Classifications

• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T17/00—Three dimensional [3D] modelling, e.g. data description of 3D objects
  • G06T17/005—Tree description, e.g. octree, quadtree
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T17/00—Three dimensional [3D] modelling, e.g. data description of 3D objects
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F17/00—Digital computing or data processing equipment or methods, specially adapted for specific functions

Definitions

• the field of the invention is that of the visualization of two- or three-dimensional images and 3D scenes. More specifically, the invention relates to a data transmission technique for viewing such images or scenes on a client terminal.
• the invention applies more particularly to the field of multi-level information retrieval of detail, in which the data are structured in a tree form, and in which each node of the tree corresponds to a specific element to be displayed, at a given level of detail. It is of particular interest for network visualization of very large environments, such as city models for example, but more generally applies to any type of data organized in a hierarchical tree, whether it concerns 3D content or progressive images for example.
• the first of these techniques consists, for the server 10, to transmit to the client 11 the entirety of the visualization data, before the client 11 can begin to reconstruct the associated representation, and possibly navigate it. It applies in particular to the transmission of 3D scenes coded in VRML ("Virtual Reality Modeling Language" for "virtual reality modeling language"), for which a progressive transmission of the data is not possible.
• VRML Virtual Reality Modeling Language
• the client 11 sends a first request Yl x to the server 10, asking it to transmit all the data present in the database, and to reconstruct a representation of a 3D image or scene.
• the server 10 responds 13j by transmitting the entire required model.
• the client 11 can then navigate H 1 in the loaded model. If the client 11 wishes to change the model, he must issue a new model request 12 2 , to which the server 10 responds with a new transmission 13 2 of all the visualization data associated with the new required model. It is only after loading the entire new model that the client 11 can begin to navigate 14 2 in its representation.
• the second known transmission technique is based on the progressive transmission of the model (2D or 3D) from the server 10 to the client 11. It concerns the images, or 3D contents, encoded according to a tree or hierarchical structure, of which the visualization requires, at a given instant, to have all the parent nodes of the active nodes (that is to say nodes rendered in the visual representation of the model at the moment considered). Indeed, in the case of a hierarchical structure, there is a strong dependence between the nodes of the structure, the client can not know the existence of an element if the parent element has not yet been transmitted. In such a context of data transmission on demand, the client must therefore request the child nodes, then wait for their transmission before they can request the following child nodes.
• the first step 21 allows the client 11 to recover 2I 2 the first node 24 of the tree it receives after sending a specific request 2I 1 to the server 10.
• the following steps 22 and 23 enable the client 11 to retrieve the information he needs according to his position in the scene or in the image.
• the first node 24 After having recovered 2I 2 the first node 24, it can determine according to its position whether or not it needs to develop it, that is to say if its position requires a more detailed representation of the scene or the scene. 'picture. It performs recursively this operation until it converges to the optimal model corresponding to its position.
• the client 11 traverses the tree after rebuilding it, to determine whether it needs the child nodes of some of the nodes that it already has: in other words, it determines whether one or more nodes of the tree should be expanded.
• the client 11 thus determines during the journey 25j of the tree that the first node 24 must be developed.
• it sends a request 2I 1 to the server 10, which sends back 22 2 the son nodes 26 and 27 required.
• a new path 25 2 of the tree enables him to determine that he needs the child nodes of the nodes 26 and 27 that he has just received.
• a disadvantage of the first transmission technique of Figure 1 is that it is not suitable for the transmission of models or complex images, associated with a large amount of visualization data. Indeed, for such models, the initial transmission time of the data, from the server to the client, is very important, and even prohibitive for the customer, who must wait for the model to fully load before he can start viewing it. According to this first technique, the user can not navigate quickly in the image or in the scene.
• the second transmission technique of FIG. 2 makes it possible to solve this problem of waiting for complete loading before being able to start the visualization.
• it has the disadvantage of generating an exchange of important messages (requests 2I 1 , 2I 2 , 2I 1 to 22 4 , 23 ls ...), especially when the desired level of detail corresponds to a deep branch of the tree structure.
• Another disadvantage of this technique is that it induces the transmission of unnecessary information, and in particular of certain nodes that are not displayed by the client. Indeed, upon receipt of a request for development of a node, the server sends the client all the child nodes of the node to be developed.
• the many client-server exchanges of the technique of Figure 2 limit the speed at which the tree structure present in the client converges to the optimal level of detail depending on the position of the user.
