Abstract:
Disclosed is a screen synthesising device which can achieve greater power saving effects when synthesising a plurality of display screens containing moving images, without affecting the display result. In the disclosed device, a synthesis destination allocation determination unit ( 105 ) determines to synthesise the display layer which is the object of determination among the plurality of display layers including a first display layer having the first frame as a synthesis destination candidate and a second display layer having the second frame as a synthesis destination candidate, in either the first frame or the second frame on the basis of layout information which shows the positional relationship between the display layer which is the object of determination and a determination reference region which corresponds to the display layer which is the object of determination, and on the basis of the synthesis processing costs of the first frame and the second frame.

Description:
TECHNICAL FIELD 
       [0001]    The claimed invention relates to a screen compositing apparatus and a screen compositing method in which a plurality of display frames including a moving image are composited. 
       BACKGROUND ART 
       [0002]    In recent years, portable terminal apparatuses have functionally evolved in terms of high-quality video reproduction, large-screen displays and multi-application compatibility, for example. Along with the development of high-function portable terminal apparatuses, it has become important to consider how to perform process for compositing and displaying drawings in a plurality of applications in a power saving manner. 
         [0003]    As a conventional technique for compositing screens in which a plurality of display frames (i.e., display layers) including a moving image are composited, a technique disclosed in Patent Literature (PTL) 1 is known. The technique of PTL1 produces a video frame, background frame, and a foreground frame separately. In the technique disclosed in PTL 1, an on-screen image is produced by sequentially superimposing the background frame, the video frame, and the foreground frame in a predetermined order. The “video frame” is configured with display layers for displaying a video image (i.e., moving image). The “background frame” is configured with display layers for displaying a background image (i.e., still image) positioned at the back (lower-layer side) of the video frame. The foreground frame is configured with display layers for displaying a front image (i.e., still image) positioned at the front (upper-layer side) of the video frame. In other words, the technology in PTL 1 discloses that the display layers positioned at the back frame side with respect to the video frame are pre-composited before compositing the background frame with the video frame. Moreover, the technology in PTL 1 discloses that the display layers positioned at the front frame side with respect to the video frame are pre-composited to produce the foreground frame. 
         [0004]    The background frames (still image) and the foreground frames (still imago) are updated less frequently than the video frame image (moving image). In other words, in the above-described conventional technology, there is no need to update the background frame and the foreground frame simultaneously with an update of the moving image frame. Accordingly, the screen compositing apparatus in the above-described technique may composite the three frames: the background frame, the moving image frame and the foreground frame immediately before final screen output. Thus, the above-described conventional technology enables power saving by reducing a process load when producing an on-screen image. 
       CITATION LIST 
     Patent Literature 
     PTL 1 
       [0000]    
       
         Japanese Patent Application Laid-Open No. 2006-301029 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0006]    In the above-described conventional technology, the screen compositing apparatus composites the three frames: the background frame, the moving image frame and the foreground frame immediately before final screen output. At this case, the screen compositing apparatus is necessary to produce the same output results as those produced in a case where forming the final screen by sequentially repeating a composition of display layers from the lowest layer (i.e., back display layer) as with an ordinary screen compositing apparatus. Therefore, when the screen compositing apparatus sets transparency of an image in a pre-composition of the foreground frame (display layers at the front side) and/or the like, it requires that the screen compositing apparatus perform a pre-compositing process while a set value of the transparency (transparency information) is maintained. The process for performing a pre-composition maintaining the set transparency value is more complicated and requires a greater amount of computation than a conventional transparent composition. Furthermore, the screen compositing apparatus maintains transparency information even after pre-composition. Accordingly, the screen compositing apparatus mast use even more memory for pre-compositing the foreground frame. 
         [0007]    On the other hand, in the pre-composition of the background frame, the screen compositing apparatus only needs to perform composition for conventional screen display. Therefore, the pre-composition of the background frame can be achieved via a process that needs less computation than that of the foreground frame even when the transparency of an image is set. Furthermore, there is no need for transparency information after the pre-composition of the background frame. Accordingly, the screen compositing apparatus requires utilization of less memory with respect to the background frames than the pre-composition results of the foreground frame. 
         [0008]    The above-described comparison in amount of computation for the pre-composition is a comparison made in a case where the same computational resources are assigned for the foreground frame and the background frame. Another screen compositing apparatus is conceivable, the configuration of which needs to have a hardware engine capable of high-speed process provided for computation in composition process for the foreground frame and needs software computation in composition process for the background frame, for example. In such an apparatus, pre-composition process for the foreground frame can be performed in a smaller amount of computation in comparison with pre-composition process for the background frame. In other words, in such an apparatus, pre-composition process for the foreground frame can be performed at a lower process cost in comparison with pre-composition process for the background frame. 
         [0009]    Similarly, a configuration is also conceivable in which a memory is arranged for storing the pre-composition results at a more rapid speed than a memory for the background frame with respect to a foreground frame, and/or the like. With respect to memory for storing the pre-composition results, a foreground frame does not necessarily require more memory than the background frame. In other words, the pre-composition of the foreground frame does not necessarily require a higher process cost than the background frame. 
         [0010]    In other words, the condition of the process loads in the pre-composition of the foreground frame and the background frame depend on the characteristics of the computational resources and the memory resources and/or the like of the screen compositing apparatus. The above-described process loads include process costs such as the amount of computation and memory capacity. 
         [0011]    On the other hand, the screen compositing apparatus in the above-described conventional technology performs the composition process based only on a display layout (e.g., the hierarchical order of display layers). Thus, a problem exists in which the process load (e.g., the above-described process cost) in the pre-composition of the foreground frame and the background frame has not been considered in the above-described conventional technology. Accordingly, there are cases where the power saving effect in the above-described conventional technology may be insufficient. 
         [0012]    An object of the claimed invention is to provide a screen compositing apparatus and a screen compositing method capable of obtaining an improved power saving effect without affecting a display result when compositing a plurality of display frames including a moving image. 
       Solution to Problem 
       [0013]    To achieve the above-described object, a screen compositing apparatus according to a first aspect of the claimed invention includes: a first composition section that composites an input display layer into a first frame; a second composition section that composites an input display layer into a second frame; a composition output section that composites the first frame and the second frame to produce a display screen; and a determination section that receives input of a plurality of display layers including a first display layer associated with the first frame used as a composition destination candidate and a second display layer associated with the second frame used as a composition destination candidate and that determines which one of the first frame and the second frame is composited with a determination, target display layer among the plurality of display layers based on layout information indicating a positional relationship between a determination reference region corresponding to the determination target display layer and the determination target display layer, and a composition process cost for each of the first frame and the second frame. 
         [0014]    A screen composition method according to a second aspect of the claimed invention includes: compositing an input display layer into a first frame; compositing an input display layer into a second frame; compositing the first frame and the second frame to produce a display screen; and receiving input of a plurality of display layers including a first display layer associated with the first frame used as a composition destination candidate and a second display layer associated with the second frame used as a composition destination candidate and determining which one of the first frame and the second frame is composited with a determination target display layer among the plurality of display layers based on layout information indicating a positional relationship between a determination reference region corresponding to the determination target display layer and the determination target display layer, and a composition process cost of each of the first frame and the second frame. 
       Advantageous Effects of invention 
       [0015]    The claimed invention enables an improved power saving effect to be obtained when compositing a plurality of display frames including a moving image without affecting a display result. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a block diagram showing the configuration of a screen compositing apparatus according to Embodiment 1 of the claimed invention; 
           [0017]      FIG. 2  is a diagram showing an example of a display layout according to Embodiment 1 of the claimed invention; 
           [0018]      FIG. 3  is a diagram showing an example of object information according to Embodiment 1 of the claimed invention; 
           [0019]      FIG. 4  is a diagram showing an example of layout information according to Embodiment 1 of the claimed invention; 
           [0020]      FIG. 5  is a diagram showing an example of composition process cost information according to Embodiment 1 of the claimed invention; 
           [0021]      FIG. 6  is a flowchart showing composition destination allocation determination process according to Embodiment 1 of the claimed invention; 
           [0022]      FIG. 7  is a diagram showing an example of composition destination determination reference region according to Embodiment 1 of the claimed invention; 
           [0023]      FIG. 8  is a block diagram showing the configuration of a screen compositing apparatus according to Embodiment 2 of the claimed invention; 
           [0024]      FIG. 9  is a flowchart showing composition destination allocation determination process according to Embodiment 2 of the claimed invention; 
           [0025]      FIG. 10  is a diagram showing an example of a display layout after division according to Embodiment 2 of the claimed invention; 
           [0026]      FIG. 11  is a diagram showing an example of object information after division according to Embodiment 2 of the claimed invention; 
           [0027]      FIG. 12  is a diagram showing an example of layout information after division according to Embodiment 2 of the claimed invention; 
           [0028]      FIG. 13  is a block diagram showing the configuration of a screen compositing apparatus according to Embodiment 3 of the present embodiment; 
           [0029]      FIG. 14  is a diagram showing an example of a type of divisible object according to Embodiment 3 of the claimed invention; 
           [0030]      FIG. 15  is a flowchart showing a composition destination allocation determination process according to Embodiment 3 of the claimed invention; 
           [0031]      FIG. 16  is a block diagram showing the configuration of a screen compositing apparatus according to Embodiment 4 of the claimed invention; 
           [0032]      FIG. 17  is a diagram showing an example of layout information according to Embodiment 4 of the claimed invention; 
           [0033]      FIG. 18  is a flowchart showing the composition destination allocation determination process according to Embodiment 4 of the claimed invention; and 
           [0034]      FIG. 19  is a flowchart showing display screen composition output process according to Embodiment 4 of the claimed invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0035]    Various embodiments of the claimed invention will be described in detail with reference to the drawings. The embodiments described below are only examples, and therefore are not considered to be limiting of the configuration of the system and the configuration of functional blocks. 
