Abstract:
Determining a displayable color in a self-overlapping region of a variable color object by determining for each point in the region, the color of each portion of the object present at that point. At each point averaging the colors present at that point and outputting the averaged colors for reproduction within the region.

Description:
FIELD OF THE INVENTION 
     The present invention relates to computer graphics and, in particular, to the generation of overlapping texture mapped regions, including parametrically defined colour blends. 
     BACKGROUND 
     The display of graphical objects involves considering many issues which influence various aspects of the image being displayed. Examples of such issues include the actual colour being displayed and the time taken to determine (render) the colour to be displayed. Where graphical objects are opaque, these issues are relatively simple. However, transparent and partly transparent objects complicate issues, particularly where the colour and/or opacity of an object changes or varies within the object boundaries. Specific problems arise where an object is self-overlapping and the displayed colour is a textural blend of varying colours from the overlapping portions. 
     It is an object of the present invention to provide a manner in which such self-overlapping regions may be faithfully rendered in a convenient fashion. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention there is disclosed a method of determining a displayable colour in a self-overlapping region of a variable colour object, said method comprising the steps of: 
     determining, for each point in the region, the colour of each portion of the object present at that point; 
     at each said point, averaging the colours present at that point; and 
     outputting said averaged colours for reproductions within said region. 
     In accordance with another aspect of the present invention there is disclosed a method of determining a displayable colour in a self-overlapping region of a variable colour object characterised by the use of a commutative operator that mixes a plurality of colours in an order independent manner. 
     In accordance with another aspect of the present invention there is disclosed a commutative operator for mixing colours having opacity components, said operator being of the form:            (         ∑       c   i            o   i     /   n           ∑       o   i     /   n         ,       over     i   =   1     n                     o   i         )     =     (         ∑       c   i          o   i           ∑     o   i         ,       over     i   =   1     n                     o   i         )       ,                          
     where              over     i   =   1     n                     o   i       =       ∑     i   =   1     n            o   i            ∏     j   =     i   +   1       n          (     1   -     o   i       )             ,                          
     In accordance with another aspect of the present invention there is disclosed a method of determining self-overlapping regions of a graphical object, characterised by using the non-zero winding fill rule to accumulate edge transitions of said object in a scan line by scan line fashion. 
     In accordance with another aspect of the present invention there is disclosed a method for rendering in a scan-line manner an arbitrary object having at least overlapping region, said method comprising the steps of: 
     for each scan line of said object 
     (a) scan converting the object on said scan line 
     (b) determining the overlapping regions of said object within said scan line; 
     (c) initializing a overlap buffer for each said overlapping regions 
     (d) for each portion of the object on said scan line 
     (da) determining if said portion is an overlapping region 
     (daa) if so, accumulating the colour of pixels of said portion in the corresponding said overlap buffer; 
     (dab) if not, writing the colour of pixels to an output buffer; 
     (e) for each overlapping region, calculating an average colour from the corresponding accumulated value in said corresponding overlap buffer; 
     (f) writing the average colours to said output buffer; and 
     (g) transferring values from said output buffer to an output device. 
     In accordance with another aspect of the present invention there is disclosed an apparatus for determining a displayable colour in a self-overlapping region of a variable colour object, said apparatus comprising: 
     colour determination means for determining, for each point in the region, the colour of each portion of the object present at that point; 
     averaging means, responsive to said colour determination means, adapted to average, at each said point, the colours present at that point; and 
     output means coupled to said averaging means for outputting said averaged colours for reproductions within said region. 
     In accordance with another aspect of the present invention there is disclosed an apparatus for determining a displayable colour in a self-overlapping region of a variable colour object characterised by comprising a commutative operator means adapted to mix a plurality of colours in an order independent manner. 
     In accordance with another aspect of the present invention there is disclosed a commutative operator means for mixing colours having opacity components, said operator means adapted to perform the operation:            (         ∑       c   i            o   i     /   n           ∑       o   i     /   n         ,       over     i   =   1     n                     o   i         )     =     (         ∑       c   i          o   i           ∑     o   i         ,       over     i   =   1     n                     o   i         )       ,                          
     where              over     i   =   1     n                     o   i       =       ∑     i   =   1     n            o   i            ∏     j   =     i   +   1       n          (     1   -     o   i       )             ,                          
     In accordance with another aspect of the present invention there is disclosed an apparatus for determining self-overlapping regions of a graphical object, characterised by comprising a non-zero winding fill rule means to accumulate edge transitions of said object in a scan line by scan line fashion. 