• a disadvantage of this technique is that the progressive refinement of the representation of the image or scene is visible to the client at the beginning of navigation, which induces a visual discomfort for the latter.
• the invention proposes a solution that does not have these disadvantages of the prior art, in the form of a method for transmitting data for viewing a content between a server and at least one terminal.
• client said data being organized in nodes of a hierarchical tree comprising at least one parent node and at least one child node, a displayable element being associated with each of said nodes.
• such a method comprises the following steps: transmission by said server to said client of a simplified representation of said tree comprising, for at least some of said nodes, only information of location of said visualizable element in said content to be viewed; selection by said client terminal, from said simplified representation, of at least one node of interest, as a function of a distance criterion from said client to said viewable element associated with said node of interest, in said content to be viewed; transmission of geometric data enabling reconstruction of said visualizable element for the selected one or more nodes of interest.
• the invention is based on a completely new and inventive approach to the transmission, between a server and one or more client terminals, of visualization data organized according to a tree structure.
• the invention is based on the initial transmission of a simplified tree structure, which does not contain the geometric data allowing the reconstruction of the image or the scene, but allows the user to view the complete hierarchical tree, and selecting the node (s) containing geometric data necessary for reconstruction of the scene or image, according to its current viewpoint. He can then generate queries that exactly match his needs.
• This transmission technique makes it possible, with respect to prior techniques, to load the tree data more quickly, and thus allows the user to start a scene or image visualization with an optimal level of detail, adapted to his point of view. .
• the invention provides the same advantage in case of rapid change of position of the user.
• the invention proposes, in an advantageous embodiment, a faster convergence to an optimal level of detail of the representation, both during the initialization and in case of rapid change of the position of the user.
• the invention thus proposes, according to this advantageous embodiment, a technique allowing a pleasant visual rendering for the user, without successive refinements of the reconstructed image or scene being visible.
• the invention is suitable for use in a network of tree-type content and optimizes viewing applications that operate in query mode.
• the invention proposes a transmission technique of visualization data organized according to a tree structure well adapted to a client-server architecture operating in query mode.
• the invention is suitable for client terminals of various processing capabilities, and in particular a client-server architecture in which a large number of client terminals access the same data server.
• such a visualization data transmission technique is inexpensive in resources, especially in bandwidth and transmission time.
• said simplified representation comprises, for each of said nodes of the tree: an identifier of said node; information making it possible to link said node to said parent node of said node and / or to said child node (s) of said node; a position of a center of said viewable element associated with said node in said content to be displayed (for example the center of gravity of the element in a Cartesian coordinate system associated with the scene or the image); a selection zone of said viewable element associated with said node.
• Such a selection area is for example a sphere of radius R if the content to be viewed is a 3D scene, or a disc of radius R if the content to be viewed is a two-dimensional image. It can also take any other form adapted to the nature of the content or the needs of the intended application.
• Information for linking a child node to a parent node can take the form of an identifier of the parent node of the node considered in the simplified tree.
• a node of said tree is selected as a node of interest if a current position of said client is included in said selection area.
• the position of the user is viewed in a Cartesian coordinate system associated with the scene or the image, and it is determined whether this position is included in the sphere of radius R associated with the considered node.
• said client terminal sends to said server a request for obtaining said display data of said nodes of interest. Thanks to the geometrical data contained in the nodes of interest, the customer will be able to reconstruct a representation of the image or the scene adapted to his point of view. These queries are targeted and perfectly match the needs of the user, which avoids any transmission of unnecessary information.
• a user can thus navigate quickly in the scene or in the reconstructed image.
• said client terminal verifies the presence of said data of said nodes of interest in a cache of said terminal, and said request is not sent to said server for said data already present in said cache.
• the server transmits only the necessary geometrical data to the user of whom it is not yet available: it avoids unnecessary network overhead by limiting the data traffic to the minimum necessary.
• said client terminal in the event of a change in said current position of said client, sends to said server a request to cancel said request for obtaining said data from at least one node which is not plus a node of interest, if said data have not yet been transmitted by said server.
• said client terminal does not traverse a node of said simplified representation to determine whether said distance criterion is verified for said browsed node that if said parent node of said searched node is a node of interest.