         [0036]    Each of the screen compositing apparatuses described in the embodiments of the claimed invention composites a plurality of display frames (display layers) which include a moving image. In this compositing process, the screen compositing apparatus separately produces a video frame, a background frame and a foreground frame in a display layout of the plurality of display frames that include a moving image. The “video frame” is a moving image frame configured with display layers for displaying a moving image. The “background frame” is a lower-layer frame configured with display layers for displaying a background image positioned at the back of the moving image frame. The “foreground frame” is an upper-layer frame configured with display layers for displaying a front image positioned at the front of the moving image frame. The background image is a graphic positioned in a layer below the moving image. Hereinafter, this image will be referred to as “lower layer GFX.” The front image is a graphic positioned in a layer above the moving image. Hereinafter, this image will be referred to as “upper layer GFX.” 
         [0037]    In other words, each of the screen compositing apparatuses according to the embodiments of the claimed invention pre-composites display frames (e.g., texts and still images) other than moving images by allocating the display frames as upper-layer and lower-layer frames. The screen compositing apparatuses produce a final display screen by sequentially compositing the lower-layer frame, the moving image frame and the upper-layer frame. 
       Embodiment 1 
       [0038]      FIG. 1  is a block diagram showing the configuration of screen compositing, apparatus  100  according to the present embodiment. 
         [0039]    In screen compositing apparatus  100  shown in  FIG. 1 , application group  101  represents a group of applications, such as application  1 , application  2 , . . . , application N, using screen compositing apparatus  100 . 
         [0040]    Object information buffer  102  stores object information indicating data of images drawn by the applications in application group  101  and actual data such as texts. The object information is information on objects in display layers, i.e., information indicating the contents of the display layers to be actually displayed in a defined display layout. 
         [0041]    For example,  FIG. 2  shows display layers included in a display layout. In addition,  FIG. 3  shows object information about the display layers shown in  FIG. 2 . In  FIG. 2 , four display layers: layers A, B, C, and D are displayed with respect to the entire output screen. As shown in  FIG. 3 , the object information stored in object information buffer  102  is configured of “layer name,” “attribute information,” “format information” and “storage address.” The “layer name” indicates a name of one of the display layers. The “attribute information” indicates an attribute of an object. The “format information” indicates a format of an object. The “storage address” indicates an storage address of an object. In the “attribute information” shown in  FIG. 3 , a “text” designates a text; an “image” designates a still image; and a “movie” designates a moving image. For example, in  FIG. 3 , as object information about layer A, attribute information “image” (still image), format information “ARGB8888,” and entity address “0x00080000” are shown. The data format of the object information about layers B to D is the same as that of the object information about layer A. 
         [0042]    Layout information buffer  103  stores layout information about depiction destination regions for images drawn by the applications in application group  101 . Layout information about a depiction destination region is image data coordinate information (information indicating a region in the display layer) and hierarchy information indicating a hierarchical order among the plurality of display layers in the display layout, and/or the like. 
         [0043]    For example,  FIG. 4  shows layout information regarding the display layers included in the display layout shown in  FIG. 2 . As shown in  FIG. 4 , the layout information stored in layout information buffer  103  is configured from “layer name,” “leftmost upper-end coordinates,” “rightmost lower-end coordinates” and “hierarchy information.” A “layer name” indicates the name of one of the display layers. The “leftmost upper-end coordinates” designate the leftmost upper-end coordinate point in the display layer. The “rightmost lower-end coordinates” designate the rightmost lower-end coordinate point in the display layer. The “hierarchy information” indicates a hierarchical order among the plurality of display layers displayed in the entire output screen. 
         [0044]    The coordinate expression in  FIG. 4  is such that the left upper end of the entire output screen shown in  FIG. 2  is expressed as an origin (0, 0) and the coordinate values increase with a change in position in the rightward direction and with a change in position in the downward direction. In the display layout shown in  FIG. 2 , each display layer is assumed to be a rectangular region expressed by leftmost upper-end coordinates and rightmost lower-end coordinates. A smaller value in the hierarchy information shown in  FIG. 4  represents an inner position (lower layer side) on the screen, while a larger value in the hierarchy information represents a front position (i.e., upper layer side) on the screen. In other words, the display layout shown in  FIG. 2  represents a hierarchical structure in which layer B (hierarchy information:  1 ), layer A (hierarchy information:  2 ), layer C (hierarchy information:  3 ) and layer D (hierarchy information:  4 ) are sequentially arranged from the innermost position (i.e., the lowermost layer) on the screen. 
         [0045]    Composition process cost DB  104  stores composition process cost information indicating the process cost of each instance of pre-composition process. Hereinafter, the process cost of each instance of the pre-composition process is referred to as the “composition process cost” or “superposition cost”. For example,  FIG. 5  shows composition process cost information stored in composition process cost DB  104 . The composition process cost information shown in  FIG. 5  is configured of a “composition target buffer name” indicating the name of the buffer for a composition target and a “composition process cost value” indicating the value of the composition process cost. In composition target buffer names shown in  FIG. 5 , FB 0  indicates upper-layer frame composition buffer  107  and FB 1  indicates lower-layer frame composition buffer  109 . Each composition process cost value shown in  FIG. 5  indicates a computation time required for the composition process in a corresponding one of the buffers. When referring to  FIG. 5 , the computation time ( 300 ) required for the composition process in FB 0  (upper-layer frame) is longer than the computation time ( 200 ) required for the composition process in FB 1  (i.e., lower-layer frame). In other words, when referring to  FIG. 5 , the composition process cost for the upper-layer frame is higher than that for the lower-layer frame. 
         [0046]    The description of the present embodiment has been made by way of example to a case where composition process cost DB  104  retains and uses values determined in advance by a system designer. In the present embodiment, the terminal may measure the process times for the upper-layer frame and the lower-layer frame at the time of screen drawing immediately after power-on in the terminal (e.g., screen compositing apparatus  100 ), and/or the like. Also, the terminal may register measurement results as composition process costs. 
         [0047]    Composition destination allocation determination section  105  selects one of upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109  as a composition destination, for the contents of each of images (i.e., display layers) drawn by the applications in application group  101 . More specifically, composition destination allocation determination section  105  refers to object information (e.g.,  FIG. 3 ), layout information (e.g.,  FIG. 4 ) and composition process cost information (e.g.,  FIG. 5 ). The object information is stored in object information buffer  102 . The layout information is stored in layout information buffer  103 . The composition process cost information is stored in composition process cost DB  104 . Composition destination allocation determination section  105  then determines whether to composite the display layer for displaying the contents of each of the images drawn in the applications in the lower frame or in the upper frame. At this time, the display layer for displaying the moving image frame is excluded from the display layers to be allocated to the composition destinations by composition destination allocation determination section  105 . 
         [0048]    Upper-layer frame composition section  106  composites in upper-layer frame composition buffer  107  the display layer (i.e., the results of drawing of an image by an application that become a composition target) input from composition destination allocation determination section  105 . In other words, upper-layer frame composition section  106  composites in upper-layer frame composition buffer  107  the display layer determined to be composited in the upper-layer frame by composition destination allocation determination section  105  (i.e., the display layer determined as upper layer GFX). 
         [0049]    Upper-layer frame composition buffer  107  holds the results of the composition of the display layer (i.e., upper layer (GFX) positioned as a layer above the moving image. 
         [0050]    Lower-layer frame composition section  108  composites the display layer (i.e., the results of drawing of an image by an application that become a composition target) input from composition destination allocation determination section  105  in lower-layer frame composition buffer  109 . In other words, lower-layer frame composition section  108  composites the display layer determined to be composited in the lower-layer frame by composition destination allocation determination section  105  (i.e., the display layer determined as lower layer GFX) in lower-layer frame composition buffer  109 . 
         [0051]    Lower-layer frame composition buffer  109  holds the results of composition of the display layer (i.e., lower layer GFX) positioned as a layer below the moving image. 
         [0052]    In a case where application group  101  includes an application having a moving image reproduction function, moving image decoding section  110  performs decoding process on encoded video data accumulated in screen compositing apparatus  100  or encoded video data transmitted via a network. Moving image decoding section  110  outputs the decoded video data to moving image frame buffer  111 . 
