Abstract:
A method of real-time cropping of a real entity in motion in a real environment and recorded in a video sequence, the real entity being associated with a virtual entity, the method comprising the following steps: extraction (S 1,  S 1 A) from the video sequence of an image comprising the real entity recorded, determination of a scale and/or of an orientation (S 2,  S 2 A) of the real entity on the basis of the image comprising the real entity recorded, transformation (S 3,  S 4,  S 3 A, S 4 A) suitable for scaling, orienting, and positioning in a substantially identical manner the virtual entity and the real entity to recorded, and substitution (S 5,  S 6,  S 5 A, S 6 A) of the virtual entity with a cropped image of the real entity, the cropped image of the real entity being a zone of the image comprising the real entity recorded delimited by a contour of the virtual entity.

Description:
FIELD OF THE INVENTION 
       [0001]    One aspect of the invention concerns a method for cropping, in real time, a real entity recorded in a video sequence, and more particularly the real-time cropping of a part of a user&#39;s body in a video sequence, using an avatar&#39;s corresponding body part. Such a method may particularly but not exclusively be applied in the field of virtual reality, in particular animating an avatar in a so-called virtual environment or mixed-reality environment. 
       STATE OF THE PRIOR ART 
       [0002]      FIG. 1  represents an example virtual reality application within the context of a multimedia system, for example a videoconferencing or online gaming system. The multimedia system  1  comprises multiple multimedia devices  3 ,  12 ,  14 ,  16  connected to a telecommunication network  9  that makes it possible to transmit data, and a remote application server  10 . In such a multimedia system  1 , the users  2 ,  11 ,  13 ,  15  of the respective multimedia devices  3 ,  12 ,  14 ,  16  may interact in a virtual environment or in a mixed reality environment  20  (depicted in  FIG. 2 ). The remote application server  10  may manage the virtual or mixed reality environment  20 . Typically, the multimedia device  3  comprises a processor  4 , a memory  5 , a connection module  6  to the telecommunication network  9 , means of display and interaction  7 , and a camera  8 , for example a webcam. The other multimedia devices  12 ,  14 ,  16  are equivalent to the multimedia device  3  and will not be described in greater detail. 
         [0003]      FIG. 2  depicts a virtual or mixed reality environment  20  in which an avatar  21  evolves. The virtual or mixed reality environment  20  is a graphical representation imitating a world in which the users  2 ,  11 ,  13 ,  15  can evolve, interact, and/or work, etc. In the virtual or mixed reality environment  20 , each user  2 ,  11 ,  13 ,  15  is represented by his or her avatar  21 , meaning a virtual graphical representation of a human being. In the aforementioned application, it is beneficial to mix the avatar&#39;s head  22 , in real-time, with a video of the head of the user  2 ,  11 ,  13  or  15  taken by the camera  8 , or in other words to substitute the head of the user  2 ,  11 ,  13  or  15  for the head  22  of the corresponding avatar  21  dynamically or in real time. Here, dynamic or in real-time means synchronously or quasi-synchronously reproducing the movements, postures, and actual appearances of the head of the user  2 ,  11 ,  13  or  15  in front of his or her multimedia device  3 ,  12 ,  14 ,  16  on the head  22  of the avatar  21 . Here, video refers to a visual or audiovisual sequence comprising a sequence of images. 
         [0004]    The document US 20091202114 describes a video capture method implemented by a computer comprising the identification and tracking of a face within a plurality of video frames in real time on a first computing device, the generating of data representative of the identified and tracked face, and the transmission of the face&#39;s data to a second computing device by means of a network in order for the second computing device to display. the face on an avatar&#39;s body. 
         [0005]    The document by SONOU LEE et al: “CFBOXTM: superimposing 3D human face on motion picture”, PROCEEDINGS OF THE SEVENTH INTERNATIONAL CONFERENCE ON VIRTUAL SYSTEMS AND MULTIMEDIA BERKELEY, Calif., USA Oct. 25-27, 2001, LOS ALAMITOS, Calif., USA, IEEE COMPUT. SOC, US LNKD D01:10.1109NSMM.2001.969723, Oct. 25, 2001 (2001-10-25), pages 644-651, XP01567131 ISBN: 978-0-7695-1402-4 describes a product named CFBOX which constitutes a sort of personal commercial film studio. It replaces the person&#39;s face with that of a user&#39;s modeled face, using, in real-time, a three-dimensional face integration technology. It also proposes manipulation features for changing the modeled face&#39;s texture to suit one&#39;s tastes. It therefore enables the creation of custom digital video. 