     In accordance with another aspect of the present invention there is disclosed an apparatus for rendering in a scan-line manner an arbitrary object having at least overlapping region, said apparatus comprising: 
     for each scan line of said object 
     (a) scan conversion means for scan converting the object on said scan line 
     (b) overlapping region detection means for determining the overlapping regions of said object within said scan line; 
     (c) buffer initialization means for initializing an overlap buffer for each said overlapping regions 
     (d) overlapping region determination means, operative in respect of each portion of the object on said scan line 
     (da) for determining if said portion is an overlapping region 
     (daa) and, if said portion is an overlapping region, adapted to accumulate the colour of pixels of said portion in the corresponding said overlap buffer; 
     (dab) and, if said portion is not an overlapping region, adapted to write the colour of pixels to an output buffer; 
     (e) calculation means, operative in respect of each overlapping region, to calculate an average colour from the corresponding accumulated value in said corresponding overlap buffer; 
     (f) writing means for writing the average colours to said output buffer; and 
     (g) transferring means for transferring values from said output buffer to an output device. 
     In accordance with another aspect of the present invention there is disclosed a computer program product including a computer readable medium incorporating a series of instructions configured to determine a displayable colour in a self-overlapping region of a variable colour object, said instructions comprising: 
     determination steps for determining, for each point in the region, the colour of each portion of the object present at that point; 
     averaging steps for at each said point, averaging the colours present at that point; and 
     outputting steps for outputting said averaged colours for reproductions within said region. 
     In accordance with another aspect of the present invention there is disclosed a computer program product for determining a displayable colour in a self-overlapping region of a variable colour object characterised by comprising commutative operator steps adapted to mix a plurality of colours in an order independent manner. 
     In accordance with another aspect of the present invention there is disclosed a computer program product comprising commutative operator steps for mixing colours having opacity components, said operator steps adapted to perform the operation:            (         ∑       c   i            o   i     /   n           ∑       o   i     /   n         ,       over     i   =   1     n                     o   i         )     =     (         ∑       c   i          o   i           ∑     o   i         ,       over     i   =   1     n                     o   i         )       ,                          
     where              over     i   =   1     n                     o   i       =       ∑     i   =   1     n            o   i            ∏     j   =     i   +   1       n          (     1   -     o   i       )             ,                          
     In accordance with another aspect of the present invention there is disclosed a computer program product for determining self-overlapping regions of a graphical object, characterised by comprising non-zero winding fill rule steps to accumulate edge transitions of said object in a scan line by scan line fashion. 
     In accordance with another aspect of the present invention, there is provided computer program product for rendering in a scan-line manner an arbitrary object having at least overlapping region, said program comprising: 
     for each scan line of said object 
     (a) scan conversion steps for scan converting the object on said scan line 
     (b) overlapping region detection steps for determining the overlapping regions of said object within said scan line; 
     (c) buffer initialization steps for initializing a overlap buffer for each said overlapping regions 
     (d) overlapping region determination steps, operative in respect of each portion of the object on said scan line 
     (da) for determining if said portion is an overlapping region 
     (daa) and, if said portion is an overlapping region, adapted to accumulate the colour of pixels of said portion in the corresponding said overlap buffer; 
     (dab) and, if said portion is not an overlapping region, adapted to write the colour of pixels to an output buffer; 
     (e) calculation steps, operative in respect of each overlapping region, to calculate an average colour from the corresponding accumulated value in said corresponding overlap buffer; 
     (f) writing steps for writing the average colours to said output buffer; and 
     (g) transferring steps for transferring values from said output buffer to an output device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A number of aspects of the prior art and a number of embodiments of the present invention will now be described with reference to the drawings in which: 
     FIGS. 1A and 1B depict prior art two-dimensional parameterized regions; 
     FIGS. 2A and 2B depict a prior art distortion to an image; 
     FIGS. 3A and 3B depict a prior art mesh technique used to improve speed of rendering at the expense of quality; 
     FIG. 4 depicts an overlapping ruled surface to which an embodiment of the present invention may be applied; 
     FIG. 5A depicts another ruled surface; 
     FIGS. 5B and 5C depict application of the embodiments to the regions of FIG. 5A; 
     FIGS. 6A to  6 C depict how the non-zero winding rule may be applied to the regions of FIG. 4; 
     FIG. 7 depicts application of the mesh techniques to the rendering of FIG.  4  and highlights mesh elements that are affected by self-overlap; 
     FIG. 8 depicts a set of scan line runs generated using the winding counting fill rule and a generated array of transition points; 
     FIG. 9 is a block diagram of a general purpose computer upon which the preferred embodiment of the present invention can be practiced; 
     FIG. 10 depicts the method steps performed in one embodiment; and 
     FIG. 11 depicts the method steps performed in a scan-line rendering embodiment. 
    
