Patent Publication Number: US-8983193-B1

Title: Techniques for automatic photo album generation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to co-pending U.S. application Ser. No. 13/628,820, filed concurrently herewith on Sep. 27, 2012, entitled “TECHNIQUES FOR USER CUSTOMIZATION IN A PHOTO MANAGEMENT SYSTEM”. The disclosure of the above application is incorporated herein by reference in its entirety. 
     FIELD 
     The present disclosure relates to digital photo storage and, more particularly, to techniques for automatically generating a photo album using a collection of photos. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     A computing device, e.g., a mobile phone, can include a camera that enables a user to capture photos. The photos can be stored in a memory of the computing device. The user can accumulate a large collection of photos over a period of time. The user can periodically upload the collection of photos to a server via a network. For example, the server can be associated with a social network. The user may also periodically divide his/her collection of photos into one or more photo albums. For example, the user may divide the collection of photos into one or more photo albums corresponding to one or more distinct events, respectively. The user may also delete one or more photos from the collection of photos or from a specific photo album. 
     SUMMARY 
     A computer-implemented technique is presented. The technique can include receiving, at a computing device including one or more processors, a plurality of photos. The technique can include extracting, at the computing device, a set of quality features from each of the plurality of photos, the set of quality features including two or more features, wherein each feature of the set of quality features corresponds to a quality of a specific photo, and wherein the set of quality features includes at least one of photometric features, saliency-based features, and content-based features for a specific photo. The technique can include extracting, at the computing device, a set of similarity features from each of the plurality of photos, each of the set of similarity features being indicative of a similarity between a specific photo and another one or more of the plurality of photos, the set of similarity features including at least one of spatial resolution, color resolution, and temporal resolution of the specific photo. The technique can include obtaining, at the computing device, a quality weight for each feature of the set of quality features by performing machine learning on a reference photo collection using an L2 regularization with an L2-loss function to obtain a set of quality weights, the reference photo collection including plurality of reference photos and a quality weight associated with each of the plurality of reference photos. The technique can include obtaining, at the computing device, a similarity weight for each feature of the set of similarity features based on an analysis of the reference photo collection to obtain a set of similarity weights, the reference photo collection including a similarity weight associated with each unique pair of reference photos in the reference photo collection. The technique can include generating, at the computing device, a quality metric for each of the plurality of photos by analyzing the set of quality features for a specific photo to obtain a set of quality scores and combining the set of quality scores using the set of quality weights to obtain the quality metric. The technique can include generating, at the computing device, a similarity matrix for the plurality of photos by analyzing the set of similarity features for each unique pair of photos of the plurality of photos to obtain a set of similarity scores and generating the similarity matrix using the set of similarity scores and the set of similarity weights. The technique can include selecting, at the computing device, a subset of the plurality of photos by performing joint global maximization of photo quality and photo diversity based on the quality metrics and the similarity matrix using a determinantal point process (DPP) including a maximum-a-posteriori (MAP) approximation algorithm. The technique can also include storing, at the computing device, the subset of the plurality of photos. 
     Another computer-implemented technique is also presented. The technique can include receiving, at a computing device including one or more processors, a plurality of photos. The technique can include extracting, at the computing device, a first set of features from each of the plurality of photos, each feature of the first set of features corresponding to a quality of a specific photo. The technique can include extracting, at the computing device, a second set of features from each of the plurality of photos, each feature of the second set of features being indicative of a similarity between a specific photo and another one or more of the plurality of photos. The technique can include obtaining, at the computing device, a quality weight for each feature of the first set of features based on an analysis of a reference photo collection to obtain a first set of weights, the reference photo collection including plurality of reference photos and a quality weight associated with each of the plurality of reference photos. The technique can include obtaining, at the computing device, a similarity weight for each of the second set of features based on an analysis of the reference photo collection to obtain a second set of weights, the reference photo collection including a similarity weight associated with each unique pair of reference photos in the reference photo collection. The technique can include generating, at the computing device, a quality metric for each of the plurality of photos by analyzing the first set of features for a specific photo to obtain a set of quality scores and combining the set of quality scores using the first set of weights to obtain the quality metric. The technique can include generating, at the computing device, a similarity matrix for the plurality of photos by analyzing the second set of features for each unique pair of photos of the plurality of photos to obtain a set of similarity metrics and generating the similarity matrix using the set of similarity metrics and the second set of weights. The technique can include selecting, at the computing device, a subset of the plurality of photos by performing joint global maximization of photo quality and photo diversity based on the quality metrics and the similarity matrix. The technique can also include storing, at the computing device, the subset of the plurality of photos. 
     In some embodiments, the first set of features for a specific photo includes at least one of photometric features, saliency-based features, and content-based features for the specific photo, wherein the photometric features for the specific photo include at least one of contrast, hue diversity, sharpness, and brightness of the specific photo, wherein the saliency-based features for the specific photo include at least one of simplicity, composition quality, and lighting quality of the specific photo, and wherein the content-based features for the specific photo include a presence of at least one of a specific person, a specific place, and a specific thing. 
     In other embodiments, extracting a specific saliency-based feature for the specific photo further includes filtering, at the computing device, the specific photo to identify a salient portion of the specific photo that encompasses greater than a predetermined amount of a salient object in the specific photo, and extracting, at the computing device, the specific saliency-based feature for the specific photo using the salient portion. 
     In some embodiments, the second set of features for a specific photo includes at least one of spatial resolution, color resolution, and temporal resolution of the specific photo. 
     In other embodiments, extracting a specific feature of the second set of features for the specific photo further includes generating, at the computing device, a hierarchical histogram for the specific photo, the hierarchical histogram including one or more visual words, modifying, at the computing device, the hierarchical histogram by adding a timestamp of the specific photo to obtain a modified hierarchical histogram, and extracting, at the computing device, the specific feature of the second set of features for the specific photo based on the modified hierarchical histogram. 
     In some embodiments, obtaining the quality weight for each feature of the first set of features further includes performing machine learning on the reference photo collection using an L2 regularization with an L2-loss function. 
     In other embodiments, selecting the subset of the plurality of photos by performing the joint global maximization of photo quality and photo diversity based on the quality metrics and the similarity matrix includes using a determinantal point process (DPP), wherein the DPP includes calculating:
 