         [0053]    Moving image frame buffer  111  stores video data input from moving image decoding section  110 . 
         [0054]    Display screen composition output process section  112  composites data from the hack of a display screen in the order described below. The order of the composition from the back of the display screen is the result of the composition of the lower-layer frame held by lower-layer frame composition buffer  109 , the video data held by moving image frame buffer  111 , and the results of composition of the upper-layer frame held by upper-layer frame composition buffer  107 . Accordingly, a final output image (i.e., display screen) is produced. The results of composition of the lower-layer frame are the lower-layer frame formed of display layers pre-composited in layers below the moving image display layer (i.e., moving image layer) (i.e., the background frame). The video data is the moving image layer. The results of the composition of the upper-layer frame are the upper-layer frame formed of display layers pre-composited as layers above the moving image layer (i.e., the foreground frame). Display screen composition output process section  112  outputs the final output image to output screen  113 . 
         [0055]    Output screen  113  presents the final output image input from display screen composition output process section  112 . 
         [0056]    Pre-composition process in screen compositing apparatus  100  will be described in detail. Hereinafter, a pre-composition of process will be described with respect to the object information shown in  FIG. 3  and the display layout shown in  FIG. 2 . The display layout shown in  FIG. 2  is formed of the layout information shown in  FIG. 4 . 
         [0057]      FIG. 6  is a flowchart for explaining pre-composition process executed by screen compositing apparatus  100  according to the present embodiment. 
         [0058]    In step (hereinafter, referred to as “ST”)  101  of  FIG. 6 , composition destination allocation determination section  105  initializes parameters to be used for determination of allocations of composition destinations. More specifically, composition destination allocation determination section  105  sets information (e.g., coordinates and hierarchy information) on the region of the moving image frame (i.e., the moving image layer, layer B in  FIG. 2 ) as values of a composition destination determination reference region serving as a determination criterion in composition destination allocation determination process. 
         [0059]    This composition destination determination reference region corresponds to determination parameters serving as a criterion for determining whether composition destination, allocation determination section  105  allocates the regions to be allocated other than the moving image layer to upper-layer frame composition buffer  107  or lower-layer frame composition buffer  109 . For example, the regions to be allocated other than the moving image layer are layers A, C, and D in  FIG. 2 . Specifically, the composition destination determination reference region is the union of the moving image layer that corresponds to the reference layer, and the display layers to be pre-composited as layers above the moving image layer. The display layers to be pre-composited as layers above the moving image layer are those determined to be composited in upper-layer frame composition buffer  107  in the determination process performed by composition destination allocation determination section  105 . 
         [0060]    More specifically, the composition destination determination reference region is the union of the reference layer and the display layer to be composited in a composition destination candidate frame based on the hierarchical order among the display layers positioned in hierarchical order between the reference layer and one of the determination target display layers. For example, a composition destination determination reference region for one of the display layers positioned in a determination target layer above the reference layer is the region of the union of the display layer and the reference layer. The display layer is composited in a composition destination candidate frame based on the hierarchical order (i.e., upper-layer frame composition buffer  107 ) in the display layers between the reference layer and the determination target display layer in the hierarchical order. 
         [0061]      FIG. 7  shows an example of a composition destination determination reference region. A composition destination determination reference region is expressed by a set of a plurality of regions. A composition destination determination reference region is expressed by the union of groups of region information in lines each indicating one region. The composition destination determination reference region shown in  FIG. 7  is shown as the union of a rectangular region defined by leftmost upper-end coordinates (0, 0), rightmost lower-end coordinates (200, 240) and hierarchy information  2 ; and another rectangular region defined by leftmost upper-end coordinates (200, 0), rightmost lower-end coordinates (400, 100) and hierarchy information  3 . Composition destination allocation determination section  105  determines whether or not the determination target region (i.e., display layer) of composition destination allocation is at the front relative to the composition destination determination reference region (i.e., on the front side or the upper-layer side). Composition destination allocation determination section  105  also determines whether or not the region of the determination target display layer of composition destination allocation has a portion in common with the composition destination determination reference region. Composition destination allocation determination section  105  determines which one of upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109  is composited with the display layer. 
         [0062]    In ST 101  shown in  FIG. 6 , composition destination allocation determination section  105  sets the region information about the moving image layer (i.e., layer B shown in  FIG. 2 ) as the composition destination determination reference region to initial values. As a result, the composition destination determination reference region is represented by region information formed of rightmost lower-end coordinates (200, 100), leftmost upper-end coordinates (800, 500) and hierarchy information  1 , based on the layout information on layer B shown in  FIG. 4 . 
         [0063]    In ST 102 , composition destination allocation determination section  105  sets as the present determination target layer of composition destination allocation the display layer in the innermost hierarchical position in the plurality of display layers forming the display layout. Referring to  FIG. 2 , composition destination allocation determination section  105  sets layer A (corresponding to hierarchy information:  2 ) in the innermost position in the plurality of display layers (i.e., layers A, C, and D) other than the moving image layer (layer B) as the present determination target layer of composition destination allocation. 
         [0064]    In ST 103 , composition destination allocation determination section  105  determines whether or not a common portion exists in the present determination target layer of composition destination allocation and the composition destination determination reference region. 
         [0065]    If a common portion exists in the present determination target layer of composition destination allocation and the composition destination determination reference region (ST 103 : YES), composition destination allocation determination section  105  performs process in ST 104 . In ST 104 , composition destination allocation determination section  105  determines whether or not the present determination target layer of composition destination allocation is positioned at the front of the region existing as the common portion of the present determination target layer for composition destination allocation and the composition destination determination reference region. In other words, composition destination allocation determination section  105  determines whether or not the hierarchy information on the present determination target layer for composition destination allocation is larger than the hierarchy information on the composition destination determination reference region in the common portion. 
         [0066]    If the present determination target layer for composition destination allocation is positioned at the front of the region existing as the common portion of the present determination target layer for composition destination allocation and the composition destination determination reference region (ST 104 : YES), composition destination allocation determination section  105  performs process in ST 106 . In other words, composition destination allocation determination section  105  determines that the present determination target layer for composition destination allocation is a region that needs to be composited in upper-layer frame composition buffer  107  and proceeds to process in ST 106 . If the present determination target layer for composition destination allocation is not positioned at the front of the region existing as the common portion of the present determination target layer for composition destination allocation and the composition destination determination reference region (ST 104 : NO), composition destination allocation determination section  105  performs process in ST 108 . In other words, composition destination allocation determination section  105  determines that the present determination target layer for composition destination allocation is a region that needs to be composited in lower-layer frame composition buffer  109  and proceeds to process in ST 108 . 
         [0067]    Thus, in a case where a common portion exists in the present determination target layer for composition destination allocation and the composition destination determination reference region (ST 103 : YES), composition destination allocation determination section  105  performs process in ST 104 . In other words, composition destination allocation determination section  105  determines in ST 104  whether the present determination target layer for composition destination allocation is a layer above or below the composition destination determination reference region. Composition destination allocation determination section  105  then determines, based on the determination result, whether to composite the layer in upper-layer frame composition buffer  107  or in lower-layer frame composition buffer  109 . 
         [0068]    On the other hand, if no common portion exists in the present determination target layer for composition destination allocation and the composition destination determination reference region (ST 103 : NO), composition destination allocation determination section  105  performs process in ST 105 . In ST 105 , composition destination allocation determination section  105  determines whether or not the composition process cost for the upper-layer frame is higher than the composition process cost for the lower-layer frame by referring to the composition process cost information stored in composition process cost DB  104 . The composition process cost for the upper-layer frame is referred to as “upper-layer cost”. For example, FB 0  shown in  FIG. 5  corresponds to the upper-layer cost The composition process cost for the lower-layer frame is referred to as “lower-layer cost”. For example, FB 1  shown in  FIG. 5  corresponds to the lower-layer cost 
         [0069]    If the upper-layer cost is equal to or lower than the lower-layer cost (ST 105 : NO), composition destination allocation determination section  105  determines that the cost of composition with the upper-layer frame is lower than the cost of composition with the lower-layer frame. Accordingly, composition destination allocation determination section  105  determines that the present determination target layer for composition destination allocation is a region to be composited in upper-layer frame composition buffer  107  and proceeds to process in ST 106 . If the upper-layer cost is higher than the lower-layer cost (ST 105 : YES), composition destination allocation determination section  105  determines that the cost of composition with the lower-layer frame is lower than the cost of composition with the higher-layer frame. Accordingly composition destination allocation determination section  105  determines that the present determination target layer for composition destination allocation is a region to be composited in lower-layer frame composition buffer  109  and proceeds to process in ST 108 . 
         [0070]    Thus, in a case where no common portion exists in the present determination target layer for composition destination allocation and the composition destination determination reference region (ST 103 : NO), composition destination allocation determination section  105  performs process in ST 105 . In ST 105 , composition destination allocation determination section  105  determines, in consideration of display results only, that the present determination target layer for composition destination allocation is a region that can be composited in either of upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109 . In other words, composition destination allocation determination section  105  determines to composite the present determination target layer for composition destination allocation in one of upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109  with which composition process can be performed at a lower cost. 