         [0006]    However, cropping the head from the video of the user captured by the camera at a given moment, extracting it, then pasting onto the avatar&#39;s head and repeating the sequence at later moments is a difficult and expensive operation, because real rendering is sought out. First, contour recognition algorithms require a high-contrast video image. This may be obtained in a studio with ad hoc lighting. On the other hand, this is not always possible with a webcam and/or in the lighting environment of a room in a home or office building. Additionally, contour recognition algorithms require heavy computing power from the processor. Generally speaking, this much computing power is not currently available on standard multimedia devices such as personal computers, laptop computers, personal digital assistants (PDAs), or smartphones. 
         [0007]    Consequently, there is a need for a method to crop a part of a user&#39;s body in a video in real time, using the corresponding part of an avatar&#39;s body with a high enough quality to afford a feeling of immersion in the virtual environment and which may be implemented with the aforementioned standard multimedia devices. 
       DESCRIPTION OF THE INVENTION 
       [0008]    One purpose o the invention is to propose a method for cropping an area of a video in real time, and more particularly cropping a part of a user&#39;s body in a video in real time by using the corresponding part of an avatar&#39;s body intended to reproduce an appearance of the user&#39;s body part, and the method comprises the steps of:
       extracting from the video sequence an image comprising the user&#39;s recorded body part,   determining an orientation and scale of the user&#39;s body part within the image comprising the user&#39;s recorded body part,   orienting and scaling the avatar&#39;s body part in a manner roughly identical to that of the user&#39;s body part, and   using a contour of the avatar&#39;s body part to form a cropped image of the image comprising the user&#39;s recorded body part, the cropped image being limited to an area of the image comprising the user&#39;s recorded body part contained within the contour.       
 
         [0013]    According to another embodiment of the invention, the real entity may be a user&#39;s body part, and the virtual entity may be the corresponding part of an avatar&#39;s body that is intended to reproduce an appearance of the user&#39;s body part, and the method comprises the steps of:
       extracting from the video sequence an image comprising the user&#39;s recorded body part,   determining an orientation of the user&#39;s body part from the image comprising the user&#39;s body part,   orienting the avatar&#39;s body part in a manner roughly identical to that of the image comprising the user&#39;s recorded body part,   translating and scaling the image comprising the user&#39;s recorded body part in order to align it with the avatar&#39;s corresponding oriented body part,   drawing an image of the virtual environment in which a cropped area bounded by a contour of the avatar&#39;s oriented body part is coded by an absence of pixels or transparent pixels; and   superimposing the virtual environment&#39;s image onto the image comprising the user&#39;s translated and scaled body part.       
 
         [0020]    The step of determining the orientation and/or scale of the image comprising the user&#39;s recorded body part may be carried out by a head tracker function applied to said image. 
         [0021]    The steps of orienting and scaling, extracting the contour, and merging may take into account noteworthy points or areas of the avatar&#39;s or user&#39;s body part. 
         [0022]    The avatar&#39;s body part may be a three-dimensional representation of said avatar body part. 
         [0023]    The cropping method may further comprise an initialization step consisting of modeling the three-dimensional representation of the avatar&#39;s body part in accordance with the user&#39;s body part whose appearance must be reproduced. 
         [0024]    The body part may be the user&#39;s or avatar&#39;s head. 
         [0025]    According to another aspect, the invention pertains to a multimedia system comprising a processor implementing the inventive cropping method. 
         [0026]    According to yet another aspect, the invention pertains to a computer program product intended to be loaded within a memory of a multimedia system, the computer program product comprising portions of software code implementing the inventive cropping method whenever the program is run by a processor of the multimedia system. 
         [0027]    The invention makes it possible to effectively crop areas representing an entity within a video sequence. The invention also makes it possible to merge an avatar and a video sequence in real time, with sufficient quality to afford a feeling of immersion in a virtual environment. The inventive method consumes few processor resources, and uses functions that are generally encoded into graphics cards. It may therefore be implement it with standard multimedia devices such as personal computers, laptop computers, personal digital assistants, or smartphones. It may use low-contrast images or images with defects that come from webcams. 