    
     DETAILED DESCRIPTION 
     Prior to discussing the present invention and its embodiments, it is appropriate to review current techniques for rendering texture mapped regions. 
     Given 
     (i) a region of space R over which a two-dimensional (2-D) parameterization x(u,v)=(x(u,v),y(u,v)) is defined, for example, a ruled surface as in FIG. 1A, or a Coons patch as in FIG. 1B, and 
     (ii) some image, texture map, blend procedural texture or the like T(u,v) that yields colour (and/or opacity value) for given (u,v) in suitable range, 
     it is possible to create an image distortion or curved colour blend or the like, by taking the colour T(u,v) at each (u,v) value and drawing it at position x(u,v), as illustrated in FIGS. 2A and 2B. 
     Conversely, it is possible to determine the (u,v) value for each point x in the region R, look up the colour T(u,v) at (u,v), and draw it at x. This presupposes that the mapping x(u,v) is invertible and can be inverted with sufficient ease. 
     An alternative approach that has been used is to provide T(u,v), an inverse distortion (u,v)=u(x,y), and to derive the colour at each (x,y) by first deriving (u,v), then looking up T(u,v), then drawing the colour at x=(x,y). In this scheme, the region defined by u acts like a “cookie” section cutter on T, then distorts the cut (cookie) section into a rectangle. This approach has the advantage that an easily invertible mapping is not required. However, it is not easy with this method to take a rectangular shape and distort it into a required irregular shape, as seen in FIG.  2 B. 
     A common technique employed to improve the speed of such techniques is to divide the parameterized region R into a mesh of points at which the parameterization is calculated, an example of which is seen in FIGS. 3A and 3B. The colour can be looked up in T at the mesh points and then linearly interpolated. The finer the mesh, the better the quality, but the slower the rendering. As shown in FIG. 3B, the mesh is often subdivided into triangles, which are the simplest polygon to render. 
     One difficulty with the first approach (where x(u,v) and T(u,v) are given) is that the region x(u,v) may self-overlap, as illustrated in FIG.  4 . In this case, x(u,v) has no unique inverse within the overlapping region, with two competing choices for the correct colour. 
     Where the overlapping shape is a two-dimensional projection of a three-dimensional object, each point will have a distance from the viewpoint. A variety of algorithms exist for determining which point in a region of overlap is the one closest to the viewpoint, and whose colour is thus typically chosen as the one to render at that point. 
     In situations where alpha-channel compositing is available, however, it is possible to define a sensible colour for the overlapping region. Suppose there are two overlapping objects, one with colour c 1 , and opacity o 1 , and the other with colour c 2  and opacity O 2 . In practice, three colour channels, e.g., (R 1 , G 1 , B 1 ) and (R 2 , G 2 , B 2 ) may be used. For simplicity, and without loss of generality, one channel will only be considered. The colour and opacity can be stored together as a tuple (c,o). It is customary in alpha-channel compositing to store the colour and the opacity multiplied together, thus representing the two colours as &lt;c 1 o 1 ,o 1 &gt; and &lt;c 2 o 2 ,o 2 &gt;. (In mathematical expressions, angle brackets will be used here to notate these “premultiplied” colours). 
     A commonly used effect that simulates the superposition of one object on the other is provided by the “over” operator: 
     &lt;c 1 o 1 ,o 1 &gt; over &lt;c 2 o 2 ,o 2 &gt;=&lt;c 1 o 1 +(1−o 1 )c 2 o 2 , o 1 +o 2 −o 1 o 2 ). 
     Note that this operator is not commutative, that is, in general, 
     &lt;c 1 o 1 ,o 1 &gt; over &lt;c 2 o 2 ,o 2 &gt;≠&lt;c 2 o 2 ,o 2 &gt; over &lt;c 1 o 1 ,o 1 &gt;. 
     In other words, given a series of two or more objects, the order in which they are superposed is significant. However, it is worthwhile noting that the resulting opacity is independent of the order of the objects. 
     Returning to the difficulty of overlapping regions, at any point p where over parameterized region R overlaps itself such as seen in FIG. 4, there are two or more sets of parametric coordinates that map to p: 
     p=x(u 1 ,v 1 )=x(u 2 ,v 2 )=. . . 
     It is possible to obtain, from these parametric coordinates, a set of competing colours that contribute to the net colour at p in some way: 
     &lt;c 1 o 1 ,o 1 &gt;=T(u 1 ,v 1 ), 
     &lt;c 2 o 2 ,o 2 &gt;=T(u 2 ,v 2 ), 
     . . . . 
     Combining these colours using the “over” operator may be unsuitable because it implies an ordering. Unless some specific application dictates an ordering, such as one based upon parameterization, in general none is imposed by the geometry or other intrinsic aspect of this process. 
     Addressing this issue, in accordance with the present invention, a number of embodiments are proposed that permit calculation of the overlapping area to obtain a textured result. 
     A first embodiment presents the opacity-weighted average:                   〈         ∑       c   i          o   i         n     ,       ∑     o   i       n       〉             (   1   )                                
     where n is the number of parametric coordinates (u i ,v i ) that map to the point p in question. Expression (1) has the effect of mixing the contributing colours and averaging their opacities. An important advantage of averaging c i o i  rather than c i  is that colours that are nearly transparent contribute only in a small way to the result, while nearly opaque colours contribute significantly. Thus, for example, while mixing 50% transparent red and 50% transparent yellow should give orange, a nearly opaque red mixed with a barely visible yellow should yield a colour not far from red. 
     The simpler form        (         ∑     c   i       n     ,       ∑     o   i       n       )                          
     is possible if desired in some application. 