• said user tests the different nodes of the hierarchical tree to determine if the geometric data they contain are necessary for viewing the content, it does not scan the child nodes of a tree. father node, if this father node is not a node of interest. It is not necessary to browse the entire tree to determine which nodes are visible. This characteristic stems directly from the inclusion property of the selection zones of the father and son nodes which will be described in more detail below.
• the invention also relates to a computer program comprising program code instructions for executing the steps of the method of transmitting data for viewing content between a server and at least one client terminal described above when said program is executed by a processor.
• the invention also relates to a content viewing data server intended for at least one client terminal, said data being organized in nodes of a hierarchical tree comprising at least one parent node and at least one child node, an element viewable being associated with each of said nodes.
• Such a server comprises: means for generating and transmitting to said client a simplified representation of said tree comprising, for at least some of said nodes, only information of location of said visible element in said content to be displayed, so that said client selects from said simplified representation, at least one node of interest, according to a distance criterion of said client to said viewable element associated with said node of interest, in said content to be viewed; and means for transmitting to said client geometric data allowing a reconstruction of said visualizable element for the selected one or more nodes of interest.
• the invention also relates to a computer program product comprising program code instructions recorded on a medium that can be used in or by a computer, said program enabling the transmission of data for viewing content between a server and at least one terminal.
• client said data being organized in nodes of a hierarchical tree comprising at least one parent node and at least one child node, a displayable element being associated with each of said nodes.
• Such a computer program includes: computer readable programming means for performing a step transmission by said server to said client of a simplified representation of said tree comprising, for at least some of said nodes, only information of location of said visualizable element in said content to be viewed; computer readable programming means for performing a selection step by said client terminal, from said simplified representation, of at least one node of interest, as a function of a distance criterion of said client to said associated displayable element; said node of interest, in said content to be viewed; - Computer readable programming means for performing a geometric data transmission step for a reconstruction of said visualizable element for the selected one or more nodes of interest.
• the invention also relates to a client terminal for viewing content from visualization data organized in nodes of a hierarchical tree comprising at least one parent node and at least one child node, a displayable element being associated with each of said nodes. nodes.
• a terminal comprises: means for receiving a simplified representation of said tree, transmitted by a server, and comprising, for at least some of said nodes, only information of location of said visible element in said content to be viewed; means for selecting, from said simplified representation, at least one node of interest, as a function of a distance criterion from said client to said viewable element associated with said node of interest, in said content to be displayed; geometric data transmission means for reconstructing said visualizable element for the selected one or more nodes of interest. 5.
• FIG. 1 already described in relation with the prior art, presents a visualization data transmission technique between a server and a client terminal, based on an integral and prior transmission of the model to be visualized;
• FIG. 2 also illustrates a prior art implementing a progressive transmission of the model to be displayed, by successive exchanges of requests between the client and the server;
• Figure 3 shows a block diagram of the transmission technique of the invention;
• FIG. 4 describes a tree structure of PBTree type associated with a 3D representation of a city, in a particular embodiment of the invention;
• FIG. 5 is a block diagram of the method for generating the description file of the tree structure of FIG. 4;
• FIG. 6 illustrates the geometric representation of the description file of the tree structure generated according to the diagram of FIG. 5;
• FIG. 7 presents a block diagram of the selection mechanism of the nodes of interest by the client terminal;
• FIG. 8 illustrates an example of a two-dimensional image in which the selection zones are not regular;
• Figures 9 and 10 show block diagrams of a server and a client terminal according to the invention.
• the first phase 30 corresponds to the initialization of the client 11. After a connection procedure 3O 1 , it receives 3O 2 , in a file, a simplified description of the scene or the image to be displayed.
• This description file of the tree is generated by the server 10 and contains only the information that allows the selection of the nodes to be displayed; thus, the file contains, for each node associated with a level of detail of the tree, its position in the frame of the scene as well as a selection area, or for each of the nodes: the index of the node; the position of the center of the node (position x, y, z of the center of gravity for example in a cartesian coordinate system associated with the image or the scene); the selection area for this node (a radius r for example if the selection area is a sphere or a disk); the index of the father node.
• the terminal 11 Upon receipt of the tree description file 3O 2 by the client, the terminal 11 decodes this file and then verifies that the selection areas of all the threads of the nodes are included in the selection area of their father. If need be, the client makes the necessary corrections to respect this property of inclusion, by widening the selection zones of the fathers nodes. Once the corrections are made, the client rebuilds the tree structure that has been transmitted. Note that this verification is not mandatory, and could be done by the server, or by third party equipment.