 S   ij   =q   i   f (Ø i ,Ø j ) q   j ,
 
where S i,j  is a combined matrix of quality and similarity features, q i  is a quality metric for an i th  element, q j  is a quality metric for a j th  element, Φ i  is a feature vector corresponding to the i th  element, Φ j  is a feature vector corresponding to the j th  element, and f(Φ i ,Φ j ) is a function for calculating a similarity between feature vector Φ i  and feature vector Φ j  using the similarity matrix.
 
     In some embodiments, the DPP further includes performing:
         (i) initializing Y={ }, J={ };   (ii) adding eigenvector v k  to J with probability λ k /(λ k +1);   (iii) while |J|&gt;0, performing:       

     
       
         
           
             
               
                 
                   ( 
                   a 
                   ) 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 selecting 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   y 
                   
                     i 
                     ⁢ 
                     
                         
                     
                   
                 
                 ⁢ 
                 from 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Y 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 with 
                 ⁢ 
                 
                   
                       
                   
                   ⁢ 
                   
                       
                   
                 
                 ⁢ 
                 
                   P 
                   ⁡ 
                   
                     ( 
                     
                       y 
                       i 
                     
                     ) 
                   
                 
               
               = 
               
                 
                   1 
                   
                      
                     J 
                      
                   
                 
                 ⁢ 
                 
                   
                     ∑ 
                     
                       v 
                       ∈ 
                       J 
                     
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     
                       ( 
                       
                         
                           v 
                           T 
                         
                         ⁢ 
                         
                           ⅇ 
                           i 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
             
             , 
           
         
       
         
         
           
             
               
                 (b) updating Y=Y∪y i , and 
                 (c) computing J⊥, orthogonal to e i  and setting J=J⊥; and 
               
             
             (iv) returning Y,
 
where Y is a selected subset of photos, v k  is an eigenvector associated with an index k, λ k  is a probability associated with the index k, y i  is an i th  column of the combined matrix S i,j  that is added to the selected subset of photos Y based on its probability P(y i ), J⊥ is a matrix orthogonal to a canonical basis vector e i , and J is an auxiliary matrix used to compensate for elements added to the selected subset of photos Y.
 
           
         
       
    
     In other embodiments, the DPP further includes determining a probability for the selected subset of photos Y by calculating: 
                 P   ⁡     (   Y   )       =       det   ⁡     (     S   Y     )         det   ⁡     (     S   +   I     )           ,         
where P(Y) is a probability that the selected subset of photos Y is a representative subset of the plurality of photos, and I is an identity matrix.
 
     In some embodiments, the DPP further includes applying a maximum-a-posteori (MAP) approximation algorithm to determine a number of iterations for performing the selection of the selected subset of photos Y, and wherein the subset of the plurality of photos includes the selected subset of photos Y after the number of iterations, wherein the MAP approximation algorithm is defined as:
         (i) receiving an input X, parameters e, and a budget B,   (ii) U←S(X) and Y←{ },   (iii) while U≠{ }, performing:       

     
       
         
           
             
               
                 
                   ( 
                   a 
                   ) 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Y 
               
               ← 
               
                 
                   Y 
                   
                     U 
                     ⁢ 
                     
                         
                     
                   
                 
                 ⁢ 
                 
                   
                     
                       arg 
                       ⁢ 
                       max 
                     
                     
                       i 
                       ∈ 
                       U 
                     
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         
                           
                             P 
                             θ 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   Y 
                                   U 
                                 
                                 ⁢ 
                                 
                                   { 
                                   i 
                                   } 
                                 
                               
                               | 
                               X 
                             
                             ) 
                           
                         
                         - 
                         
                           
                             P 
                             θ 
                           
                           ⁡ 
                           
                             ( 
                             
                               Y 
                               | 
                               X 
                             
                             ) 
                           
                         
                       
                       
                         
                           cos 
                           ⁢ 
                           t 
                         
                         ⁡ 
                         
                           ( 
                           i 
                           ) 
                         
                       
                     
                     ) 
                   
                 
               
             
             , 
             and 
           
         
       
         
         
           
             
               
                 (b) U←U\{i|cost(Y)+cost(i)&gt;B}; and 
               
             
             (iv) returning Y,
 
where X represents a portion of the combined matrix S i,j  including parameters ⊖, which represent quality weights and/or similarity weights, B represents a maximum amount of time or computational resources, U represents an auxiliary matrix, cost represents a required time or computational resources, and Y indicates the number of iterations.
 
