S&F Ref: 824534 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name and Address Canon Kabushiki Kaisha, of 30-2, Shimomaruko 3-chome, of Applicant : Ohta-ku, Tokyo, 146, Japan Actual Inventor(s): Joseph Leigh Belbin Address for Service: Spruson & Ferguson St Martins Tower Level 35 31 Market Street Sydney NSW 2000 (CCN 3710000177) Invention Title: Efficient rendering of page descriptions containing grouped layers The following statement is a full description of this invention, including the best method of performing it known to me/us: 5845c(987957_1) -1 EFFICIENT RENDERING OF PAGE DESCRIPTIONS CONTAINING GROUPED LAYERS Technical Field The present application is concerned with rendering graphical objects, and in 5 particular with efficiently rendering grouped graphical objects in a pixel sequential renderer. Background A computer application typically provides a page to a device for printing and/or display in the form of a page description, represented using a Page Description Language 10 (PDL). The PDL page representation provides descriptions of graphical objects to be rendered onto the page, as opposed to a raster image of the page to be printed. These object descriptions are typically interpreted and placed into an intermediate format page representation known as a display list. Simple PDL page representations comprise object descriptions with a well-defined 15 rendering order, or Z-order, which is preserved in the display list representation. A conventional 'painter's algorithm' renderer then renders, or rasterizes, the display list objects one after the other in the specified Z-order to a page buffer. However, some PDLs support more complex page structures where the rendering order of the objects is not solely based upon the object's Z-order. The PDL interpreter or painter's algorithm renderer is 20 faced with the task of defining a rendering order from such complex page structures so as to preserve the intended appearance of the rendered page. One known method stores the display list in a tree data structure. The structure of the tree determines the order in which objects are rendered. A second known method uses the effect associated with a layer (object) to 25 determine the priority or Z-order position of the layer. 986653 1 A9ARu rnpri ni -2 A third known method is used when the page representation groups multiple layers into complex layers or groups, which may in turn be grouped into further complex layers. The method composites grouped layers together and then treats the result as a single object in a new display list. 5 Another rendering approach involves rendering the display list in pixel sequential order, whereby a sequence of active object fills is generated and evaluated, or composited, for each pixel on the page in raster order. Pixel sequential rendering is generally more efficient than painter's algorithm rendering in that each pixel is only processed once and there is no need for a full page buffer to hold intermediate rendering results. 10 Fill compositing sequences generated during pixel-sequential rendering do not in general include fills for all of the objects in the display list, but only the fills for objects which are active for a given pixel. The ordering of these fills within the fill compositing sequence determines the order of evaluation. If the fill compositing sequence contains fills for objects that form part of a complex layer, current methods have difficulty translating 15 the complex layer structure of the page representation to a fill sequence that does not contain fills for every object in the display list. Summary The embodiments of the present invention allow group boundaries to be determined within fill compositing sequences generated during efficient, pixel sequential rendering of 20 page representations containing objects arranged in a group hierarchy, so that the fill compositing sequence can be evaluated correctly at every pixel on the page. In accordance with one aspect of the present invention there is provided A method of rendering a page description comprising a plurality of graphical objects arranged in a group hierarchy, each said graphical object comprising a fill, said method comprising: 986653_1 824534 sped 01 -3 (a) generating, from said page description, an intermediate format representation of said page description, said intermediate format representation comprising one or more non-overlapping regions of said page, each said region referencing a fill compositing sequence referencing one or more said fills, at least one said fill 5 corresponding to an object that belongs to a group in said group hierarchy; (b) inserting, using said group hierarchy, group boundary information into a fill compositing sequence referencing said at least one fill, and (c) rendering said intermediate format page representation using said group boundary information inserted into said fill compositing sequence. 