• the second phase (31 to 33) corresponds to the visualization of the scene.
• the client 11 takes care of the selection of the nodes 35 to be displayed, according to the position of the user in the scene or the image (expressed also in the form of coordinates (x, y, z) in a Cartesian coordinate system associated with the scene or image). It uses (34j to 34 3) the description file which is transmitted by the server 10 to the boot 30. If the current position of the user 11 is included in the selection area of a node 35 is justify- this is selected. Every moment, depending on the loaded data and the required nodes, the client sends requests to the server to receive the visualization data.
• the information that the client needs for the visualization of the scene or the image can be geometric representations, compressed images, links to textures, wavelet coefficients, etc.
• the client 11 travels 34 ! the simplified representation of the received tree 3O 2 of the server 10, and determines that it needs the geometrical data contained in the nodes referenced 35 1 to 35 3 . It sends to the server 10 a request 3I 1 to obtain this data, which the server 10 transmits 3I 2 .
• the client 10 can then view the elements of the scene or the image associated with the nodes referenced 35 1 to 35 3 . For example, if the scene is a city model, the client 11 displays the buildings he sees according to his position.
• the client terminal 34 After a first displacement 15j of the user in the scene, the client terminal 34 travels again 2 the simplified tree, testing each of the nodes of the tree, and determines that it also needs the geometric data in nodes son 35 4 to 35 7 of the nodes 35 2 and 35 3 previously received. It then sends the server 10 a request 32 t to obtain the only missing data, which the server sends 32 2 .
• the client reconstructs, on reception, the representation of all the elements associated with the nodes referenced 3S 1 to 35 7 , from the received data (35 4 to 35 7 ), and data already present in its cache (3S 1 to 3). 35 3 ).
• the user starts a third step 33 of visualization, by a new path 34 3 of the simplified tree, which allows him to determine that in view of his new position, only the nodes referenced 3S 1 to 35 5 constitute for it nodes of interest. Since all the geometrical data contained in these nodes of interest 35 1 to 35 5 are already stored in the cache of the client terminal 11, no exchange with the server is necessary for this third display step 33: the client 11 uses the data that he has already loaded.
• This transmission technique illustrated in FIG. 3 applies to all types of tree data, and in particular to multilevel representations of detail of 3D scenes and two-dimensional images. Indeed, a multi-level representation of detail makes it possible to adapt the complexity of the scene or the image in relation to the point of view.
• a tree structure is used to store the different levels of detail in the scene. The root node represents the coarsest level of detail, and the leaf nodes correspond to the most detailed models in the scene (initial models). Several pieces of information can be stored in the tree nodes.
• PBTree proposed by J. Royan et al. (in “PBTree - A new progressive and hierarchical representation for network-based navigation in urban environments", VMV 2003, Kunststoff, Germany) stores simplifications made on buildings represented in 2D 1/2 (footprint, altitude, height) ; VRML detail levels are used to store a facet index for each level of detail of a 3D object.
• image detail representations such as MPEG4 (Motion Picture Expert Group) VTC and JPEG 2000 (Joint Picture Expert Group)
• the tree structure is implicit, and is based on a division of the image.
• image in nested pixel areas for example, the image is divided into four zones, associated with four nodes of the tree, then each of these mother zones is in turn divided into four daughter zones associated with four child nodes of the tree, and so on).
• Information is transmitted to refine a particular area of the image.
• FIG. 4 shows the tree structure (PBTree) associated with such a representation.
• PBTree tree structure
• Each 4O 1 node of this PBTree stores a 2D 1/2 building model at a given level of detail.
• the simplifications used to generate this multi-level representation of details of a set of buildings are three in number: the removal of a vertex from the footprint; the merging of two adjacent buildings; the merger of two non-adjacent buildings.
• the set of nodes referenced 41 groups all the mergeable nodes, and the set of nodes referenced 42 represents the list of active nodes (that is, whose associated contents are displayed on the terminal and which are likely to be refined).
• the city model is described by means of a vector that defines the footprints of the buildings associated with each of the nodes.
• Each node must contain a 2D 1/2 representation that allows a 3D reconstruction of the associated building and includes the following elements: a tree node structure; a footprint of a building (a polygon indexed to the vector above); height ; - an altitude; a roof model; possibly one or more parameters allowing a more complex 3D modeling of the building.