           
         
       
    
     A computing device is also presented. The computing device can include an input/output device, one or more processors, and a memory. The input/output device can be configured to receive a plurality of photos. The one or more processors can be configured to extract a first set of features from each of the plurality of photos, each feature of the first set of features corresponding to a quality of a specific photo. The one or more processors can be configured to extract a second set of features from each of the plurality of photos, each feature of the second set of features being indicative of a similarity between a specific photo and another one or more of the plurality of photos. The one or more processors can be configured to obtain a quality weight for each feature of the first set of features based on an analysis of a reference photo collection to obtain a first set of weights, the reference photo collection including plurality of reference photos and a quality weight associated with each of the plurality of reference photos. The one or more processors can be configured to obtain a similarity weight for each of the second set of features based on an analysis of the reference photo collection to obtain a second set of weights, the reference photo collection including a similarity weight associated with each unique pair of reference photos in the reference photo collection. The one or more processors can be configured to generate a quality metric for each of the plurality of photos by analyzing the first set of features for a specific photo to obtain a set of quality scores and combining the set of quality scores using the first set of weights to obtain the quality metric. The one or more processors can be configured to generate a similarity matrix for the plurality of photos by analyzing the second set of features for each unique pair of photos of the plurality of photos to obtain a set of similarity metrics and generating the similarity matrix using the set of similarity metrics and the second set of weights. The one or more processors can also be configured to select a subset of the plurality of photos by performing joint global maximization of photo quality and photo diversity based on the quality metrics and the similarity matrix. The memory can be configured to store the subset of the plurality of photos. 
     In some embodiments, the first set of features for a specific photo includes at least one of photometric features, saliency-based features, and content-based features for the specific photo, wherein the photometric features for the specific photo include at least one of contrast, hue diversity, sharpness, and brightness of the specific photo, wherein the saliency-based features for the specific photo include at least one of simplicity, composition quality, and lighting quality of the specific photo, and wherein the content-based features for the specific photo include a presence of at least one of a specific person, a specific place, and a specific thing. 
     In other embodiments, the one or more processors are configured to extract a specific saliency-based feature for the specific photo by filtering the specific photo to identify a salient portion of the specific photo that encompasses greater than a predetermined amount of a salient object in the specific photo, and extracting the specific saliency-based feature for the specific photo using the salient portion. 
     In some embodiments, the second set of features for a specific photo includes at least one of spatial resolution, color resolution, and temporal resolution of the specific photo. 
     In other embodiments, the one or more processors are configured to extract a specific feature of the second set of features for the specific photo by generating a hierarchical histogram for the specific photo, the hierarchical histogram including one or more visual words, modifying the hierarchical histogram by adding a timestamp of the specific photo to obtain a modified hierarchical histogram, and extracting the specific feature of the second set of features for the specific photo based on the modified hierarchical histogram. 
     In some embodiments, the one or more processors are configured to select the subset of the plurality of photos by performing the joint global maximization of photo quality and photo diversity based on the quality metrics and the similarity matrix includes using a determinantal point process (DPP), wherein the DPP includes calculating:
 
 S   ij   =q   i   f (Ø i ,Ø j ) q   j ,
 
where S i,j  is a combined matrix of quality and similarity features, q i  is a quality metric for an i th  element, q j  is a quality metric for a j th  element, Φ i  is a feature vector corresponding to the i th  element, Φ j  is a feature vector corresponding to the j th  element, and f(Φ i ,Φ j ) is a function for calculating a similarity between feature vector Φ i  and feature vector Φ j  using the similarity matrix.
 
     In other embodiments, the DPP further includes performing:
         (i) initializing Y={ }, J={ };   (ii) adding eigenvector v k  to J with probability λ k /(λ k +1);   (iii) while |J|&gt;0, performing:       

     
       
         
           
             
               
                 
                   ( 
                   a 
                   ) 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 selecting 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   y 
                   i 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 from 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Y 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 with 
                 ⁢ 
                 
                   
                       
                   
                   ⁢ 
                   
                       
                   
                 
                 ⁢ 
                 
                   P 
                   ⁡ 
                   
                     ( 
                     
                       y 
                       i 
                     
                     ) 
                   
                 
               
               = 
               
                 
                   1 
                   
                      
                     J 
                      
                   
                 
                 ⁢ 
                 
                   
                     ∑ 
                     
                       v 
                       ∈ 
                       J 
                     
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     
                       ( 
                       
                         
                           v 
                           T 
                         
                         ⁢ 
                         
                           ⅇ 
                           i 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
             
             , 
           
         
       
         
         
           
             
               
                 (b) updating Y=Y∪y i , and 
                 (c) computing J⊥, orthogonal to e i  and setting J=J⊥; and 
               
             
             (iv) returning Y,
 
where Y is a selected subset of photos, v k  is an eigenvector associated with an index k, λ k  is a probability associated with the index k, y i  is an i th  column of the combined matrix S i,j  that is added to the selected subset of photos Y based on its probability P(y i ), J⊥ is a matrix orthogonal to a canonical basis vector e i , and J is an auxiliary matrix used to compensate for elements added to the selected subset of photos Y.
 
           
         
       
    
     In some embodiments, the DPP further includes determining a probability for the selected subset of photos Y by calculating: 
                 P   ⁡     (   Y   )       =       det   ⁡     (     S   Y     )         det   ⁡     (     S   +   I     )           ,         
where P(Y) is a probability that the selected subset of photos Y is a representative subset of the plurality of photos, and I is an identity matrix.
 
