# Patent

## Title
System and method for generating pixel values for pixels in an image using strictly deterministic methodologies for generating sample points

## Abstract

A computer graphics system generates a pixel value for a pixel in an image, the pixel being representative of a point in a scene as recorded on an image plane of a simulated camera. The computer graphics system comprises a sample point generator and a function evaluator. The sample point generator is configured to generate a set of sample points representing at least one simulated element of the simulated camera, the sample points representing elements of, illustratively, for sample points on the image plane, during time intervals during which the shutter is open, and on the lens, a Hammersley sequence, and, for use in global illumination, a scrambled Halton sequence. The function evaluator is configured to generate at least one value representing an evaluation of a selected function at sample points generated by the sample point generator, those values corresponding to the pixel value.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application for patent is a Continuation of U.S. patent application Ser. No. 09/884,861 filed Jun. 19, 2001 (issued as U.S. Pat. No. 7,227,547) which in turn claims the benefit of U. S. Provisional Patent Apps. Ser. Nos. 60/212,286 filed Jun. 19, 2000 and 60/265,934 filed Feb. 1, 2001 (both expired). This application also incorporates by reference herein U.S. patent application Ser. No. 08/880,418, filed Jun. 23, 1997 (issued as U.S. Pat. No. 6,529,193), Martin Grabenstein, et al., entitled “System And Method For Generating Pixel Values For Pixels In An Image Using Suicily Deterministic Methodologies For Generating Sample Points, (herein after “Grabenstein application”). ”


FIELD OF THE INVENTION
The invention relates generally to the field of computer graphics, and more, particularly to systems and methods for generating pixel values for pixels in the image.
BACKGROUND OF THE INVENTION
In computer graphics, a computer is used to generate digital data that represents the projection of surfaces of objects in, for example, a three-dimensional scene, illuminated by, one or more light sources, onto a two-dimensional image plane, to simulate the recording of the scene by, for example, a camera. The camera may include a lens four projecting the image of the scene onto the image plane, or it may comprise a pinhole camera in which case no lens is used. The two-dimensional image is in the form of an array of picture elements (which are variable termed “pixels” or “pels”), and the digital data generated for each pixel represents the color and luminance of the scene as projected onto the image plane at the point of the respective pixel in the image plane. The surfaces the objects may have any of a number of characteristics, including shape, color, specularity, texture, and so forth, which are preferably rendered in the image as closely as possible, to provide a realistic-looking image.
Generally, the contributions of the light reflected from the various points in the scene to the pixel value representing the color and intensity of a particular pixel are expressed in the form of the one or more integrals of relatively complicated functions. Since the integrals used in computer graphics generally will not have a closed-form solution, numerical methods must be used to evaluate them and thereby generate the pixel value. Typically, a conventional “Monte Carlo” method has been used in computer graphics to numerically evaluate the integrals. Generally, in the Monte Carlo method, to evaluate an integral




  
    
      
        
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where f(x) is a real function on the real numerical interval from zero to 1, inclusive, first a number “N” statistically-independent random numbers xi, i=1, . . . , N, are generated over the interval. The random numbers xi are used as sample points for which sample values f(xi) are generated for the function f(x), and an estimate f for the integral is generated as





  
    
      
        
          
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As the number of random numbers used in generating the sample points f(xi) increases, the value of the estimate f will converge toward the actual value of the integral f. Generally, the distribution of estimate values that will be generated for various values of “N,” that is, for various numbers of samples, of being normal distributed around the actual value with a standard deviation σ which can be estimated by





  
    
      
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if the values xi used to generate the sample values f(xi) are statistically independent, that is, if the values xi are truly generated at random.