10 Other aspects are also disclosed. Brief Description of the Drawings One or more embodiments of the present invention will now be described with reference to the following diagrams, in which: Fig. I is a block diagram of a pixel rendering system on which the embodiments of 15 the invention may be practised; Fig. 2 is a data flow diagram of the group boundary determination method according to an embodiment of the present invention; Fig. 3 shows a schematic flow diagram of the display list creation process used in the method of Fig. 2; 20 Fig. 4 is an illustration of an exemplary page to be rendered; Fig. 5 shows the display list representation for the page from Fig. 4; Fig. 6 shows the tiled fillmap representation of the page from Fig.4; Fig. 6 shows Level Information for the objects in the page shown in Fig. 4; Fig. 7 shows a table of fillmap fill compositing sequences for the page of Fig. 4; 986653 1 824534 sweci 01 -4 Fig. 8 shows a schematic flow diagram of the complex layer boundary determination process used in the method of Fig. 2; Fig. 9 shows a schematic flow diagram of the Group End process used in Fig. 8; Fig. 10 shows a schematic flow diagram of the Group Change process used in 5 Fig. 8; Fig. 11 shows the result of the group boundary determination process of Fig. 8 on the page from Fig. 4; and Fig. 12 shows the result of the group boundary determination process of Fig. 8 on a second exemplary page to be rendered. 10 Detailed Description including Best Mode Fig. I shows a schematic block diagram of a pixel rendering system 100 for rendering computer graphic object images on which the embodiments of the present invention may be practised. The pixel rendering system 100 comprises a personal computer 110 connected to a printer system 160 through a network 150. The network 150 15 may be a typical network involving multiple personal computers, or may be a simple connection between a single personal computer and printer system 160. The personal computer 110 comprises a host processor 120 for executing a software application 130, such as a word processor or graphical software application. The printer system 160 comprises a controller processor 170 for executing a 20 controlling program 140, a pixel rendering apparatus 180, memory 190, and a printer engine 195 coupled via a bus 175. The pixel rendering apparatus 180 is preferably in the form of an ASIC coupled via the bus 175 to the controller processor 170, and the printer engine 195. However, the pixel rendering apparatus 180 may also be implemented in software executed in the controller processor 170. 986653_1 824534 sped 01 -5 In the pixel rendering system 100, the software application 130 creates page-based documents where each page contains objects such as text, lines, fill regions, and image data. The software application 130 sends a high level description of the page (for example a PDL file) to the controlling program 140 executing in the controller processor 170 of the 5 printer system 160 via the network 150. The controlling program 140 receives the description of the page from the software application 130. Objects in the page description are placed into an intermediate format page representation called a display list, which is in turn used by the controlling program 140 to generate a further intermediate page representation comprising a fillmap, a 10 table of fill compositing sequences and a group hierarchy, which are described below. The controlling program 140 then processes the group hierarchy and table of fill compositing sequences to generate complex layer, or group, boundary positions for each fill compositing sequence. This complex layer boundary generating process is also described below. The program executing on the controller processor 170 is also responsible for 15 providing memory 190 for the pixel rendering apparatus 180, initialising the pixel rendering apparatus 180, and instructing the pixel rendering apparatus 180 to start rendering the page. The pixel rendering apparatus 180 then uses the fillmap and table of fill compositing sequences, now including group boundary information, to render the page to 20 pixels. The output of the pixel rendering apparatus 180 is colour pixel data, which may be used by printer engine 195. Figure 2 shows a dataflow diagram of the method 210 used by the controlling program 140 to generate a fillmap and table of fill compositing sequences with group boundary information from the page description provided by the software application 130. 25 The page description contains objects arranged in a group hierarchy. The generation 986653_1 824534 speci 01 -6 method 210 comprises a display list creation process 220, a fillmap generation process 230 and a complex layer boundary determination process 240. The display list creation process 220 receives a Page Description 250, and produces intermediate page data 260 consisting of a display list and a group hierarchy for the page. This display list is passed to the 5 fillmap generation process 230, which produces fillmap data 270 comprising a fillmap and a table of fill compositing sequences with no complex layer boundaries, and the group hierarchy from intermediate page data 260. The table of fill compositing sequences together with the group hierarchy is then passed to the complex layer boundary determination process 240, which returns a table of fill compositing sequences with 10 complex layer boundaries 280. Figure 3 is a schematic flow diagram showing the display list creation process 220 in more detail. Here each page description object is processed and the Display List and Group Hierarchy are created. Each display list object contains the rendering information required for fillmap generation