         [0071]    If the present determination target layer for composition destination allocation is positioned at the front of the region existing as the common portion of the present determination target layer for composition destination allocation, and the composition destination determination reference region (ST 104 : YES), composition destination allocation determination section  105  performs process in ST 106 . Likewise, if the upper-layer cost is equal to or lower than the lower-layer cost (ST 105 : NO), composition destination, allocation determination section  105  performs process in ST 106 . In ST 106 , composition destination allocation determination section  105  performs process for adding to the composition destination determination reference region the region information on the region determined presently to be composited in upper-layer frame composition buffer  107  (i.e., the present determination target layer for composition destination allocation). In other words, composition destination allocation determination section  105  sets the union of the present determination target region (i.e., display layer) for composition destination allocation and the present composition destination determination reference region as a now composition destination determination reference region. 
         [0072]    This is because a case is considered in which the present determination target layer for composition destination allocation in composition destination allocation determination process performed after the present composition destination allocation determination process is a region (i.e., display layer) having no portion in common with the present composition destination determination reference region. More specifically, it is intended that composition destination allocation determination section  105  composites this display layer in upper-layer frame composition buffer  107  if this display layer has a portion in common with the present determination target region for composition destination allocation, and if this display layer is positioned at the front relative to the present determination target region for composition destination allocation. In other words, in composition destination allocation determination process performed next time by composition destination allocation determination section  105 , composition destination allocation determination section  105  performs composition destination allocation determination process based on composition process cost only on the display layer having no portion in common with the new composition destination determination reference region. 
         [0073]    In ST 107 , upper-layer frame composition section  106  composites the present determination target layer for composition destination allocation in upper-layer frame composition buffer  107 . 
         [0074]    On the other hand, if the present determination target layer for composition destination allocation is not positioned at the front of the region existing as the common portion of the present determination target layer for composition destination allocation and the composition destination determination reference region (ST 104 : NO), the process advances to process in ST 108 . On the other hand, if the upper-layer cost is higher than the lower-layer cost (ST 105 : YES), the process advances to process in ST 108 . In ST 108 , lower-layer frame composition section  108  composites in lower-layer frame composition buffer  109  the present determination target layer for composition destination allocation. 
         [0075]    In ST 109 , composition destination allocation determination section  105  determines whether or not any one of the display layers to be subjected to composition process exists at the front relative to the present determination target layer for composition destination allocation. If one of the display layers to be subjected to composition process exists at the front relative to the present determination target layer of composition destination allocation (ST 109 : YES), composition destination allocation determination section  105  proceeds to process in ST 110 . In ST 110 , composition destination allocation determination section  105  resets the display layer positioned at the front next to the present determination target layer for composition destination allocation, as the next determination target layer for composition destination allocation. Composition destination allocation determination section  105  then returns to ST 103  to perform composition destination allocation determination process and composition process on the display layer. On the other hand, if no display layer exists at the front relative to the present determination target layer for composition destination allocation (ST 109 : NO), composition destination allocation determination section  105  ends the process. 
         [0076]    An example of composition destination allocation determination, process and composition process on layer A shown in  FIG. 2  will be described. Layer A is a layer higher in position in the hierarchical order relative to layer B (i.e., reference layer). In addition, layer A is a display layer associated with upper-layer frame composition buffer  107  as the composition destination candidate when only the hierarchical order is considered. Also, the composition destination determination reference region corresponding to layer A is the same as layer B. 
         [0077]    Referring to  FIG. 2 , layer A has a portion in common with layer B. Accordingly, composition destination allocation determination section  105  determines that layer A is to be composited in upper-layer frame composition buffer  107 , since layer A is positioned as a layer above layer B. 
         [0078]    In the following case, composition destination allocation determination section  105  composites the determination target display layer (i.e., layer A) in the composition destination candidate selected from upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109 . This case is where the region of the determination target display layer (i.e., layer A) has a portion in common with the composition destination determination reference region including the reference layer. The composition destination candidate for the determination target display layer is upper-layer frame composition buffer  107  when considering only the hierarchical order. 
         [0079]    An example of composition destination allocation determination process and composition process on layer. C shown in  FIG. 2  will be described. Layer C is a display layer higher in position in the hierarchical order relative to layer B (i.e., reference layer). In addition, layer C is a display layer, whose composition destination candidate is upper-layer frame composition buffer  107  if only the hierarchical order is considered. A composition destination determination reference region associated with layer C is the union of layer B (i.e., reference layer) and layer A (i.e., the layer between layer C and layer B in the hierarchical order, which is to be composited in upper-layer frame composition buffer  107 ). 
         [0080]    Referring to  FIG. 2 , layer C has no portion in common with be composition destination determination reference region. Accordingly, composition destination allocation determination section  105  determines that layer C is to composited in lower-layer frame composition buffer  109  with which composition process can be performed at a lower cost (see  FIG. 5 ), of upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109 . 
         [0081]    In other words, in the following case, composition destination allocation determination section  105  composites the determination target display layer (i.e., layer C) in the frame associated with a lower composition process cost among upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109 . This case is where the region of the determination target display layer (i.e., layer C) and the composition destination determination reference region including the reference layer have no common portion. The frame that can be composited at a lower composition process cost is lower-layer frame composition buffer  109 . 
         [0082]    Thus, layer C (hierarchy information:  3 ) is a display layer to be composited in the upper-layer frame above layer  13  (hierarchy information:  1 ) if only the hierarchical order is considered. However, layer C is composited in lower-layer frame composition buffer  109  in which composition process can be performed at a lower cost. In other words, in the following case, screen compositing apparatus  100  can select the buffer in which composition process can be performed at a lower cost as a buffer to be composited. This case is where screen compositing apparatus  100  determines that it is not necessarily required to composite the display layer in upper-layer frame composition buffer  107  even display layer C positioned on the surface side (front side) relative to the moving image layer by considering the final display screen. 
         [0083]    The description has been made of the display layers (including the above-described layers A and C) higher in position in the hierarchical order than layer B (i.e., reference layer) shown in  FIG. 2 . However, the object of process is not limited to them. Screen compositing apparatus  100  performs the same process even on any display layer lower in position in the hierarchical order than layer B (i.e., display layer). In other words, screen compositing apparatus  100  also performs the same process as that described above on a display layer (not shown) associated with lower-layer frame composition buffer  109  used as a composition destination candidate when only the hierarchical order is considered. 
         [0084]    As described above, screen compositing apparatus  100  determines which one of upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109  is composited with each determination target display layer among a plurality of display layers. Screen compositing apparatus  100  makes this determination based on layout information and the composition process costs in upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109 . The layout information indicates the positional relationship between the determination target display layers and determination reference regions associated with the determination target display layers. In this way, screen compositing apparatus  100  can reduce the load of composition process therein without affecting the final display screen. In other words, screen compositing apparatus  100  can reduce the load of composition process therein without changing the hierarchical order in the display layout. 
         [0085]    The present embodiment has been described with respect to a case where composition destination allocation determination section  105  makes composition destination determination for each layer when pre-composition process for updating the contents of upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109  is performed. According to the present embodiment, an implementation of the claimed invention is conceivable in which composition destination allocation determination section  105  performs only composition destination determination for each layer in advance at a point in time when a need arises for updating of the results of composition destination determination for the layer. The point in time when a need arises for updating of the results of composition destination determination for each layer is a point in time when the layout information on the layer is changed, for example. 
         [0086]    As a result, the screen compositing apparatus according to the present embodiment can obtain an improved power saving effect without affecting display results at the time of compositing a plurality of display frames including a moving image. 
       Embodiment 2 
       [0087]      FIG. 8  is a block diagram showing the configuration of screen compositing apparatus  200  according to the present embodiment. In  FIG. 8 , the same reference numerals are used for the same components as those in Embodiment 1 ( FIG. 1 ). The description for the same components will not be repeated. 
         [0088]    In screen compositing apparatus  200  shown in  FIG. 8 , composition destination allocation determination section  201  selects a composition destination for the contents of each of images (i.e., display layers) drawn by the applications in application group  101 , as does composition destination allocation determination section  105  in Embodiment 1. For example, composition destination allocation determination section  201  shown in  FIG. 8  selects one of upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109  as a composition destination for the display layer. At this time, composition destination allocation determination section  201  furthermore determines whether or not the determination target display layer of composition destination allocation can be divided. Composition destination allocation determination section  201  divides the display layer determined to be divisible. Based on the results of this division, composition destination allocation determination section  201  registers changed object information in object information buffer  102  and registers changed layout information in layout information buffer  103 . 
         [0089]    Pre-composition process in screen compositing apparatus  200  will be described in detail. Description will be made below of pre-composition process with respect to the object information shown in  FIG. 3  and the display layout shown in  FIG. 2 , as is the description of Embodiment 1. The display layout shown in  FIG. 2  is formed of the layout information shown in  FIG. 4 . 