• This representation allows a progressive transmission, and dependent on the point of view, towards the customer.
• the implementation described below relates more specifically to the selection of the nodes to be transmitted according to the position of the user.
• FIG. 5 presents a block diagram of the method for generating the description file of the tree structure, which comprises six successive steps organized in two main phases. This generation is implemented by the server, either on storage of visualization data of an object, or on receipt of a first request from a user wishing to access this object.
• the first phase 50 consists of a calculation of the selection zones associated with each of the nodes. It begins with a loading 52 of the entire tree representing the object (scene or image - in this case, a city) that the client terminal wishes to view. In an iterative loop, the server then traverses each of the nodes of the loaded tree in an arbitrary path of order. For each of the nodes traversed, the server determines 54 the selection area of the node, that is to say the geographical area in which the user will have to be for the need to load the contents of the node. In a particular embodiment of the invention, it is proposed to use, to define this selection area, a geometric error which is calculated according to the original model. More precisely, the selection area is a sphere whose radius is chosen according to a geometric error with respect to the original model, by using a generic cost function with multiple parameters.
• the server saves the center of each of the selection areas that it has just calculated, and then checks that all the nodes of the tree have been processed. If not, it loops back to the referenced step 53 of travel of the tree; if so, it moves to the second phase of the process.
• This second phase 51 for generating the simplified representation of the hierarchical tree consists of a verification of the inclusion property of the zone of the son nodes in that of the fathers nodes. Indeed, in this particular embodiment, it is required that this inclusion constraint be verified for all the nodes of the tree.
• This second phase 51 is optional.
• the server goes through each of the nodes of the tree, but the order of the route is now imposed: we start by traversing the nodes of the bottom of the tree, that is to say the leaf nodes, for finish with the root. For each of the nodes traveled, 58 is checked if this node has one or more son. If not, we return to the previous step 57 of course of the nodes of the tree. If so, we proceed to the next step 59 of checking the inclusion of the selection areas of all the child nodes in the selection area of the parent node. The progeny test 58 indicates that there can be no modification of the selection areas associated with leaf nodes of the tree.
• the step 59 of checking the inclusion property of the selection zones of the child and father nodes implements the following calculation.
• O p be the center of the selection zone of the parent node p
• O n be the center of the selection zone of the child node n.
• R p be the radius of the selection zone of the parent node p
• R n the radius of the selection zone of the child node n.
• Ad n is the distance between p and O O n. It must be verified that:
• the selection area of the parent node is widened so that the inclusion property is satisfied.
• FIG. 6 illustrates the geometric representation of the description file of the tree structure generated according to the diagram of FIG. 5.
• the displayable elements associated with each of the nodes have a position in the description space of the city.
• FIG. 6 illustrates the various nodes 6I 1 to IA x of the tree structure making it possible to describe the city in question, as well as the area of selection.
• FIG. 6 illustrates the inclusion property of the selection zones of the nodes 6I 1 to IA x . Note that a node and the associated selection area have the same reference numerical, to the index, the index associated with the node being 1, and the index associated with the selection zone being 2.
• the selection areas 72 2, 73 2 and 74 2 associated with the leaf nodes 72 ls 73j and 74 1 are included in the selection area 67 2 associated with the referenced node 6I 1 .
• the selection areas 65 2 66 2 and 67 2 associated with the son nodes 65 ls 6O 1 and 67! are included in the selection area 62 2 associated with the node referenced 6I 1 .
• the selection areas 62 2, 63 2 and 64 2 associated with the nodes 62 ls 63 1 and 64 1 are included in the selection area 6I 2 associated with the root node 1 6I.
• selection fields are circles.
• selection areas are spheres. It is important to note that the centers of these spheres are not necessarily located on the same plane (unless there is no relief in the city). It is therefore necessary to consider that FIG. 6 is a view from above in the case considered of representation of a city.
• a file which contains, for each of the nodes of the hierarchical tree: the index of the node in the tree; the position of the center of gravity of the node (in the form of Cartesian coordinates (x, y, z) in a coordinate system associated with the representation of the city); the area of selection for this node (expressed as a radius r, the selection area being a sphere centered on the center of gravity of the node); the index of the father node.
• Figure 7 illustrates the selection mechanism of the nodes of interest, implemented by the client terminal to view the city, on receipt of this file.