     Secondly, a more sophisticated approach is to mix the colour as in Expression (1), but to simulate the effect of “over” in increasing the opacity as more and more contributing colours are superposed using the following Expression No. (2):                  (           ∑                               c   i                       o   i     /   n             ∑                               o   i     /   n         ,       over     i   =   1     n                     o   i         )     =     (           ∑                               c   i                     o   i             ∑                             o   i         ,       over     i   =   1     n                     o   i         )       ,           (   2   )                                
     where              over     i   =   1     n                     o   i       =       ∑     i   =   1     n                       o   i                       ∏     j   =     i   +   1       n                     (     1   -     o   j       )             ,                          
     i.e., the opacity of &lt;x,o 1 &gt; over &lt;x, o 2 &gt;. . . over &lt;x, o n &gt;. An advantage of this second, albeit more complicated method, is shown in FIGS. 5A,  5 B and  5 C. 
     FIG. 5A shows a ruled surface in which opacity o(u,v)=v. FIG. 5B shows an opacity profile along line L of FIG. 5A derived by averaging contributing opacities (as per Expression (1)). FIG. 5C shows an opacity profile along line L of FIG. 5A derived by accumulating contributing opacities with “over” (as per Expression (2)). As can be seen, Expression (2) yields a result without the discontinuities of FIG.  5 B. 
     A description of how such techniques can be implemented follows, with primary emphasis on scan line rendering. The methods described can be generalised easily to band or frame buffer rendering. 
     The simplest approach is to store, for each pixel, running sums          C   =       ∑     i   =   1     k                       c   i          o   i           ,     
          A   =       ∑     i   =   1     k                       o   i        k         ,                          
     and, for the second method,        B   =       over     i   =   1     k                       o   i     .                              
     Initially, for every pixel the values are set such that A=B=C=k=0. 
     As each pixel is rendered by whatever means, for example by dividing the region into a mesh and linearly interpolating intermediate colours, A, B and C are accumulated, and k is incremented. When rendering is finished, Expression (1) or (2) is calculated:                  〈         ∑       c   i          o   i         n     ,       ∑     o   i       n       〉     =     〈       C   k     ,     A   k       〉            
        or           (   3   )                 (         ∑       c   i          o   i           ∑     o   i         ,           over           i              o   i         )     =     {             (       C   A     ,   B     )     ,     A   ≠   0                     (     0   ,   0     )                   A     =   0.                     (   4   )                                
     An inefficiency with this technique is that in regions where there is only one contributing colour, extra storage and complicated arithmetic are not needed for evaluation. If the regions where overlaps occur can be identified, it is then possible to limit the storage and calculation of running sums and final division to the pixels within those regions. Pixels outside can be written into a scan line (or frame, etc.) buffer directly. 
     One method for identifying those regions is to generate an outline for the region R and scan convert the region R using the non-zero winding method and maintaining a count of the number of contributing colours in a region. This will provide, for each scan line, information about which pixels have one contributing region, two contributing regions, and so on, as seen in FIGS. 6A to  6 C. 
     FIG. 6A shows a directed outline derived from the ruled surface R of FIG.  4 . Applying scan conversion using the non-zero winding method returns the number of portions of R that contribute to each pixel, as shown in FIG.  6 B. On each scan line, as seen in FIG. 6B, non-zero winding scan conversion identifies a series of segments each having an associated count as seen in FIG.  6 C. 
     In the case where the region R has been subdivided into a mesh and linear interpolation applied, care must be taken in cases where a sequence of linearly interpolated pixels cross a boundary of a segment returned by the “non-zero winding” method of scan conversion. FIG. 7 illustrates an example where this happens, with a triangular mesh derived from the ruled surface of FIG.  4 . The shaded triangles span regions with different winding count, as seen from FIG.  6 . In those cases, the linear interpolation must be broken into two (or more, as required) loops. The portions relating to winding count 1 are determined simply by writing the interpolated colours, while those with greater winding count are determined by accumulating partial sums (and later calculating the final colour and opacity). 
     Further details may now be discussed about how rendering would take place. 
     For each scan line, a set of runs is returned from the scan conversion of the outline of region R, as in FIG.  6 C. These can be processed into a set of transition points where the fill count changes between less than 2 and 2 or more, with the transition points stored in an array, X, as shown in FIG.  8 . It will be observed that regions with fill count of 2 or more lie between points in array X indexed by even numbers and points indexed. An additional final entry containing a very large number (denoted ∞) is required by the method described above. 
     A scan line without any occurrence of fill count 2 or more only has X[ 0 ]=∞. 
     The next thing to do is to initialize buffers in regions where the fill count is 2 or more. 
     When rendering each triangle, the process described by the following pseudocode may be implemented. 
     