         [0090]      FIG. 9  is a flowchart for explaining pre-composition process executed by screen compositing apparatus  200  according to the present embodiment. In  FIG. 9 , the same reference characters are used for the same process as in Embodiment 1 ( FIG. 6 ). The description for the same process will not be repeated. 
         [0091]    When the present determination target layer for composition destination allocation in ST 104  shown in  FIG. 9  is positioned at the front of the region existing as the common portion of the present determination target layer for composition destination allocation and the composition destination determination reference region (ST 104 : YES), the process advances to the process in ST 201 . Composition destination allocation determination section  201  in ST 201  determines whether or not the upper-layer cost is higher than the lower-layer cost by referring to the composition process cost information stored in composition process cost DB  104 , as it does in ST 105 . For example, composition destination allocation determination section  201  in ST 201  determines whether or not the upper-layer cost (e.g., FB 0 ) is higher than the lower-layer cost (e.g., FB 1 ) by referring to the composition process cost information shown in  FIG. 5 . 
         [0092]    When the upper-layer cost is equal to or lower than the lower-layer cost (ST 201 : NO), composition destination allocation determination section  201  determines that the composition process cost required for the upper-layer frame is lower than that required for the lower-layer frame. Composition destination allocation determination section  201  determines that the present determination target layer for composition destination allocation is a region to be composited in upper-layer frame composition buffer  107 , and proceeds to process in ST 106 . In other words, the determination made by composition destination allocation determination section  201  when the upper-layer cost is equal to or lower than the lower-layer cost is that the present determination target layer for composition destination allocation is to be composited in upper-layer frame composition buffer  107  regardless of the relationship between the present determination target layer for composition destination allocation and the composition destination determination reference region (i.e., whether or not there is a common portion). 
         [0093]    On the other hand, when the upper-layer cost is higher than the lower-layer cost (ST 201 : YES), composition destination allocation determination section  201  determines in ST 202  whether or not the present determination target layer for composition destination allocation and the composition destination determination reference region have a region other than the common portion. 
         [0094]    If the present determination target layer for composition destination allocation and the composition destination determination reference region have no region other than the common portion (ST 202 : NO), the process advances to process in ST 106 . In other words, composition destination allocation determination section  201  determines that the present determination target layer for composition destination allocation is a region that needs to be composited in upper-layer frame composition buffer  107 . 
         [0095]    On the other hand, if the present determination target layer for composition destination allocation and the composition destination determination reference region have a region other than the common portion (ST 202 : YES), the process advances to process in ST 203 . In ST 203 , composition destination allocation determination section  201  divides the present determination target layer for composition destination allocation into at least the above-described common portion and the region other than the common portion. In addition, composition destination allocation determination section  201  registers object information and layout information on the display layer after division in object information buffer  102  and layout information buffer  103 , respectively. Composition destination allocation determination section  201  resets the region of the above-described common portion in the divided portions of the display layer as the next determination target layer for composition destination allocation. The process then returns to the process in ST 103 . 
         [0096]    A case where the present determination target layer for composition destination, allocation in the display layout shown in  FIG. 2  is layer A will be described. In other words, the composition destination determination reference region is the same as layer B shown in  FIG. 2  (i.e., rightmost lower-end coordinates (200, 100), leftmost upper-end coordinates (800, 500), and hierarchy information  1 ). 
         [0097]    In this case, composition destination allocation determination section  201  determines that layer A has a portion in common with the composition destination determination reference region (the same region as layer B) (ST 103 : YES). Composition destination allocation determination section  201  also determines that layer A is positioned at the front (upper layer side) relative to the composition destination determination reference region (sT 104 ; YES). As a result, composition destination allocation determination section  201  determines that at least a portion of layer A or the entire layer A needs to be composited in upper-layer frame composition buffer  107 . 
         [0098]    Composition destination allocation determination section  201  then compares the upper-layer cost and the lower-layer cost by referring to the composition process cost information ( FIG. 5 ) stored in composition process cost DB  104  (ST 201 ). When the upper-layer cost is higher than the lower-layer cost in  FIG. 5  (ST 201 : YES), the process advances to ST 202 . In other words, the composition process cost required for the lower-layer frame is lower than that required for the upper-layer frame. Composition destination allocation determination section  201  then determines whether or not layer A and the composition destination determination reference region (i.e., the same region as layer B) have a region other than the common portion (ST 202 ). 
         [0099]    As shown in  FIG. 2 , the region of layer A has a portion in common with layer B and has no portion in common with layer B (i.e., a region other than the common portion). Accordingly, composition destination allocation determination section  201  divides layer A into at least the region of the portion in common with layer B and the region other than the portion in common with layer B (ST 203 ). Since each display layer is expressed as a rectangular region, composition destination allocation determination section  201  divides layer A into layer A 2  that corresponds to the portion in common with layer B, and layers A 0  and A 1  that are regions other than the portion in common with layer B as shown in  FIG. 10 . 
         [0100]    Composition destination allocation determination section  201  registers object information on layer A 0  to layer A 2  resulting from the division of layer A in object information buffer  102  in place of the object information on layer A, as shown in  FIG. 11 . Similarly, composition destination allocation determination section  201  registers layout information on layer A 0 , layer A 1  and layer A 2  as a result of division of layer A in layout information buffer  103 , in place of the layout information on layer A, as shown in  FIG. 12 . 
         [0101]    Composition destination allocation determination section  201  performs composition destination allocation determination process on the display layers indicated by the updated layout information (refer to  FIG. 12 ). More specifically, composition destination allocation determination section  201  first performs composition destination allocation determination process by setting layer A 2  shown in  FIG. 10  as a present determination target layer for composition destination allocation. Subsequently, composition destination allocation determination section  201  performs composition destination allocation determination process by sequentially setting layers A 0  and A 1  shown in  FIG. 10  as a determination target layer for composition destination allocation. 
         [0102]    Layer A 2  shown in  FIG. 10  (the region of the portion in common with the composition destination determination reference region) is thereby determined as a region to be composited in upper-layer frame composition buffer  107 . On the other hand, layers A 0  and A 1  shown in  FIG. 10  (the regions other than the portion in common with the composition destination determination reference region) are determined as a region to be composited in lower-layer frame composition buffer  109 . 
         [0103]    Accordingly, screen compositing apparatus  200  divides the display layer in the case when determining that the entire display layer is to be composited in upper-layer frame composition buffer  107  (ST 103 : YES, ST 104 : YES), and when the upper-layer cost is higher than the lower-layer cost (ST 201 : YES). The display layer in this case is divided into a display layer that needs to be composited in upper-layer frame composition buffer  107  and a display layer that does not affect the final display contents even if it is composited in lower-layer frame composition buffer  109 . The display layer that needs to be composited in upper-layer frame composition buffer  107  is the region of the portion in common with the composition destination determination reference region, for example. The display layer that does not affect the final display contents even if it is composited in lower-layer frame composition buffer  109  is a region other than the portion in common with the composition destination determination reference region, for example. 
         [0104]    In other words, screen compositing apparatus  200  determines which one of upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109  is composited with the region of the portion in common with the composition destination determination reference region. Screen compositing apparatus  200  makes this determination based on whether the region is a layer above or below the moving image layer (i.e., reference layer) in the hierarchical order. On the other hand, screen compositing apparatus  200  determines that the region other than the portion in common with the composition destination determination reference region is to be composited in a frame associated with a lower composition process cost among upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109 . 
         [0105]    In this manner, screen compositing apparatus  200  can reduce the region for which the composition destination buffer is to be determined according to the hierarchical order to a minimum (i.e., only the composition buffer for the region of the portion in common with the composition destination determination reference region is determined). In other words, screen compositing apparatus  200  can increase the region that allows the composition destination buffer to be determined according to the composition process cost to a maximum (i.e., the composition buffer for the region other than the region of the portion in common with the composition destination determination reference region is determined). For the region other than the region restricted by hierarchical order among the display layers in the display layout, screen compositing apparatus  200  can perform composition destination allocation determination process by considering the composition process cost. Screen compositing apparatus  200  can minimize the composition process cost in this way. 
         [0106]    Thus, according to the present embodiment, the screen compositing apparatus can have a power saving effect without affecting display results when compositing a plurality of display frames including a moving image, which is even greater than that of Embodiment 1. 
       Embodiment 3 
       [0107]      FIG. 13  is a block, diagram showing the configuration of screen compositing apparatus  300  according to the present embodiment. In  FIG. 13 , the same reference numerals are used for the same structural elements as those in Embodiment 2 (refer to  FIG. 8 ). Accordingly, descriptions of the same structural elements using the same reference numerals will be omitted. 
         [0108]    In screen compositing apparatus  300  shown in  FIG. 13 , divisible object type DB  301  stores information for making determination in composition destination allocation determination process in composition destination allocation determination section  302  as to whether or not a layout division of a display layout can be made. Divisible object type DB  301  stores information indicating the type (i.e., an attribute of a display content in a display layer) of object capable of layout division of a display layout in composition destination allocation determination process. For example.  FIG. 14  shows an example of information indicating the type of object capable of layout division of a display layer. In other words, referring to  FIG. 14 , process for layout division of a display layer can be performed when the object information on the determination target display layer for composition destination allocation is “still image (image).” 