     In other embodiments, the DPP further includes applying a maximum-a-posteriori (MAP) approximation algorithm to determine a number of iterations for performing the selection of the selected subset of photos Y, and wherein the subset of the plurality of photos includes the selected subset of photos Y after the number of iterations, wherein the MAP approximation algorithm is defined as:
         (i) receiving an input X, parameters ⊖, and a budget B,   (ii) U←S(X) and Y←{ },   (iii) while U≠{ }, performing:       

     
       
         
           
             
               
                 
                   ( 
                   a 
                   ) 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Y 
               
               ← 
               
                 
                   Y 
                   
                     U 
                     ⁢ 
                     
                         
                     
                   
                 
                 ⁢ 
                 
                   
                     
                       arg 
                       ⁢ 
                       max 
                     
                     
                       i 
                       ∈ 
                       U 
                     
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         
                           
                             P 
                             θ 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   Y 
                                   U 
                                 
                                 ⁢ 
                                 
                                   { 
                                   i 
                                   } 
                                 
                               
                               | 
                               X 
                             
                             ) 
                           
                         
                         - 
                         
                           
                             P 
                             θ 
                           
                           ⁡ 
                           
                             ( 
                             
                               Y 
                               | 
                               X 
                             
                             ) 
                           
                         
                       
                       
                         
                           cos 
                           ⁢ 
                           t 
                         
                         ⁡ 
                         
                           ( 
                           i 
                           ) 
                         
                       
                     
                     ) 
                   
                 
               
             
             , 
             and 
           
         
       
         
         
           
             
               
                 (b) U←U\{i|cost(Y)+cost(i)&gt;B}; and 
               
             
             (iv) returning Y,
 
where X represents a portion of the combined matrix S i,j  including parameters ⊖, which represent quality weights and/or similarity weights, B represents a maximum amount of time or computational resources, U represents an auxiliary matrix, cost represents a required time or computational resources, and Y indicates the number of iterations.
 
           
         
       
    