       
         
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                   
               
             
             
               
                 Determine x start , x end , the range of pixels in the current scan line required to 
               
             
          
           
               
                   
                 fill the triangle 
               
               
                   
                 i ← 0 
               
               
                   
                 while x start  ≧ X[i] do Increment i 
               
               
                   
                 while x end  ≧ X[i] do 
               
               
                   
                 begin 
               
             
          
           
               
                   
                 if is odd then  
               
             
          
           
               
                   
                 Generate data for pixels x start  to X[i] − 1 and 
               
             
          
           
               
                   
                 accumulate data in buffers, for example, sums A, B, C and 
               
               
                   
                 k defined above. 
               
             
          
           
               
                   
                 else 
               
             
          
           
               
                   
                 Generate data for pixels x start  to X[i] − 1 and 
               
             
          
           
               
                   
                 write data directly into output buffer. 
               
             
          
           
               
                   
                 x start  ← X[i] 
               
               
                   
                 Increment i 
               
             
          
           
               
                   
                 end 
               
               
                   
                 if i is odd then 
               
             
          
           
               
                   
                 Generate data for pixels x start  to x end  − 1 and 
               
             
          
           
               
                   
                 accumulate data in buffers, for example, sums A, B, C 
               
               
                   
                 and k defined above. 
               
             
          
           
               
                   
                 else 
               
             
          
           
               
                   
                 Generate data for pixels x start  to x end  − 1 and 
               
             
          
           
               
                   
                 write data directly in output buffer. 
               
               
                   
                   
               
             
          
         
       