         [0109]    Composition destination allocation determination section  302  selects a composition destination for the contents of each of images (display layers) drawn by the applications in application group  101 , as does composition destination allocation determination section  201  in Embodiment 2. For example, composition destination allocation determination section  302  shown in  FIG. 13  selects one of upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109  as a composition destination for the display layer. In addition, composition destination allocation determination section  302  determines whether or not a layout division of the determination target display layer for composition destination allocation can be made, as does composition destination allocation determination section  201  in Embodiment 2. At this time, composition destination allocation determination section  302  refers to the information stored in divisible object type DB  301  to determine whether or not a layout division of the determination target display layer for composition destination allocation can be made. Composition destination allocation determination section  302  divides the display layer determined to be divisible. Composition destination allocation determination section  302  registers object information that has changed in object information buffer  102  and registers layout information that has changed in layout information buffer  103  according to the division results. 
         [0110]    Hereinafter, the pre-composition process in screen compositing apparatus  300  will be described in detail. Similarly to Embodiment 2, the pro-composition process with respect to the object information shown in  FIG. 3  and the display layout shown in  FIG. 2  will be described in detail. The display layout shown in  FIG. 2  is formed of the layout information shown in  FIG. 4 . 
         [0111]      FIG. 15  is a flowchart explaining pre-composition process executed by screen compositing apparatus  300  according to the present embodiment. In  FIG. 15 , the same reference characters are used for the same process as those in Embodiment 2 ( FIG. 9 ). The description for the same process will not be repeated. 
         [0112]    When the present determination target layer for composition destination allocation and the composition destination determination reference region have a region other than the common portion in ST 202  shown in  FIG. 15  (ST 202 : YES), the process advances to process in ST 301 . In ST 301 , composition destination allocation, determination section  302  determines whether or not an attribute of the present determination target layer for composition destination allocation is included in the object type stored in divisible object type DB  301 . 
         [0113]    In the following case, composition destination allocation determination section  302  determines that the present determination target layer for composition destination allocation is divisible, and proceeds to process in ST 203 . This case is where the attribute of the present determination target layer for composition destination allocation is included in the object type stored in divisible object type DB  301  (ST 301 : YES). In the other case, i.e., in a case where the attribute of the present determination target layer for composition destination allocation is not included in the object type stored in divisible object type DB  301  (ST 301 : NO), composition destination allocation determination section  302  determines that the present determination target layer for composition destination, allocation is not divisible, and proceeds to process in ST 106 . 
         [0114]    A case where the present determination target layer for composition destination allocation, in the display layout shown in  FIG. 2  is layer A will be described. In other words, the composition destination determination reference region is the same as layer B shown in  FIG. 2  (i.e., rightmost lower-end coordinates (200, 100), leftmost upper-end coordinates (800, 500), hierarchy information  1 ). 
         [0115]    In this case, composition destination allocation determination section  302  determines that layer A has a portion in common with the composition destination determination reference region (the same region as layer B) (ST 103 : YES). Composition destination allocation determination section  302  also determines that layer A is positioned at the front (upper layer side) relative to the composition destination determination reference region (ST 104 : YES). As a result, composition destination allocation determination section  302  determines that at least a portion of layer A or the entire layer A needs to be composited in upper-layer frame composition buffer  107 . 
         [0116]    Also, by referring to the composition process cost information (see  FIG. 5 ) stored in composition process cost DB  104 , composition destination allocation determination section  302  determines that the upper-layer cost is higher than the lower-layer cost (ST 201 : YES). Composition destination allocation determination section  302  also determines that layer A and the composition destination determination reference region (the same region as layer B) have a region other than the common portion (ST 202 : YES). 
         [0117]    In other words, composition destination allocation determination section  302  requires that part (i.e., the portion in common with the composition destination reference region) of the region of layer A shown in  FIG. 2  be composited in upper-layer frame composition buffer  107 . On the other hand, composition destination, allocation determination section  302  can composite the other region of layer A (i.e., the region other than the portion in common with the composition destination reference region) in lower-layer frame composition buffer  109 . In this manner, screen compositing apparatus  300  can reduce the composition process cost. 
         [0118]    Composition destination allocation determination section  302  determines whether or not an attribute of layer A is included in the object type stored in divisible object type DB  301  (ST 301 ). The attribute information on layer A is an “image,” as shown in  FIG. 3 . The type of object stored in divisible object type DB  301  is “image,” as shown in  FIG. 14 . Therefore, composition destination allocation determination section  302  determines that layer A is divisible. Composition destination allocation determination section  302  then makes a layout division of layer A (ST 203 ), as in Embodiment 2 (see  FIG. 10 ). For example, composition destination allocation determination section  302  makes a layout division of layer A such that layer A is divided into the portion in common with the composition destination determination reference region (i.e., layer A 2 ) and the region other than the portion in common with the composition destination determination reference region (i.e., layer A 0  and layer A 1 ). 
         [0119]    A case where the present determination target layer for composition destination allocation in the display layout shown in  FIG. 2  is layer D will be described. In this case, composition destination allocation determination, process with respect to layer A and layer C shown in  FIG. 2  is completed. Therefore, the composition destination determination reference region is expressed by the union of layer B and layer A 2  (the portion in common with layer A and layer B) shown in  FIG. 10 . 
         [0120]    Composition destination allocation determination section  302  requires that part (the portion in common with the composition destination determination reference range) of the region of layer D shown in  FIG. 2  be composited in upper-layer frame composition buffer  107  as it does in the above-described case with respect to layer A. On the other hand, composition destination allocation determination section  302  determines that the other region of layer. D (i.e., the region other than the portion in common with the composition destination determination reference region) can be composited in lower-layer frame composition buffer  109 . In this manner, screen compositing apparatus  300  can reduce the composition process cost. 
         [0121]    Composition destination allocation determination section  302  determines whether or not an attribute of layer D is stored in divisible object type DB  301  (ST 301 ). The attribute information on layer D is “text,” as shown in  FIG. 3 . The type of object stored in divisible object type DB  301  is an “image,” as shown in  FIG. 14 . Therefore, composition destination allocation determination section  302  determines that layer D is indivisible, and composites layer D in upper-layer frame composition buffer  107  (ST 106 , ST 107 ). 
         [0122]    Thus, screen compositing apparatus  300  determines whether or not the display layer is to be divided, by referring to the attribute of the display contents of the determination target display layer for composition destination allocation, in the following case, screen compositing apparatus  300  performs layout division of the display layer, similar to that of Embodiment 2. This case is where screen compositing apparatus  300  determines that layout division of the display layer is possible. By layout division, the display layer is divided into a display layer that needs to be composited in upper-layer frame composition buffer  107  and a display layer that does not affect the final display contents even when it is composited in lower-layer frame composition buffer  109 . The display layer that needs to be composited in upper-layer frame composition buffer  107  is the region of the portion in common with the composition destination determination reference region, and/or the like. Meanwhile, the display layer that does not affect the final display contents even when it is composited in lower-layer frame composition buffer  109  is a region other than the portion in common with the composition destination determination reference region, for example. On the other hand, in a case where screen compositing apparatus  300  determines that layout division of the display layer cannot be performed, screen compositing apparatus  300  composites the entire display layer in the composition destination buffer selected based on the hierarchy information. 
         [0123]    Screen compositing apparatus  300  avoids layout division of a display layer in such a format that process for dividing the display contents (i.e., object) accompanying layout division is complicated and/or the like (e.g., objects other than still image (“image”) in  FIG. 14 ). In other words, screen compositing apparatus  300  performs layout division of display layers based on the characteristics of screen compositing apparatus  300  and characteristics and/or the like with respect to each object attribute. 
         [0124]    Thus, according to the present embodiment, the screen compositing apparatus can have a power saving effect while taking into consideration an object attribute of a display layer and without affecting display results when compositing a plurality of display frames including a moving image, which is even greater than that of Embodiment 1. 
         [0125]    The present embodiment has been described with respect to a case where screen compositing apparatus  300  separately executes the determination process in ST 103  and the determination process in ST 202  shown in  FIG. 15 . Determination in ST 103  is made as to whether or not the present determination target layer for composition destination allocation has a portion in common with the composition destination determination reference region. Determination in ST 202  is made as to whether or not the present determination target layer for composition destination allocation and the composition destination determination reference region have a region other than the common portion. However, screen compositing apparatus  300  may perform the determination process in ST 103  and the determination process in ST 202  at once. Moreover, screen compositing apparatus  300  may be configured so that the result of each of these determination process is referred to at a point in time when the process is required (a point in time corresponding to process in ST 103  in  FIG. 15  or a point in time corresponding to process in ST 202 ). 