and rendering, and also has a reference to the immediate 15 parent group for the object. A group hierarchy consists of all of the groups contained in a page description, with each group containing a reference to its immediate parent group. Processing begins at step 305 and proceeds directly to step 310. At step 310, the ActiveGroup variable is set to NULL. Processing continues to step 315, where the CurZ variable is set to 0. From step 315, processing continues to step 320, where it is 20 determined whether there are more objects that require processing. If it is determined in step 320 that there are no further objects requiring processing, then processing proceeds to step 325, where process 220 is terminated. If it is determined in step 320 that there are further objects to process, processing proceeds to step 330, where the next object in the page description 250 is obtained. 25 Processing then continues to step 335. In step 335, the object is examined to determine if 986653_1 824534 soeci 01 -7 the object is a Start Complex Layer Object. If so, processing proceeds to step 340. In step 340 a new Group Object is created with its own identifier and parent group variable, and added to the Group Hierarchy. Processing continues to step 345, where the ParentGroup variable of the new Group Object is set to be ActiveGroup. Processing then continues to 5 step 350, where the ActiveGroup variable is set to the current Group Object identifier. Processing then returns to step 320 to obtain the next object in the page description. If it is determined in step 335 that the page description object is not a Start Complex Layer object, processing proceeds to step 355. Step 355 determines whether the page description object is an End Complex Layer object. If the object is an End Complex 10 Layer object, then processing proceeds to step 360, where the Group Object which corresponds to the ActiveGroup variable is obtained. Processing continues to step 365, where the ActiveGroup variable is set to the ParentGroup variable of the Group Object obtained in step 360. Processing then returns to step 320 to obtain the next object in the page description. 15 If it is determined in step 355 that the page description object is not an End Complex Layer object, processing proceeds to step 370. In step 370, the Z_Index of the Object is set to the CurZ variable. Processing continues to step 375, where the Cur_Z variable is incremented, and then to step 380. In step 380 the Object's ParentGroup variable is set to the ActiveGroup variable. Processing continues to step 385, where the 20 Object is added to the Display List. Processing then returns to step 320 to obtain the next object in the page description. Figs. 4 and 5 illustrate the operation of a display list creation process 220 on an exemplary page 410 containing 3 objects - an opaque flat object 420, an opaque low resolution source bitmap object 430, and a semi-transparent flat object 440. The opaque 25 low resolution source bitmap object 430, and semi-transparent flat object 440 form a 986653 1 824534 sped 01 -8 complex layer. Fig. 5 shows the display list 510 and group hierarchy 520 corresponding to the page 410. The display list 510 consists of three display list objects, each of which corresponds to an object on the page. Display list object one 530 corresponds to the opaque flat object 420, display list object two 540 corresponds to the opaque low 5 resolution source bitmap object 430, and display list object three 550 corresponds to the semi-transparent flat object 440. The group hierarchy 520 for the page 410 consists of group one 560, which is not contained within any other groups, and therefore the reference to the parent group is NULL. Group one 560 contains display list objects two (540) and three (550). Each of these display list objects has a reference to group one (560) in 10 addition to its level information. The display list is used to by the fillmap generation process 230 to generate a fillmap for the page. A fillmap is a region based representation of a page. A fillmap maps a region within the page to a fill compositing sequence which will be composited to generate the colour of each pixel within that region. Multiple regions within a fillmap can 15 map to the same fill compositing sequence. Regions within the fillmap do not overlap and hence each pixel in the rendered page can only belong to a single region. Each region within the fillmap is defined by a set of fillmap edges which activate the fill compositing sequence referenced by the region. Each region in a fillmap references a fill compositing sequence from which pixels in the region derive their colour. A tiled fillmap is a fillmap 20 comprising one or more fillmap tiles, each of which is a fillmap tile corresponding to a tile of the page. For a tiled fillmap, each fillmap tile potentially comprises a plurality of regions. Fill compositing sequences are preferably shared between fillmap tiles. A tiled fillmap representation of the example page 410 is illustrated in Fig. 6. Fig. 6 shows the example page 410 represented as a tiled fillmap 610. Fillmap tile 620 is 25 at position (1, 2) expressed in (column, row) form. The contents of fillmap tile 620 are 986653_1 824534 speci 01 -9 shown expanded on the right hand side of Fig. 6. The four regions 