         [0028]    Other advantages will become clear from the detailed description of the invention that follows. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0029]    The present invention is depicted by nonlimiting examples in the attached Figures, in which identical references indicate similar elements: 
           [0030]      FIG. 2  depicts a virtual or mixed reality environment in which an avatar evolves; 
           [0031]      FIGS. 3A and 3B  are a functional diagram illustrating one embodiment of the inventive method for the real-time cropping of a user&#39;s head recorded in a video sequence; and 
           [0032]      FIGS. 4A and 4B  are a functional diagram illustrating another embodiment of the inventive method for the real-time cropping of a user&#39;s head recorded in a video sequence. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]      FIGS. 3A and 3B  are a functional diagram illustrating one embodiment of the inventive method for the real-time cropping of a user&#39;s head recorded in a video sequence. 
         [0034]    During a first step S 1 , at a given moment an image  31  is extracted EXTR from the user&#39;s video sequence  30 . Video sequence refers to a succession of images recorded, for example, by the camera (see  FIG. 1 ). 
         [0035]    During a second step S 2 , a head tracker function HTFunc is applied to the extracted image  31 . The head tracker function makes it possible to determine the scale E and orientation O of the user&#39;s head. It uses the noteworthy position of certain points or areas of the face  32 , for example the eyes, eyebrows, nose, cheeks, and chin. Such a head tracker function may be implemented by the software application “faceAPI” sold by the company Seeing Machines. 
         [0036]    During a third step S 3 , a three-dimensional avatar head  33  is oriented ORI and scaled ECH in a manner roughly identical to that of the extracted image&#39;s head, based on the determined orientation O and scale E. The result is a three-dimensional avatar head  34  whose size and orientation comply with the image of the extracted head  31 . This step uses standard rotating and scaling algorithms. 
         [0037]    During a fourth step S 4 , the three-dimensional avatar head  34  whose size and orientation comply with the image of the extracted head is positioned ROSI like the head in the extracted image  31 . The result is that the two heads are identically positioned compared to the image. This step uses standard translation functions, with the translations taking into account noteworthy points or areas of the face, such as eyes, eyebrows, nose, cheeks, and/or chin as well as noteworthy points encoded for the avatar&#39;s head. 
         [0038]    During the fifth step S 5 , the positioned three-dimensional avatar head  35  is projected PROJ onto a plane. A projection function on a standard plan, for example a transformation matrix, may be used. Next, only the pixels from the extracted image  31  that are located within the contour  36  of the projected three-dimensional avatar head are selected PIX SEL and saved. A standard function ET may be used. This selection of pixels forming a cropped head image  37 ; a function of the avatar&#39;s projected head and the image resulting from the video sequence at the given moment. 
         [0039]    During a sixth step S 6 , the cropped head image  37  may be positioned, applied, and substituted SUB for the head  22  of the avatar  21  evolving within the virtual or mixed reality environment  20 . This way, the avatar features, within the virtual environment or mixed reality environment, the actual head of the user in front of his or her multimedia device, at roughly the same given moment. According to this embodiment, as the cropped head image is pasted onto the avatar&#39;s head, the avatar&#39;s elements, for example its hair, are covered by the cropped head image  37 . 
         [0040]    As an alternative, the step S 6  may be considered optional when the cropping method is used to filter a video sequence and extracts only the user&#39;s face from it. In this case, no image of a virtual environment or mixed-reality environment is displayed. 
         [0041]      FIGS. 4A and 4B  are a functional diagram illustrating one embodiment of the inventive method for the real-time cropping of a user&#39;s head recorded in a video sequence. In this embodiment, the area of the avatar&#39;s head  22  corresponding to the face is encoded in a specific way in the three-dimensional avatar head model. It may, for example, be the absence of corresponding pixels or transparent pixels. 
         [0042]    During a first step S 1 A, at a given moment an image  31  is extracted EXTR from the user&#39;s video sequence  30 . 
         [0043]    During a second step S 2 A, a head tracker function HTFunc is applied to the extracted image  31 . The head tracker function makes it possible to determine the orientation O of the user&#39;s head. It uses the noteworthy position of certain points or areas of the face  32 , for example the eyes, eyebrows, nose, cheeks, and chin. Such a head tracker function may be implemented by the software application “faceAPI” sold by the company Seeing Machines. 