     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is an illustration of an example computing network including a computing device according to some implementations of the present disclosure; 
         FIG. 2  is a functional block diagram of the computing device of  FIG. 1  including a photo management module according to some implementations of the present disclosure; 
         FIG. 3  is a functional block diagram of the photo management module of  FIG. 2 ; and 
         FIG. 4  is a flow diagram of an example technique for automatically generating a photo album using a collection of photos according to some implementations of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As previously described, a user may periodically divide a collection of photos into one or more photo albums. This task, however, can be both difficult and time consuming. The collection of photos can be automatically divided into one or more photo albums, which can significantly reduce the time spent by the user to manage the photos/albums. This can be referred to as “automatic photo album generation.” Some automatic photo album generation techniques, however, are incapable of generating photo albums that meet the typical user&#39;s standards or expectations for photo album representativeness. In particular, these techniques fail to take into account a plurality of different user criteria, such as photo quality and photo similarity. 
     Accordingly, techniques are presented for automatically generating a photo album using a collection of photos. The techniques can receive a plurality of photos (the photo collection) and can automatically select a subset of the photos (the photo album) having a high degree of representativeness by jointly maximizing both photo quality and photo diversity. The techniques can extract quality features and similarity features for each of the plurality of photos. The techniques can obtain weights for the various quality features and similarity features based on an analysis of a reference photo collection. The reference photo collection can include a plurality of reference photos, a quality score for each of the plurality of reference photos, and a similarity score associated with each unique pair of reference photos in the plurality of reference photos. The analysis of the reference photo collection can be performed by a server to obtain the quality and similarity scores. The techniques can then generate a quality metric for each of the plurality of photos and can generate a similarity matrix for the plurality of photos. The techniques can then perform joint global maximization of photo quality and photo diversity using the quality metrics and the similarity matrix in order to select the subset of the plurality of photos. The techniques can then store the subset of the plurality of photos in a memory. 
     Referring now to  FIG. 1 , an example computing network  100  is illustrated. The example computing network  100  can include a computing device  104  according to some implementations of the present disclosure. As shown, the computing device  104  can be a mobile phone. It should be appreciated, however, that the computing device  104  can be any suitable computing device including one or more processors, such as a desktop computer, a laptop computer, a server device, or a tablet computer. A user  108  can interact with the computing device  104 . In some implementations, the computing device  104  can include a camera (not shown) that is configured to capture photos. The photos can be stored at the computing device  104 . The computing device  104  can be configured to communicate with other devices, e.g., a server  112 , via a network  116 . 
     The server  112  can be configured to store data associated with the user  108 . For example, the server  112  can be configured to receive and store a collection of photos associated with the user  108 . The collection of photos can be divided into one or more photo albums. The network  116  can include a local area network (LAN), a wide area network (WAN), e.g., the Internet, or a combination thereof. The server  112  and/or the computing device  104  can communicate with a database  120  via the network  116 . The database  120  can store a reference photo collection composed of one or more reference photos to be used as reference by the techniques of the present disclosure. The database  120 , therefore, can also be referred to as reference photo collection  120 . For example, the reference photo collection  120  can be a public photo database that stores one or more public photos, e.g., for a social network. 
     Referring now to  FIG. 2 , a functional block diagram of the computing device  104  is illustrated. The computing device  104  can include an input/output device  200 , a processor  204 , a memory  208 , and a photo management module  212 . It should be appreciated that while one processor  204  is shown, the term processor  204  used herein is meant to also include two or more processors operating in a parallel or distributed architecture. It should also be appreciated that the one or more processors can wholly or partially execute the photo management module  212 . 
     The input/output device  200  can control input to and/or output from the computing device  104 . It should be appreciated that while one input/output device  200  is shown, the term input/output device  200  used herein is meant to also include a plurality of different input/output devices. The input to the computing device  104  can include user input, e.g., from the user  108 , or other input received from another device, e.g., the server  112  via the network  116 . For example, the input/output device  200  can include a touch display configured to receive the user input and/or a transceiver configured to receive the other input. The output from the computing device  104  can include information for a user, e.g., for the user  108 , or other information for transmission to another device, e.g., the server  112  via the network  116 . As previously mentioned, the input/output device  200  can include the touch display, which can be configured to display the information for the user  108 , and/or the transceiver, which can be configured to transmit the other information. 
     The processor  204  can control operation of the computing device  104 . The processor  204  can execute functions including, but not limited to, loading an operating system of the computing device  104 , controlling communication between the computing device  104  and other devices, e.g., the server  112  via the network  116 , and/or controlling storage/retrieval operations at the memory  208 . Further, as previously mentioned, the processor  204  can also wholly or partially execute the photo management module  212 . 
     The memory  208  can be configured to store data at the computing device  104 . For example, the memory  208  can store a plurality of photos at the computing device  104 . The memory  208  can be any suitable type of storage medium, such as a hard disk drive, flash memory, or dynamic random access memory (DRAM). It should be appreciated that while a single memory  208  is shown, the term memory  208  used herein is also meant to include a plurality of different types of memory. For example only, the computing device  104  can include both volatile memory, e.g., random access memory (RAM), and non-volatile memory, e.g., read-only memory (ROM). 
     The photo management module  212  can execute the techniques of the present disclosure. Specifically, the photo management module  212  can receive a plurality of photos (a photo collection) and can automatically select a subset of the plurality of photos (a photo album) having a high degree of representativeness by jointly maximizing both photo quality and photo diversity. For example, the photo management module  212  can receive the plurality of photos from the memory  208  and can store the selected subset of the plurality of photos in the memory  208 . The photo management module  212  is shown in  FIG. 3  and is described in detail below. 
     Referring now to  FIG. 3 , a functional block diagram of the photo management module  212  is illustrated. The photo management module  212  can include a photo receiving module  300 , a quality feature extraction module  304 , a similarity feature extraction module  308 , a quality feature weighting module  312 , a similarity feature weighting module  316 , a quality metric generation module  320 , a similarity matrix generation module  324 , a joint global maximization module  328 , and a photo subset selection module  332 . As previously mentioned, the processor  204  (or processors) of the computing device  104  can wholly or partially execute the photo management module  212  and its various sub-modules listed above. 
     The photo receiving module  300  can receive the plurality of photos. The plurality of photos may include N photos, where N is an integer greater than one. The photo receiving module  300  can receive the plurality of photos from the memory  208  and/or via the input/output device  200 . The input/output device  200  may receive the plurality of photos from a camera associated with the computing device  104  and/or from another device connected to the computing device  104 . For example, the other device connected to the computing device  104  may include the server  112  (via the network  116 ) or a local device connected via a peripheral cable, such as a universal serial bus (USB) cable. The photo receiving module  300  can further include memory, e.g., RAM, for at least temporarily storing the plurality of photos. 
     The quality feature extraction module  304  can extract one or more features from each of the plurality of photos to obtain a first set of features. Each of the first set of features can correspond to a quality of a specific photo. The first set of features, therefore, can also be referred to as a set of quality features. The first set of features for a specific photo can include at least one of photometric features, saliency-based features, and content-based features for the specific photo. The photometric features for the specific photo can include at least one of contrast, hue diversity, sharpness, and brightness of the specific photo. 