• the client who selects the nodes to be viewed. It is he who transmits the requests to the server so that he sends him the data he needs to visualize the tree structure. Indeed, the simplified representation of the tree (or description file of the tree structure) does not contain the geometric information to reconstruct the city, so the client must ask the server once he has selected a node.
• the selection of the nodes and the transmission of the requests are deported to an intermediate equipment of the network, endowed, for example, with larger processing capacities than the client terminal.
• the selection of the nodes of interest by the client terminal comprises two main phases, namely a phase 76 for generating a list of potentially interesting nodes (LNP), from a list of nodes to be tested (LNT), and a second phase 77 refining the LNP list to select the nodes to load or transmit.
• the client terminal first loads the simplified representation of the tree, then tests 75 if the position of the user in the scene has changed since the last selection of nodes of interest.
• the answer to this question is affirmative, and allows the client terminal to enter the first phase 76 of treatment.
• the step referenced 76 4 consists of checking whether the current position of the user is included in the selection area of the current node. Let O be the position of the observer, the center of the selection zone of the current node and Rn the radius of this zone. If I On-O I ⁇ Rn, there is inclusion. Then moves on to the next step 76 5 of adding the current node in the LNP list of potential nodes, ie nodes that are potentially of interest. Then 76 6 is tested if this node is a leaf. If not, add the child nodes of this node to the end of the LNT list. If the user's position is not in the selection area of the current node
• the second processing phase 77 can be considered as a correction phase of the LNP list to avoid redundancies of transmission between the server and the client.
• Each of the nodes of the LNP list is scanned 77 1 , and for each one checks 77 2 if this node is present in the cache of the client (the use of the cache is necessary to avoid transmitting the same information several times). If the node is already in the cache, remove the node 77 3 of the LNP. Then, or in the event of a negative response to the test referenced 77 2 , 77 4 is determined if there are still nodes to check in the LNP. If so, we go back to the first step 77 ! of the second phase 77.
• the client terminal has, with the corrected LNP list, all the nodes of interest whose content is necessary for viewing the city. It can then send the server a request to obtain the geometric data associated with each of these nodes.
• the step referenced 78 is the step of transmitting the geometric data of the node if necessary (if these data are not in the cache). Then return to the referenced step 75 test new position until all the geometric data of the nodes of interest are loaded. To optimize the processing of FIG. 7, it is possible to avoid testing the child nodes if the parent node is not selected as the node of interest. With this optimization, it is not necessary to browse the entire tree to determine the nodes whose content is visible to the user (knowing that the selection areas of the child nodes are included in that of their father).
• Figures 9 and 10 show two block diagrams of the server and the client terminal of the invention.
• the server comprises a memory M 91, and a processing unit 90 equipped with a processor P, which is controlled by the computer program Pg 92.
• the processing unit 90 receives as input visualization data 93 of an image or a 3D scene, organized under hierarchical tree form, from which the processor P generates, according to the instructions of the program Pg 92, a simplified representation 95 of the tree does not contain the geometric information of the nodes.
• the processing unit 90 also has an input intended to receive the requests 94 from a client terminal, from which the processor P triggers the transmission of the simplified representation 95, or of geometric data 96 enabling the terminal to reconstruct the content to be read. visualize according to his point of view.
• the client terminal comprises a memory M 101, and a processing unit 100 equipped with a processor P, which is controlled by the computer program Pg 102.
• the processing unit 100 receives as input the simplified representation 95 of the hierarchical tree generated by the server, from which the processor P selects, according to the instructions of the program Pg 102, one or more nodes of interest according to the point of view of the user, and issues a request 103 corresponding to the server.
• the processing unit 100 also has an input intended to receive geometric data 96 supplied by the server, from which the processor P reconstructs the content to be viewed 104 according to its point of view.
• the technique of the invention allows a faster loading of tree data. You do not have to go through all visible nodes to see if they need to be expanded or merged. Nor is it necessary to transmit the nodes recursively. In addition, in the case of a very fast movement of the user in the scene to be viewed, it is possible to cancel the transmission of nodes become obsolete given the new position.
• the content to be viewed by the client terminal is a two-dimensional image
• the selection areas associated with the image are regular
• Figure 8 shows this example of use: knowing the "mapping" of the image, the user can request the information he needs according to its position relative to the image.
• the different zones 81, 82, 83 of FIG. 8 could correspond to different compression formats (JPEG, JPEG 2000, etc.).
• tiles of different sizes according to the zones.

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