC’11] ||87.5||85.3||91.2||88.0|
|TSD-k [BMVC’11] ||84.8||87.3||90.7||87.6|
The class-wise classification accuracies of the MIT-67, UIUC 8-Sports, Scene-15 and NYU datasets are shown in the form of confusion matrices in Fig. 7 and 9 respectively. Note the very strong diagonal in all confusion matrices. The majority () of the mistakes are made for the closely related classes e.g., coast-opencountry (Fig. 6(a)), croquet-bocce (Fig. 6(b)), bedroom-livingroom (Fig. 6(c)), dentaloffice-operatingroom (Fig. 9) and library-bookstore (Fig. 9). We also show examples of miss-classified images in Fig. 6. The results show that the classes with significant visual and semantic similarities are confused amongst each others e.g., childrenroom-kindergarten and movietheatre-auditorium (Fig. 6).
In order to visualize which patches contributed most towards a correct classification, we plot the heat map of the patch contribution scores in Fig. 8. It turns out that the most distinctive patches, which carry valuable information, have a higher contribution towards the correct prediction of a scene class. Moreover, mid-level patches carry an intermediate level of scene details and contextual relationships between objects which help in the scene classification process.
Iv-D Ablative Analysis
To analyze the effect of the different components of the proposed scheme on the final performance, we conduct an ablation study. Table VI summarizes the results achieved on the MIT-67 Scenes dataset when different components were replaced or removed from the final framework.
It turns out that the supervised and unsupervised codebooks individually perform reasonably well. However, their combination gives state of the art performance. For the unsupervised codebook, k-means clustering performs slightly better however, at the cost of a considerable amount of computational resources (GB RAM for the MIT-67 dataset) and processing time ( day for the MIT-67 dataset). In contrast, the random sampling of MRPs gives a comparable performance with a big boost in computational efficiency. Feature encoding from a single large codebook does not only produce a lower performance, but it also requires more computational time and memory. In our experiments, feature encoding from one single large codebook requires almost twice the time ( sec/image) taken by multiple smaller codebooks ( sec/image). The resulting features performed best when the max-pooling operation was applied to combine them.
|Variants of Our Approach||Accuracy (%)|
|Supervised + Unsupervised|
|Single large codebook|
|Multiple smaller codebooks|
|Sparse linear coding|
|Classifier similarity metric coding|
This paper proposed a robust feature representation based on discriminative mid-level convolutional activations, for highly variable indoor scenes. To suitably contrive the convolutional activations for indoor scenes, the paper proposed to break their inherently preserved global spatial structure by encoding them in a number of of multiple codebooks. These codebooks are composed of distinctive patches and of the semantically labeled elements. For the labeled elements, we introduced the first large-scale dataset of object categories of indoor scenes. Our approach achieves state-of-the-art performance on five very challenging datasets.
This research was supported by the SIRF and IPRS scholarships from the University of Western Australia (UWA) and the Australian Research Council (ARC) grants DP110102166, DP150100294 and DE120102960.
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