630, 640, 650 and 660 in fillmap tile 620 reference the fill compositing sequences with indices one, two, three, and four respectively. All fill compositing sequences referenced by the fillmap are stored in the table of 5 fill compositing sequences. A fill compositing sequence is a sequence of Z-ordered levels, where each level contains attributes such as a fill, the opacity of the level and a raster operation which determines how to mix the colour data of the level with that of other overlapping levels. A fill is used by the renderer to calculate the output pixel colour at any given location in the page. 10 Fig. 7 shows the table of fill compositing sequences 710 for the tiled fillmap 610. Each level in a fill compositing sequence contains level information and group information; however the group boundaries for the fill compositing sequence have not yet been generated. Fillmap generation process 230 is carried out according to known methods and will 15 not be further described here. Figure 8 is a schematic flow diagram of the complex layer boundary determination process 240 used in the method 210 of Fig. 2. Process 240 is preferably carried out on each fill compositing sequence in the table of fill compositing sequences in the fillmap data 270 that references one or more grouped levels. Processing begins at step 805, and 20 continues immediately to step 810. In step 810 a stack of Group References is created, called GroupStack. The stack has the following operators: Peek: This operator provides the caller with a reference to the top element in the stack, but does not remove the reference from the stack. Pop: This operator provides the caller with a reference to the top element in the 25 stack, and removes the reference from the stack. 986653_1 824534_speciOl -10 Push: This operator takes a reference and places it on the top of the stack. Note that when GroupStack is created in step 810, it is empty of any references. Processing continues from step 810 to 815, which determines whether there are any levels in the current fill compositing sequence which have not been processed. If all of the 5 levels in the fill compositing sequence have been processed, processing proceeds to step 820, where End Boundaries are inserted into the fill compositing sequence for any Groups remaining in the GroupStack and the GroupStack is cleared. Step 820 is described in detail below with reference to Fig. 9. Processing then continues to step 825, where processing of the current fill compositing sequence is terminated. 10 If it is determined in step 815 that there are more levels in the current fill compositing sequence that require processing, process 240 proceeds to step 830. In step 830, the variable CurLevel is set to reference the Level with the lowest z-order in the fill compositing sequence which has not been processed. Processing then continues to step 835, which determines whether CurLevel belongs to a complex layer by examining its 15 group information. If it is determined that CurLevel does not belong to a group, processing continues to step 840, where End Boundaries are inserted into the fill compositing sequence for any Groups remaining in the GroupStack and the GroupStack is cleared as in step 820. Processing then returns to step 815. If it is determined in step 835 that CurLevel does belong to a group, then 20 processing proceeds from step 835 to step 845. In step 845, the variable CurGroup is set to reference the group corresponding to CurLevel. Processing continues to step 850, where the variable ActiveGroup is set to reference the group at the top of the GroupStack obtained via the 'peek' stack operation. Processing then continues to step 855, where it is determined whether the CurGroup is equal to ActiveGroup. If it is 25 determined that CurGroup is equal to ActiveGroup then processing returns to step 815. 986653_1 824534_speci01 -11 If it is determined that CurGroup is not equal to ActiveGroup then processing proceeds from step 855 to sub-process 860, where the Start and End Boundaries required for the new group are inserted into the fill compositing sequence and the GroupStack is updated. Subprocess 860 is described in more detail below with reference to Fig. 10. Processing 5 then returns to step 815. Fig. 9 is a schematic flow diagram of a subprocess 900 for inserting Group End Boundaries and clearing the GroupStack. This subprocess 900 is performed in steps 820 and 840 in process 240 of Fig. 8. Processing begins at step 910, and proceeds directly to step 920. In step 920, it is determined whether the Group Stack is empty. If so, processing 10 proceeds to step 930, where the subprocess 900 is terminated. If it is determined in step 920 that the Group stack is not empty, processing proceeds to step 940, where the CurGroup variable is set by popping the GroupStack. Processing then continues to step 950, where an End Boundary corresponding to CurGroup is inserted into the fill compositing sequence. Processing then returns to step 920. 