     The contrast feature can measure a level of contrast of the specific photo. In some implementations, the contrast feature may be a computation of a length of the red-green-blue (RGB) color histogram interval that covers a predetermined amount of the histogram&#39;s total mass. For example, the predetermined amount may be 90%, which does not include 5% intervals at each end of the histogram. The hue diversity feature may involve a computation of a histogram of hue values that have saturation and brightness within a predefined range. The hue diversity feature can use this histogram to quantify how many dominant hue values are present, which can be expressed as a score proportional to the quantity. 
     The saliency-based features for the specific photo can include at least one of simplicity, composition quality, and lighting quality of the specific photo. Saliency-based features can quantify whether the specific photo includes a clear, salient object. Extracting a specific saliency-based feature for the specific photo, therefore, can further include filtering the specific photo to identify a salient portion of the specific photo that encompasses greater than a predetermined amount of the salient object in the specific photo. For example, the predetermined amount may be 90%. The specific saliency-based feature for the specific photo can then be extracted from the specific photo using the salient portion. 
     The simplicity feature can use the salient portion to quantify how much area the object covers with respect to a total area of the specific photo. The composition quality feature can use a center portion of the salient portion to assess how much the specific photo conforms to a “rule of thirds” for photography. The rule of third suggests that the object should be oriented at a distance of one third the total area from each of the borders of the specific photo for increased aesthetic value. The lighting quality feature can measure a difference between an amount of brightness of the object and an amount of brightness in a background of the specific photo. 
     The content-based features for the specific photo can include a presence of at least one of a specific person, a specific place, and a specific thing. For example, a content-based feature may be a presence of the user  108  in a specific photo or a presence of a specific landmark. In general, the presence of human faces is a strong indication of photo desirability. A single face, however, may be more desirable than a photo that includes a plurality of different faces, each of which may be hard to distinguish. 
     The similarity feature extraction module  308  can extract one or more second features from each of the plurality of photos. Each of the second set of features can indicate a similarity between a specific photo and another one or more of the plurality of photos. The second set of features, therefore, can also be referred to as a set of similarity features. The second set of features for a specific photo can include at least one of spatial resolution, color resolution, and temporal resolution of the specific photo. 
     Extracting a specific feature of the second set of features for the specific photo further can include generating a hierarchical histogram for the specific photo, the hierarchical histogram including one or more visual words. Each visual word can include texton and color. The texton can indicate human texture perception. For example, the color may be in the hue-saturation-value (HSV) color space. It should be appreciated, however, that other color spaces can be used, e.g., the hue-saturation-lightness (HSL) color space. Furthermore, it should be appreciated that other image features can be used, to either replace or augment the visual words. For example, scale-invariant feature transform (SIFT) and/or histogram of orientated gradients (HOG) features can be used. Alternatively, TinyImage features can be used. Using TinyImage features can include down-sampling the specific photo to a predetermined resolution, e.g., 32×32 pixels, concatenating the pixel values to obtain a feature vector, e.g., 32×32×3 or 3072 dimensions, and then applying K-means clustering to the feature vector with random initial cluster centers. 
     Temporal resolution may also be included. If visual words are used, the hierarchical histogram can be modified by adding a timestamp of the specific photo to obtain a modified hierarchical histogram. The similarity feature extraction module  308  can then extract the specific feature of the second set of features for the specific photo based on the modified hierarchical histogram. Temporal resolution can be important because pictures taken close in time tend to be more similar, whereas pictures far apart in time, e.g., in different seasons, can be very different. The timestamp can include a day and a month that the specific photo was captured. In some implementations, the timestamp can be normalized within the day, e.g., a value between 0 and 1, and within the month, e.g., another value between 0 and 1. The timestamp can also be weighted with respect to the other similarity features. For example, the timestamp may account for approximately 17% of the similarity metric. 
     The quality feature weighting module  312  can obtain a quality weight for each of the one or more first features. Specifically, the quality feature weighting module  312  obtain a quality weight for each of the one or more first features based on an analysis of the reference photo collection  120  to obtain a first set of weights. The first set of weights can also be referred to as a set of quality weights. As previously mentioned, the reference photo collection  120  may be a public photo database indicative of quality preferences of a typical user. The reference photo collection  120  can include a plurality of reference photos and a quality weight for each reference photo in the plurality of reference photos. The analysis of the reference photo collection  120  can include performing machine learning of the first set of weights using an L2 regularization with an L2-loss (squared loss) function. For example, the analysis of the reference photo collection  120  can be performed by the server  112 , which may have greater processing capabilities than the computing device  104 . 
     The similarity feature weighting module  316  can obtain a similarity weight for each of the one or more second features. Specifically, the similarity feature weighting module  316  can obtain the similarity weight for each of the one or more second features based on an analysis of the reference photo collection  120  to obtain a second set of weights. The second set of weights can also be referred to as a set of similarity weights. As previously mentioned, the reference photo collection  120  may be a public photo database indicative of similarity preferences of a typical user. The reference photo collection  120  can include a similarity weight for each unique pair of reference photos in the plurality of reference photos. The analysis of the reference photo collection  120  can include any suitable techniques. For example, the computing device  104  or the server  112  may use the same or similar techniques as described above with respect to the quality feature weighting module  312 . 
     The quality metric generation module  320  can generate a quality metric for each of the plurality of photos. The quality metric can also be referred to as a quality score. The quality metric generation module  320  can generate the quality metric for a specific photo by analyzing the first set of features for a specific photo to obtain a set of quality scores. The quality metric generation module  320  can then combine the set of quality scores using the first set of weights to obtain the quality metric for the specific photo. In other words, a single quality metric can be generated a specific photo of the plurality of photos by combining the quality metrics relating to the set of quality features for the specific photo according to the set of quality weights, respectively. In some implementations, the set of quality features includes two or more features. It should be appreciated, however, that the set of quality features may include a single feature, which may not involve the step of combining in order to obtain the quality metric. 
     The similarity matrix generation module  324  can determine a similarity matrix L i,j  for the plurality of photos. For example, the similarity matrix can include one or more similarity metrics for each photo of the plurality of photos, with reference to each of a remainder of the plurality of photos. The similarity matrix generation module  324  can determine the similarity matrix by analyzing the second set of features for a specific pair of photos to obtain a set of similarity metrics. The similarity matrix generation module  324  can then generate the similarity matrix using the set of similarity metrics and the second set of weights. The similarity matrix, therefore, can be an N×N matrix that indicates a similarity between each of the N photos and each of a remainder of the N photos, i.e., the other N−1 photos. In other words, the similarity matric can indicate a similarity between each unique pair of photos of the plurality of photos. The value for each element in the similarity matrix may be a value between 0 and 1. Note, however, that a similarity between a specific photo and itself (or a duplicate) in the similarity matrix may be a value of 1. 
     The joint global maximization module  328  can perform joint global maximization of photo quality and photo diversity for the plurality of photos. The joint global maximization module  328  can generate a combined matrix S i,j  that represents a combination of the quality metrics and the similarity matrix. For example, the combined matrix S i,j  can be calculated as follows:
 