         [0044]    During a third step S 3 A, the virtual or mixed reality environment  20  in which the avatar evolves  21  is calculated and a three-dimensional avatar head  33  is oriented ORI in a manner roughly identical to that of the extracted image&#39;s head based on the determined orientation O. The result is a three-dimensional avatar head  34 A whose orientation is complies with the image of the extracted head  31 . This step uses a standard rotation algorithm. 
         [0045]    During a fourth step S 4 A, the image  31  extracted from the video sequence is positioned POSI and scaled ECH like the three-dimensional avatar head  34 A in the virtual or mixed reality environment  20 . The result is an alignment of the image extracted from the video sequence  38  and the avatar&#39;s head in the virtual or mixed reality environment  20 . This step uses standard translation functions, with the translations taking into account noteworthy points or areas of the face, such as eyes, eyebrows, nose, cheeks, and/or chin as well as noteworthy points encoded for the avatar&#39;s head. 
         [0046]    During a fifth step S 5 A, the image of the virtual or mixed reality environment  20  in which the avatar  21  evolves is drawn, taking care not to draw the pixels that are located outside the area of the avatar&#39;s head  22  that corresponds to the oriented face, as these pixels are easily identifiable thanks to the specific coding of the area of the avatar&#39;s head  22  that corresponds to the face and by simple projection. 
         [0047]    During a sixth step S 6 A, the image of the virtual or mixed reality environment  20  and the image extracted from the video sequence comprising the user&#39;s translated and scaled head  38  are superimposed SUP. Alternatively, the pixels of the image extracted from the video sequence comprising the user&#39;s translated and scaled head  38  which are behind the area of the avatar&#39;s head  22  that corresponds the oriented face are integrated into the virtual image at the depth of the deepest pixels in the avatar&#39;s oriented face. 
         [0048]    This way, the avatar features, within the virtual environment or mixed reality environment, the actual face of the user in front of his or her multimedia device, at roughly the same given moment. According to this embodiment, like the image of the virtual or mixed reality environment  20  that comprises the avatar&#39;s cropped face is superimposed onto the image of the user&#39;s translated and scaled head  38 , the avatar&#39;s elements, for example its hair, are visible and cover the user&#39;s image. 
         [0049]    The three-dimensional avatar head  33  is taken from a three-dimensional digital model. It is fast and simple to calculate, regardless of the orientation of the three-dimensional avatar head for standard multimedia devices. The same holds true for projecting it onto a plane. Thus, the sequence as a whole gives a quality result, even with a standard processor. 
         [0050]    The sequence of steps S 1  to S 6  or S 1 A to S 6 A may then be reiterated for later moments. 
         [0051]    Optionally, an initialization step (not depicted) may be performed a single time prior to the implementation of sequences S 1  to S 6  or S 1 A to S 6 A. During the initialization step, a three-dimensional avatar head is modeled in accordance with the user&#39;s head. This step may be performed manually or automatically from an image or from multiple images of the user&#39;s head taken from different angles. This step makes it possible to accurately distinguish the silhouette of the three-dimensional avatar head that will be best suited for the inventive real-time cropping method. The adaptation of the avatar to the user&#39;s head based on photo may be carried out by means of a software application such as, for example, “FaceShop” sold by the company Abalone. 
         [0052]    The Figures and their above descriptions illustrate the invention rather than limit it. In particular, the invention has just been described in connection with a particular example that applies to videoconferencing or online gaming. Nonetheless, it is obvious for a person skilled in the art that the invention may be extended to other online applications, and generally speaking all applications that require an avatar that reproduces the user&#39;s head in real-time, for example a game, a discussion forum, remote collaborative work between users, interaction between users to communicate via sign language, etc. It may also be extended to all applications that require the real-time display of the user&#39;s isolated face or head. 
         [0053]    The invention has just been described with a particular example of mixing an avatar head and a user head. Nonetheless, it is obvious for a person skilled in the art that the invention may be extended to other body parts, for example any limb, or a more specific part of the face such as the mouth, etc. it also applies to animal body parts, or objects, or landscape elements, etc. 
         [0054]    Although some Figures show different functional entities as distinct blocks, this does not in any way exclude embodiments of the invention in which a single entity performs multiple functions, or multiple entities perform a single function. Thus, the Figures must be considered as a highly schematic illustration of the invention. 
         [0055]    The symbols of references in the claims are not in any way limiting. The verb “comprise” does not exclude the presence of other elements besides those listed in the claims. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.