     
     After all triangles have been rendered, calculate output pixels can be determined for regions where the fill count is 2 or more, for example, by evaluating Expression (1) or (2) using terms A, B, C and k. 
     An alternative to          over     i   =   1     n                     o   i                            
     for opacity is simply            ∑     i   =   1     n                     o   i       ,                          
     clamped at the maximum opacity, usually defined to be 1. 
     It will be apparent from the foregoing that a means for generating overlapping texture mapped regions and in particular overlapping parametrically defined colour blends is provided that affords practical results whilst keeping the amount of memory and processing comparable to prior art methods. The various alternatives for texture determination may be selected depending upon specific requirements of the desired result. 
     The preferred embodiment of the invention can be practised using a conventional general-purpose (host) computer system, such as the computer system  40  shown in FIG. 9, wherein the various method steps performed are implemented as an application program software executed on the computer system  40 . The computer system  40  comprises a computer module  41 , input devices such as a keyboard  42  and mouse  43 , output devices including a printer  13  and a display device  11 . A Modulator-Demodulator (Modem) transceiver device  52  is used by the computer module  41  for communicating to and from a communications network, for example, connectable via a telephone line or other functional medium. The modem  52  can be used to obtain access to the Internet, and other network systems. 
     The computer module  41  typically includes at least one processor unit  45 , a memory unit  46 , for example, formed from semiconductor random access memory (RAM) and read only memory (ROM), input/output (I/O) interfaces including a video interface  47 , and an I/O interface  48  for the keyboard  42 , a mouse  43  and optionally a joystick (not illustrated). A storage device  49  is provided and typically includes a hard disk drive  53  and a floppy disk drive  54 . A CD-ROM drive  55  is typically provided as a non-volatile source of data. The components  45  to  49  and  53  to  55  of the computer module  41 , typically communicate via an interconnected bus  50  and in a manner which results in a conventional mode of operation of the computer system  40  known to those in the relevant art. Examples of computers on which the embodiments can be practised include IBM-PC&#39;s and compatibles, Sun Sparcstations or alike computer systems evolved therefrom. Typically, the application program of the preferred embodiment is resident on a hard disk drive  53  and read and controlled using the processor  45 . Intermediate storage of the program and the print list and any data fetched from the network may be accomplished using the semiconductor memory  46 , possibly in concert with the hard disk drive  53 . In some instances, the application program may be supplied to the user encoded on a CD-ROM or floppy disk, or alternatively could be read by the user from the network via the modem device  52 . 
     FIG. 10 indicates the various method steps  100  implemented in one embodiment. The method  100  commences at step  102  where a graphic object forming part of the image to be rendered is examined to determined those regions of the object that are self-overlapping. As indicated above, this is preferably performed using the non-zero winding method. This is followed by step  104  where a determination is made as to whether any such regions require processing. If so, step  106  follows which examines if any portions of the region require processing. A single overlap provides two portions and a double overlap gives three, and so on. If so, the colour at each point (eg. pixel) within the position is examined at step  108  and at step  110  that colour is accumulated with colours at the same point locations from other portions in the region. After step  110 , control returns to step  106 . It will be apparent from this process that the averaging over the portions in a region can be performed progressively without a need to simultaneously store each pixel value for each location for each portion. When step  106  determines all portions in a region have been processed, the average colour for each point in the region is calculated at step  112 . When there are no more regions, step  104  passes control to step  114  where pixel data for the object including all overlapping and non-overlapping regions are output. The pixel data may be output for display by either the video display  11  or the printer  13 . Alternatively, the data may be stored in the memory  46  or using the devices  49 . Further, the pixel data may be output to a computer network via the modem  52 . 
     The graphical object being rendered is typically variable in colour and/or opacity. For example the object may be formed of a distorted, tiled or texture-mapped pixel-based image, or any combination of such. The object may also be a spatially or parametrically defined blend of colour and/or opacity. The object may also be a spatially or parametrically defined procedural texture. 
     FIG. 11 shows a method  200  for rendering an arbitrary object having overlapping regions in a scan-line manner. The method  200  commences with step  202  with an initialization for scan conversion of the scan line. Step  204  determines if there are lines remaining for scan conversion and, on a positive response (yes) step  206  scan converts the next scan line. Step  208  which follows determines the overlapping regions within the scan line, and step  210  initializes temporary buffers for the overlapping regions. Step  212  commences a loop which operates on each portion of the object on the current scan line. Step  214  which follows determines if the current portion is an overlapping region determined from step  208 . If so, step  216  follows which accumulates the colour of pixels of that portion in the temporary buffer for that overlapping region. Control then returns to step  214  to examine if there are any more portions on the current scan line. If, from step  214 , the region is not an overlapping region, step  218  follows which writes the pixel colour values directly to an output buffer. When step  212  determines there are no more portions of the object on the current scan line, step  220  follows to calculate the average colour for each pixel in each overlapping region. Step  222  then writes the average colours to the output buffer and step  224  follows by sending the completed (i.e. filled) output buffer to an output device, such as a printer or display, for reproduction. Control then returns to step  204  for consideration of the next scan line. When all scan lines have been converted, step  226  ceases scan conversion. 
     The foregoing describes only a number of embodiments of the present invention and modification may be made thereto without departing from the scope of the present invention. 
     For example, the weighted average opacity determined using Expression (1) may not always be desired. In some instances an unweighted (direct) average may give a desired effect. Further, the colours at the overlap may be combined, in an alternative to average, by for example accumulating their values. Where appropriate, a predetermined clamp level can be used to ensure at least a minimum level of translucency rather than allowing the texture to become wholly opaque.