 S   ij   =q   i   f (Ø i ,Ø j ) q   j ,
 
where S i,j  is the combined matrix of quality and similarity features, q i  is a quality metric for an i th  element, q i  is a quality metric for a j th  element, Φ i  is a feature vector corresponding to the i th  element, Φ j  is a feature vector corresponding to the j th  element, and f(Φ i ,Φ j ) is a function for calculating a similarity between feature vector Φ i  and feature vector Φ j  using the similarity matrix.
 
     The photo subset selection module  332  can select a subset of the plurality of photos based on the results of the joint global maximization module  328 . More specifically, the photo subset selection module  332  can select the subset of the plurality of photos using on the combined matrix S i,j . The subset of the plurality of photos can include Y photos, where Y is an integer less than or equal to N. The photo subset selection module  332  can implement a determinantal point process (DPP). DPPs can provide tractable algorithms for exact inference (including computing marginal probabilities and sampling), which can be useful for subset selection where diversity is preferred. 
     The photo subset selection module  332  can perform the DPP as follows. First, the DPP can include performing the following process (hereinafter Process 1):
         (i) initializing Y={ }, J={ };   (ii) adding eigenvector v k  to J with probability λ k /(λ k +1);   (iii) while |J|&gt;0, performing:       

     
       
         
           
             
               
                 
                   ( 
                   a 
                   ) 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 selecting 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   y 
                   i 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 from 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Y 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 with 
                 ⁢ 
                 
                   
                       
                   
                   ⁢ 
                   
                       
                   
                 
                 ⁢ 
                 
                   P 
                   ⁡ 
                   
                     ( 
                     
                       y 
                       i 
                     
                     ) 
                   
                 
               
               = 
               
                 
                   1 
                   
                      
                     J 
                      
                   
                 
                 ⁢ 
                 
                   
                     ∑ 
                     
                       v 
                       ∈ 
                       J 
                     
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     
                       ( 
                       
                         
                           v 
                           T 
                         
                         ⁢ 
                         
                           e 
                           i 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
             
             , 
           
         
       
         
         
           
             
               
                 (b) updating Y=Y∪y i , and 
                 (c) computing J⊥, orthogonal to e i  and setting J=J⊥; and 
               
             
             (iv) returning Y,
 
where Y is a selected subset of photos, v k  is an eigenvector associated with an index k, λ k  is a probability associated with the index k, y i  is an i th  column of the combined matrix S i,j  that is added to the selected subset of photos Y based on its probability P(y i ), J⊥ is a matrix orthogonal to a canonical basis vector e i , and J is an auxiliary matrix used to compensate for elements added to the selected subset of photos Y.
 
           
         
       
    
     Next, the DPP can include determining a probability for the selected subset of photos Y by calculating: 
                 P   ⁡     (   Y   )       =       det   ⁡     (     S   Y     )         det   ⁡     (     S   +   I     )           ,         
where P(Y) is a probability that the selected subset Y is a representative subset of the plurality of photos, and I is an identity matrix. In some implementations, the photo subset selection module  332  may select the subset Y if the probability P(Y) is greater than a predetermined threshold indicating an acceptable representativeness of a typical user. If the probability P(Y) is less than the predetermined threshold, however, the photo subset selection module  332  can repeat Process 1 above to determine another subset Y.
 
     In some implementations, the DPP can include applying a maximum-a-posteriori (MAP) approximation algorithm to determine a number of iterations for performing the selection of the selected subset of photos Y (Process 1), and wherein the subset of the plurality of photos includes the selected subset of photos Y after the number of iterations. For example, the MAP approximation algorithm (hereinafter Process 2) may be defined as follows:
         (i) receiving an input X, parameters ⊖, and a budget B,   (ii) U←S(X) and Y←{ },   (iii) while U≠ { } , performing:       

     
       
         
           
             
               
                 
                   ( 
                   a 
                   ) 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Y 
               
               ← 
               
                 
                   Y 
                   
                     U 
                     ⁢ 
                     
                         
                     
                   
                 
                 ⁢ 
                 
                   
                     
                       arg 
                       ⁢ 
                       max 
                     
                     
                       i 
                       ∈ 
                       U 
                     
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         
                           
                             P 
                             θ 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   Y 
                                   U 
                                 
                                 ⁢ 
                                 
                                   { 
                                   i 
                                   } 
                                 
                               
                               | 
                               X 
                             
                             ) 
                           
                         
                         - 
                         
                           
                             P 
                             θ 
                           
                           ⁡ 
                           
                             ( 
                             
                               Y 
                               | 
                               X 
                             
                             ) 
                           
                         
                       
                       
                         
                           cos 
                           ⁢ 
                           t 
                         
                         ⁡ 
                         
                           ( 
                           i 
                           ) 
                         
                       
                     
                     ) 
                   
                 
               
             
             , 
             and 
           
         
       
         
         
           
             
               
                 (b) U←U\{i|cost(Y)+cost(i)&gt;B}; and 
               
             
             (iv) returning Y,
 
where X represents a portion of the combined matrix S i,j  including parameters ⊖, which represent quality weights and/or similarity weights, B represents a maximum amount of time or computational resources (i.e., a budget), U represents an auxiliary matrix, cost represents a required time or computational resources, and Y indicates the number of iterations. It should be appreciated, however, that other MAP approximation algorithms can be used instead of Process 2.
 