         [0126]    The present embodiment has been described with respect to a case where divisible object type DB  301  stores a type of divisible object. However, divisible object type DB  301  may conversely store a type of indivisible object. In other words, composition destination allocation determination section  302  may determine whether or not layout division of the determination target display layer for composition destination allocation can be made based on the type of indivisible object. In addition, the type of divisible object stored in divisible object type DB  301  may be changed during use. The type of divisible object stored in divisible object type may be changed during use, for example, by considering the sum of the sizes of regions that have been subjected to layout division and the sum of the sizes of the regions that have been composited in frames or the sum of composition times. 
         [0127]    The screen compositing apparatus in the present embodiment has been described with respect to a case where an attribute of the display contents of the present determination target display layer for composition destination allocation is used as a determination criterion when determining whether or not the display layer is divisible. In the claimed invention, however, an attribute of the display contents of the determination target display layer for composition destination allocation may be used as a criterion for determining a composition destination for the display layer. In other words, the screen compositing apparatus may determine which one of the upper-layer frame composition buffer and the lower-layer frame composition buffer is composited with the display layer thereof based on an attribute of the display contents of the determination target display layer for composition destination allocation. For example, high-speed and/or high-quality process of still image data can be performed at the time of process for composition in the upper-layer frame, depending on the configuration of the screen compositing apparatus. In some cases, however, a function and performance high enough to enable high-speed and/or high-quality process at the time of process for composition in the lower-layer frame are not provided. In such a case, the screen compositing apparatus sets an attribute “still image” of the object to be processed as determination criterion and selects the upper-layer frame as a composition destination for the object, to be processed. Thus, the screen compositing apparatus determines the composition destination buffer for each display layer so that the display layer is composited in a frame in which more appropriate composition process is performed with respect to an attribute of the display contents of the display layer. 
       Embodiment 4 
       [0128]      FIG. 16  is a block diagram showing the configuration of screen compositing apparatus  400  according to the present embodiment, in  FIG. 16 , the same reference numerals are used for the same components as those in Embodiment 3 ( FIG. 13 ). The description for the same components will not be repeated. 
         [0129]    In screen compositing apparatus  400  shown in  FIG. 16 , layout information buffer  401  stores layout information similar to that in layout information buffer  103  in Embodiments 1 to 3. Layout information similar to that in layout information buffer  103  is image data coordinate information (information indicating the region of each display layer) and hierarchy information, and/or the like. Layout information buffer  401  stores timestamp information as the layout information and the above-described layout information. The timestamp information is information for designating timing (display composition timing) of performing process for display composition of display layers in display screen composition output process section  403  described hereinafter. 
         [0130]    For example, as shown in  FIG. 17 , the layout information stored in layout information buffer  401  is configured of “layer name,” “leftmost upper-end coordinates,” “rightmost lower-end coordinates,” “hierarchy information,” and “timestamp information.” A value indicated as timestamp information in  FIG. 17  (“ 0 ” in  FIG. 17 ) denotes a timestamp designating the timing of the display composition of each display layer. A case where there is no need to assign timestamp information is indicated by a symbol (hyphen (“-”)) indicating that there is no information. 
         [0131]    Composition destination allocation determination section  402  selects a composition destination for the contents of each of images (i.e. display layers) drawn by the applications in application group  101 , as does composition destination allocation determination section  302  in Embodiment 3. For example, composition destination allocation determination section  402  shown in  FIG. 16  selects one of upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109  as a composition destination for the display layer. Moreover, composition destination allocation determination section  402  determines whether or not a layout division of the determination target display layer for composition destination allocation can be made, by referring to the information stored in divisible object type DB  301 , as does composition destination allocation determination section  302  in Embodiment 3. 
         [0132]    Composition destination allocation determination section  402  divides the display layer that has been determined as being divisible, as does composition destination allocation determination section  302  in Embodiment 3. Composition destination allocation determination section  402  registers changed object information in object information buffer  102  and registers changed layout information in layout information buffer  103  based on the results of the division. At this time composition destination allocation determination section  402  assigns timestamp information for designating suitable display composition timing to each display layer after division. Composition destination allocation determination section  402  registers the timestamp information as layout information in layout information buffer  401 . Composition destination allocation determination section  402  assigns the same time stamp information (i.e., the same display composition timing) to a plurality of display layers as a result of division of a certain display layer. 
         [0133]    Display screen composition output process section  403  produces a final output image according to the timestamp information in the layout information stored in layout information buffer  401  when performing composition output process. The final output image is formed by compositing the results of composition of a lower-layer frame held in lower-layer frame composition buffer  109 , video data held in moving image frame buffer  111  and the results of composition of an upper-layer frame held in upper-layer frame composition buffer  107 . Display screen composition output process section  112  outputs the final output image to output screen  113 . 
         [0134]    Pre-composition process in screen compositing apparatus  400  will be described in detail. Hereinafter, the pre-composition process will be described with respect to the object information shown in  FIG. 3  and the display layout shown in  FIG. 2 , as is the description of Embodiment 3. 
         [0135]      FIG. 18  is a flowchart for explaining the pre-composition process executed by screen compositing apparatus  400  according to the present embodiment. In  FIG. 18 , the same reference characters are used for the same process as those in Embodiment 3 ( FIG. 15 ). The description for the same process will not be repeated. 
         [0136]    When an attribute of the present determination target layer for composition destination allocation is included in the object type stored in divisible object type DB  301  in ST 301  shown in  FIG. 18  (ST 301 : YES), the process moves to process in ST 401 . In ST 401 , composition destination allocation determination section  302  divides the present determination target layer for composition destination allocation into at least the region of the portion in common with the composition destination determination reference region and the region other than the common portion. Composition destination allocation determination section  402  assigns the same timestamp information to all the display layers after division. Composition, destination allocation determination section  402  registers object information and layout information on the display layers after division in object information buffer  102  and layout information buffer  103 , respectively. 
         [0137]    As a result, even though the display layer is divided into the plurality of display layers (i.e., regions), the same timestamp is assigned to the plurality of display layers after division. Moreover, composition destination allocation determination section  402  resets the region of the above-described common portion in the divided display layers as the next determination target layer for composition destination allocation. The process then returns to process in ST 103 . 
         [0138]    Display screen composition output process in display screen composition output process section  403  will be described in detail.  FIG. 19  is a flowchart for explaining display screen composition output process executed by display screen composition output process section  403 . 
         [0139]    Display screen composition output process section  403  in ST 501  which is shown in  FIG. 19  refers to the layout information stored in layout information buffer  401 . Information display screen composition output process section  403  determines whether or not time stamp information is assigned to a display layer composited in upper-layer frame composition buffer  107  and a display layer composited in lower-layer frame composition buffer  109  based on the layer information. 
         [0140]    When the time stamp information is assigned to the display layer composited in upper-layer frame composition buffer  107  and the display layer composited in lower-layer frame composition buffer  109  (ST 501 : YES), the process advances to process in ST 502 . In ST 502 , information display screen composition output process section  403  compares the time stamp information assigned to the display layer composited in upper-layer frame composition buffer  107  and the time stamp information assigned to the display layer composited in lower-layer frame composition buffer  109 . For example, information display screen composition output process section  403  determines whether or not the two timestamp information items are equal to each other. Information display screen composition output process section  403  continues process in ST 502  (i.e., timestamp information comparison process) in a case where the time stamp information assigned to the display layer composited in upper-layer frame composition buffer  107  and the time stamp information assigned to the display layer composited in lower-layer frame composition buffer  109  are not equal to each other (ST 502 : NO). 
         [0141]    On the other hand, when no timestamp information is assigned to the display layer composited in upper-layer frame composition buffer  107  and the display layer composited in lower-layer frame composition buffer  109  (ST 501 : NO), the process advances to process in ST 503 . In addition, in a case where the time stamp information assigned to the display layer composited in upper-layer frame composition buffer  107  and the time stamp information assigned to the display layer composited in lower-layer frame composition buffer  109  are equal to each other (ST 502 : YES), the process advances to process in ST 503 , in ST 503 , information display screen composition output process section  403  performs composition process from the hack side of the display screen. In other words, information display screen composition output process section  403  performs composition process in the order of the results of composition in the lower-layer frame, the results of composition of video data, and the results of composition in the upper-layer frame. The results of composition in the lower-layer frame are held in lower-layer frame composition buffer  109 . The video data is held in moving image frame buffer  111 . The results of composition in the upper-layer frame are held in upper-layer frame composition buffer  107 . Output screen  113  displays the final display screen. 
         [0142]    In other words, in the following case, information display screen composition output process section  403  determines that the display layers (i.e., data) to be simultaneously displayed are completed in upper-layer frame composition buffer  107  and lower-layer frame composition buffer  109 . This case is where the timestamp information items assigned to the display layers respectively composited in the frames are equal to each other (ST 502 : YES). 
         [0143]    A case when the present determination target layer for composition destination allocation in the display layout shown in  FIG. 2  is layer A will be described. In other words, the composition destination determination reference region is the same as layer B shown in  FIG. 2  (i.e.) rightmost lower-end coordinates (200, 100), leftmost upper-end coordinates (800, 500), hierarchy information  1 ). 