15 Fig. 10 is a schematic flow diagram showing the boundary updating subprocess 860 of process 240 in Fig. 8 in more detail. Processing begins at step 1005 and proceeds directly to 1010. In step 1010, it is determined whether CurGroup is a descendant of ActiveGroup. CurGroup is a descendant of ActiveGroup if ActiveGroup can be reached by moving directly up the Group Hierarchy, beginning at CurGroup. If it is 20 determined that CurGroup is a descendant of ActiveGroup, processing proceeds to step 1015. In step 1015 Start Boundaries are inserted into the fill compositing sequence, beginning at but not including ActiveGroup and moving down through the Group Hierarchy until Cur Group is reached. A Start boundary corresponding to CurGroup is inserted into the fill compositing sequence. Processing then continues to step 1020. In 25 step 1020, Group references are pushed onto the Group_Stack, beginning at but not 986653_1 824534 speci 01 -12 including Active_Group and moving down through the Group Hierarchy until CurGroup is reached. A group reference corresponding to CurGroup is pushed onto the GroupStack. Processing then continues to step 1025, where the subprocess 860 terminates. 5 If it is determined in step 1010 that CurGroup is not a descendant of Active_Group, processing proceeds from step 1010 to step 1030. In step 1030 it is determined whether ActiveGroup is a descendant of CurGroup. If it is determined that ActiveGroup is a descendant of CurGroup, processing proceeds to step 1035. In step 1035, End Boundaries are inserted into the fill compositing sequence, beginning at and 10 including Active_Group and moving up through the Group Hierarchy until CurGroup is reached. An End boundary corresponding to CurGroup is not inserted into the fill compositing sequence. Processing then continues to step 1040. In step 1040, Group references are popped off the GroupStack for each Group in the Group hierarchy between ActiveGroup and CurGroup, including ActiveGroup but not including CurGroup. 15 Processing then continues to step 1025, where the subprocess 860 terminates. If it is determined in step 1030 that ActiveGroup is not a descendant of CurGroup, processing proceeds from step 1030 to step 1045. In step 1045, the variable AncestorGroup is set to the lowest Group in the Group Hierarchy which is an ancestor of both ActiveGroup and CurGroup. Processing then continues to step 1050, in which End 20 Boundaries are inserted into the fill compositing sequence, beginning at and including ActiveGroup and moving up through the Group Hierarchy until AncestorGroup is reached. An End boundary corresponding to AncestorGroup is not inserted into the fill compositing sequence. Processing then continues to step 1055. In step 1055, Group references are popped off the GroupStack for each Group in the Group hierarchy between 25 ActiveGroup and AncestorGroup, including ActiveGroup but not including 986653_1 824534_speciOl -13 AncestorGroup. Processing then continues to step 1060. In step 1060 Start Boundaries are inserted into the fill compositing sequence, beginning at but not including AncestorGroup and moving down through the Group Hierarchy until CurGroup is reached. A Start boundary corresponding to CurGroup is inserted into the fill 5 compositing sequence. Processing then continues to step 1065. In step 1065 Group references are pushed onto the GroupStack, beginning at but not including AncestorGroup and moving down through the Group Hierarchy until CurGroup is reached. A group reference corresponding to CurGroup is pushed onto the GroupStack. Processing then continues to step 1025, where the subprocess 860 terminates. 10 Fig. 11 shows the result of the complex layer boundary determination process 240 for the exemplary page 410 given in Fig. 4. Prior to complex layer boundary determination, the data 270 consists of a fillmap 1110, a table 1120 of fill compositing sequences with no boundary information (same as the table 710 in Fig. 7), and a group hierarchy 1130 (same as the group hierarchy 520 in Fig. 5). After the complex layer 15 boundary determination process 240, the data 280 consists of the fillmap 1140, which is unchanged from fillmap 1110, and an updated table 1150 of fill compositing sequences containing boundary information. Fig. 12 shows the result of the complex layer boundary determination process 240 for a second exemplary page with a more complex fill compositing sequence. The 20 fillmap 1210 for the page consists of a single region. The table 1220 of fill compositing sequences consists of a single fill compositing sequence, whose constituent levels reference a number of groups. The Group hierarchy 1230 consists of five groups. Groups two and three are nested within group one, while Group five is nested within group four. After the complex layer boundary determination process 240, the data 280 consists of the 986653_1 824534_speciOl -14 fillmap 1240, which is unchanged from fillmap 1210, and an updated table 1250 of fill compositing sequences containing group boundary information. Industrial Applicability The arrangements described are applicable to the computer and data processing 5 industries and particularly for the efficient rendering of page descriptions containing grouped layers. The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive. 10 (Australia Only) In the context of this specification, the word "comprising" means "including principally but not necessarily solely" or "having" or "including", and not "consisting only of'. Variations of the word "comprising", such as "comprise" and "comprises" have correspondingly varied meanings. 986653 1 824534 sneci 01