           
         
       
    
     As previously mentioned, after determining the optimal number of iterations, the photo subset selection module  332  can perform Process 1 for the optimal number of iterations. The resulting subset of photos Y after the last one of the optimal number of iterations can be the subset of the plurality of photos (the photo album). The photo subset selection module  332  can then output the subset of the plurality of photos. The subset of the plurality of photos can be stored in the memory  208  and/or can be output via the input/output device  200 . For example, the input/output device  200  can display the subset of the plurality of photos to the user  108 . Additionally or alternatively, for example, the input/output device  200  can output the subset of the plurality of photos to another device, such as the server  112  (via the network  116 ) or another local device, e.g., via a USB cable. 
     Referring now to  FIG. 4 , a flow diagram for an example technique  400  for automatically generating a photo album using a collection of photos is illustrated. At  404 , the computing device  104  can receive a plurality of photos. At  408 , the computing device  104  can extract a first set of features from each of the plurality of photos. Each feature of the first set of features can correspond to a quality of a specific photo and, therefore, can also be referred to as a set of quality features. In some implementations, the set of quality features includes two or more features. At  412 , the computing device  104  can extract a second set of features from each of the plurality of photos. Each feature of the second set of features can indicate a similarity between a specific photo and another one or more of the plurality of photos and, therefore, can also be referred to as a set of similarity features. 
     At  416 , the computing device  104  can determine a weight for each feature of the first set of features based on an analysis of the reference photo collection  120  to obtain a first set of weights. The reference photo collection  120  can include plurality of reference photos and a quality weight associated with each of the plurality of reference photos. In some implementations, the computing device  104  (or the server  112 ) can determine the first set of weights by performing machine learning on the reference photo collection  120  using an L2 regularization with an L2-loss function. At  420 , the computing device  104  can determine a weight for each feature of the second set of features (the set of similarity features) based on an analysis of the reference photo collection  120  to obtain a second set of weights. The reference photo collection  120  can include a similarity weight associated with each unique pair of reference photos in the reference photo collection  120 . 
     At  424 , the computing device  104  can generate a quality metric for each of the plurality of photos by analyzing the first set of features for a specific photo to obtain a set of quality scores and combining the set of quality scores using the first set of weights to obtain the quality metric. At  428 , the computing device  104  can generate a similarity matrix for the plurality of photos by analyzing the second set of features for each unique pair of photos of the plurality of photos to obtain a set of similarity scores and generating the similarity matrix using the set of similarity scores and the second set of weights. At  432 , the computing device  104  can select a subset of the plurality of photos by performing joint global maximization of photo quality and photo diversity based on the quality metrics and the similarity matrix. In some implementations, the computing device  104  can perform the joint global maximization using a DPP, which may include a MAP approximation algorithm. At  436 , the computing device  104  can store the subset of the plurality of photos. 
     The technique  400  can then end or return to  404  for one or more additional cycles. Furthermore, while the technique  400  is shown to be executed at the computing device  104 , it should be appreciated that the technique  400  can be executed in whole or in part at the server  112 . 
     Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known procedures, well-known device structures, and well-known technologies are not described in detail. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The term “and/or” includes any and all combinations of one or more of the associated listed items. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     As used herein, the term module may refer to, be part of, or include: an Application Specific Integrated Circuit (ASIC); an electronic circuit; a combinational logic circuit; a field programmable gate array (FPGA); a processor or a distributed network of processors (shared, dedicated, or grouped) and storage in networked clusters or datacenters that executes code or a process; other suitable components that provide the described functionality; or a combination of some or all of the above, such as in a system-on-chip. The term module may also include memory (shared, dedicated, or grouped) that stores code executed by the one or more processors. 
     The term code, as used above, may include software, firmware, byte-code and/or microcode, and may refer to programs, routines, functions, classes, and/or objects. The term shared, as used above, means that some or all code from multiple modules may be executed using a single (shared) processor. In addition, some or all code from multiple modules may be stored by a single (shared) memory. The term group, as used above, means that some or all code from a single module may be executed using a group of processors. In addition, some or all code from a single module may be stored using a group of memories. 
     The techniques described herein may be implemented by one or more computer programs executed by one or more processors. The computer programs include processor-executable instructions that are stored on a non-transitory tangible computer readable medium. The computer programs may also include stored data. Non-limiting examples of the non-transitory tangible computer readable medium are nonvolatile memory, magnetic storage, and optical storage. 
     Some portions of the above description present the techniques described herein in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. These operations, while described functionally or logically, are understood to be implemented by computer programs. Furthermore, it has also proven convenient at times to refer to these arrangements of operations as modules or by functional names, without loss of generality. 
     Unless specifically stated otherwise as apparent from the above discussion, it is appreciated that throughout the description, discussions utilizing terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     Certain aspects of the described techniques include process steps and instructions described herein in the form of an algorithm. It should be noted that the described process steps and instructions could be embodied in software, firmware or hardware, and when embodied in software, could be downloaded to reside on and be operated from different platforms used by real time network operating systems. 
     The present disclosure also relates to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored on a computer readable medium that can be accessed by the computer. Such a computer program may be stored in a tangible computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, application specific integrated circuits (ASICs), or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus. Furthermore, the computers referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability. 
     The algorithms and operations presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may also be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatuses to perform the required method steps. The required structure for a variety of these systems will be apparent to those of skill in the art, along with equivalent variations. In addition, the present disclosure is not described with reference to any particular programming language. It is appreciated that a variety of programming languages may be used to implement the teachings of the present disclosure as described herein, and any references to specific languages are provided for disclosure of enablement and best mode of the present invention. 
     The present disclosure is well suited to a wide variety of computer network systems over numerous topologies. Within this field, the configuration and management of large networks comprise storage devices and computers that are communicatively coupled to dissimilar computers and storage devices over a network, such as the Internet. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.