         [0144]    In this case, composition destination allocation determination section  402  determines that layer A is divisible. Composition destination allocation determination section  402  performs a layout division of layer A in the same way as in Embodiment 2 ( FIG. 10 ) (ST 401 ). For example, composition destination allocation determination section  402  performs a layout division of layer A such that layer A is divided into the region of the portion (layer A 2 ) in common with the composition destination determination reference region (layer B) and the region (layer A 0 , layer A 1 ) other than the portion in common with the composition destination determination reference region 
         [0145]    At the time of updating the layout information in layout information buffer  401 , composition destination allocation determination section  402  assigns the same timestamp information to layers A 0 , A 1 , and A 2 , which are the results of division of layer A. For example, the same timestamp information (“0”) is assigned to layers A 0 , A 1 , and A 2 , which are the results of division of layer A. 
         [0146]    Layers A 0  and A 1  (i.e., the region other than the portion in common with the composition destination determination reference region) are composited in lower-layer frame composition buffer  109 , as in Embodiment 2. In addition, layer A 2  (i.e., the region of the portion in common with the composition destination determination reference region) is composited in upper-layer frame composition buffer  107 . 
         [0147]    Thus, timestamps are assigned to layer A 2  composited in upper-layer frame composition buffer  107  and layer A 0  and layer A 1  composited in lower-layer frame composition buffer  109  (ST 501 : YES). When these timestamp information items are equal to each other (ST 502 : YES), display screen composition output process section  403  performs composition display process so that layers A 0 , A 1 , and A 2  are displayed at the same timing (ST 503 ). 
         [0148]    Accordingly, output screen  113  can display layers A 0 , A 1 , and A 2 , which were originally one display layer (layer A), at the same timing. 
         [0149]    Thus, screen compositing apparatus  400  can increase the region (i.e., region other than the portion in common with the composition destination determination reference region) for which the composition destination buffer can be determined based on the composition process cost to a maximum. Screen compositing apparatus  400  can minimize the composition process cost in this way. 
         [0150]    Furthermore, screen compositing apparatus  400  assigns the same timestamp information to display layers after division at the time of layout division of a display layer. Screen compositing apparatus  400  performs process for composition display of a final display screen based on the timestamp information. Accordingly, even in the case of dividing a display layer for the purpose of reducing the composition process cost, screen compositing apparatus  400  can control display of divided display layers by correct timing. In other words, screen compositing apparatus  400  can maximize the effect of reducing the composition process cost while maintaining the quality of reproduction of display results. 
         [0151]    Thus, according to the present embodiment, the screen compositing apparatus can have a power saving effect without affecting display results when compositing a plurality of display frames including a moving image, which is even greater than that of Embodiment 1. 
         [0152]    The present embodiment has been described with respect to a case where screen compositing apparatus  400  performs the following process in a similar way to Embodiment 3: determining whether or not layout division of a determination target display layer for composition destination allocation can be performed, by comparing object attribute information on the display layer and divisible object type DB  301 . However, process for determining whether or not layout division can be performed through comparison of object attribute information is not indispensable to screen compositing apparatus  400  according to the present embodiment. For example, screen compositing apparatus  400  according to the present embodiment may perform the same operation as that in Embodiment 2 by omitting this determination process. 
         [0153]    The embodiments of the claimed invention have been described above. 
         [0154]    The abovementioned embodiments have described a case where the time elapsed during the composition process in each frame is used as an example of a composition process cost value (see  FIG. 5 ). However, the composition process cost value is not limited to the time elapsed during the composition process in each frame. For example, the amount of computation required for process for composition into each frame, the memory access time or the memory access count required for composition process, or a numeric value indicator, and/or the like, computed from these values may alternatively be used as a composition process cost value. 
         [0155]    in the screen compositing apparatus according to the claimed invention, selection of the composition target buffer that minimizes the composition process cost relatively is sufficient. Therefore, the composition process cost value may not be a numeric value but information which simply indicates a magnitude relationship in the composition process cost. In the described embodiments, the value of the composition process cost with respect to each buffer itself or the magnitude relationship in the composition process cost value may be changed by considering the sum of the sizes of the display layers (i.e., regions) composited in frames and the sum of composition process times to the present point in time. Moreover, in the described embodiments, a larger value for an indicator used as the composition process cost value that needs to be interpreted as a reduced composition process cost is conceivable, depending on the characteristics of indicator. However, the screen compositing apparatus according to the claimed invention is assumed to be designed so as to suitably operate based on a value used as an indicator. 
         [0156]    Furthermore, the embodiments have been described with respect to a case where the cost of the process for composition of the upper-layer frame is higher than the cost of the process for composition of the lower-layer frame ( FIG. 5 ). In the described embodiments, however, the same effects can also be obtained in a case where the cost of the process for composition of the lower-layer frame is higher than the cost of the process for composition of the upper-layer frame. In addition, the screen compositing apparatus in ST 102  which is shown in  FIG. 6 ,  FIG. 9 ,  FIG. 15  and  FIG. 18  may set the display layer at the front position in the hierarchical order (i.e., uppermost layer) as the first determination target layer for composition destination allocation. In such a case, the screen compositing apparatus performs the same process as that in ST 106  immediately before ST 108  instead of process in ST 106  shown in  FIG. 6 ,  FIG. 9 ,  FIG. 15  and  FIG. 18 . This is for the purpose of compositing a display layer in lower-layer frame composition buffer  109  in subsequent composition destination, allocation, determination process in the following case. This case is where even a region (i.e., this display layer) having no portion in common with the present composition destination determination reference region, has a portion in common with the present determination target region for composition destination allocation and is located at an inner position relative to the present determination target region for composition destination allocation. The composition destination determination reference region in this case is a display layer in a hierarchical position between the determination target display layer and the reference layer. In addition, the composition destination determination reference region is the union of a display layer composited in lower-layer frame composition buffer  109  and the reference layer. 
         [0157]    Also, the embodiments have been described with respect to a case where a moving image layer is used as a reference layer for the determination as to which one of an upper-layer buffer (i.e., upper-layer frame composition buffer) and a lower-layer buffer (i.e., lower-layer frame composition buffer) is composited with each display layer as a composition destination. In the claimed invention, however, the reference layer used to determine which one of the upper-layer buffer and the lower-layer buffer is composited with each display layer as a composition destination is not limited to the moving image layer. In the described embodiments, a display layer which is most frequently updated among a plurality of display layers in a display layout, e.g., a computer graphic frame in which an image is drawn by computation on each frame, a camera input frame, and/or the like, may be used as a reference layer. In such a case, moving image decoding section  110  shown in  FIG. 1 ,  FIG. 8 ,  FIG. 13  and  FIG. 16  functions as a functional block fear performing process for drawing an image on a target layer instead of moving image decoding process. In addition, moving image frame buffer  111  in  FIG. 1 ,  FIG. 8 ,  FIG. 13  and  FIG. 16  functions as a buffer for accumulating the results of drawing of images in a target layer. Furthermore, a special region (moving image layer in the described embodiments) serving as a reference for determining which one of the upper-layer buffer and the lower-layer buffer is composited with each display layer as a composition destination is not limited to a single layer. Such a special region may alternatively be formed of a plurality of display layers. 
         [0158]    Each section described in the embodiments is formed typically as art integrated circuit by large scale integration (LSI). Each section may be individually formed as one chip, or one chip may be formed so as to include part or all the sections. While each section is assumed to be an LSI, the LSI may be called an IC, a system LSI, a super LSI or an ultra LSI, depending on the degree of integration therewith as selected from different degrees of integration. The technique for circuit integration is not limited to LSI. A technique using a special-purpose circuit or a general-purpose processor may alternatively be used. In the described embodiments, each section may be formed by using programmable field programmable gate array (FPGA) after making the LSI. In the described embodiments, each section may be formed by using a configurable processor, which is reconfigurable in connections and settings of internal circuit cells of the LSI. Moreover, in the present embodiments, each section may be formed by a circuit integration technique that may replace LSI as a result of progress in semiconductor technology or different technology derived therefrom. In the described embodiments, integration may be performed by using a technique in biotechnology, and/or the like. 
         [0159]    The disclosure of Japanese Patent Application No. 2010-153987, filed on Jul. 6, 2010, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0160]    The claimed invention has improved power saving performance and is useful for display composition process, and/or the like for portable terminals. Moreover, the claimed invention is applicable to use in video apparatuses involving composition of moving images and graphics, e.g., television receivers, and video reproduction apparatuses. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100 ,  200 ,  300 ,  400  Screen compositing apparatus 
           101  Application group 
           102  Object information buffer 
           103 ,  401  Layout information buffer 
           104  Composition process cost DB 
           105 ,  201 ,  302 ,  402  Composition destination allocation determination section 
           106  Upper-layer frame composition section 
           107  Upper-layer frame composition buffer 
           108  Lower-layer frame composition section 
           109  Lower-layer frame composition buffer 
           110  Moving image decoding section 
           111  Moving image frame buffer 
           112 ,  403  Display screen composition output process section 
           113  Output screen 
           301  Divisible object type DB