Generally, it has been believed that random methodologies like the Monte Carlo method are necessary to ensure that undesirable artifacts, such as Moirè patterns and aliasing and the like, which are not in the scene, will not be generated in the generated image. However, several problems arise from use of the Monte Carlo method in computer graphics. First, since the sample points xi used in the Monte Carlo method are randomly distributed, they may clump in various regions over the interval over which the integral is to be evaluated. Accordingly, depending on the set of random numbers which are generated, in the Monte Carlo method for significant portions of the interval there may be no sample points xi for which sample values f(xi) are generated. In that case, the error can become quite large. In the context of generating a pixel value in computer graphics, the pixel value that is actually generated using the Monte Carlo method may not reflect some elements which might otherwise be reflected if the sample points xi were guaranteed to be more evenly distributed over the interval. This problem can be alleviated somewhat by dividing the interval into a plurality of sub-intervals, but it is generally difficult to determine a priori the number of sub-intervals into which the interval should be divided, and, in addition, in a multi-dimensional integration region (which would actually be used in computer graphics, instead of the one-dimensional interval described here) the partitioning of the region into sub-regions of equal size can be quite complicated.
In addition, since the method makes use of random numbers, the error | f−f| (where |x| represents the absolute value of the value “x”) between the estimate value f and actual value f is probabilistic, and, since the error values for various large values of “N” are close to normal distribution around the actual value f, only sixty-eight percent of the estimate values f that might be generated are guaranteed to lie within one standard deviation of the actual value f.
Furthermore, as is clear from equation (3), the standard deviation (y decreases with increasing numbers of samples N, proportional to the reciprocal of square root of “N” (that is, 1/√{square root over (N)}). Thus, if it is desired to reduce the statistical error by a factor of two, it will be necessary to increase the number of samples N by a factor of four, which, in turn, increases the computational load that is required to generate the pixel values, for each of the numerous pixels in the image.
Additionally, since the Monte Carlo method requires random numbers, an efficient mechanism for generating random numbers is needed. Generally, digital computers are provided with so-called “random number” generators, which are computer programs which can be processed to generate a set of numbers that are approximately random. Since the random number generators use deterministic techniques, the numbers that are generated are not truly random. However, the property that subsequent random numbers from a random number generator are statistically independent should be maintained by deterministic implementations of pseudo-random numbers on a computer.
The Grabenstein application describes a computer graphics system and method for generating pixel values for pixels in an image using a strictly deterministic methodology for generating sample points, which avoids the above-described problems with the Monte Carlo method. The strictly deterministic methodology described in the Grabenstein application provides a low-discrepancy sample point sequence which ensures, a priori, that the sample points are generally more evenly distributed throughout the region over which the respective integrals are being evaluated. In one embodiment, the sample points that are used are based on a so-called Halton sequence. See, for example, J. H. Halton, Numerische Mathematik, Vol. 2, pp. 84-90 (1960) and W. H. Press, et al., Numerical Recipes in Fortran (2d Edition) page 300 (Cambridge University Press, 1992). In a Halton sequence generated for number base “p,” where base “p” is a selected prime number, the “k-th” value of the sequence, represented by Hpk is generated by use of a “radical inverse” operation, that is, by

    
    
        (1) writing the value “k” as a numerical representation of the value in the selected base “p,” thereby to provide a representation for the value as DMDM−1 . . . D2D1, where Dm (m=1, 2, . . . , M) are the digits of the representation,
        (2) putting a radix point (corresponding to a decimal point for numbers written in base ten) at the least significant end of the representation DMDM−1D2 D1 written in step (1) above, and
        (3) reflecting the digits around the radix point to provide 0. D1D2 . . . DM−1DM, which corresponds to Hpk.

It will be appreciated that, regardless of the base “p” selected for the representation, for any series of values, one, two, . . . “k,” written in base “p,” the least significant digits of the representation will change at a faster rate than the most significant digits. As a result, in the Halton sequence Hp1, Hp2, . . . Hpk, the most significant digits will change at the faster rate, so that the early values in the sequence will be generally widely distributed over the interval from zero to one, and later values in the sequence will fill in interstices among the earlier values in the sequence. Unlike the random or pseudo-random numbers used in the Monte Carlo method as described above, the values of the Halton sequence are not statistically independent; on the contrary, the values of the Halton sequence are strictly deterministic, “maximally avoiding” each other over the interval, and so they will not clump, whereas the random or pseudo-random numbers used in the Monte Carlo method may clump.

    
    


It will be appreciated that the Halton sequence as described above provides a sequence of values over the interval from zero to one, inclusive along a single dimension. A multi-dimensional Halton sequence can be generated in a similar manner, but using a different base for each dimension.
A generalized Halton sequence, of which the Halton sequence described above is a special case, is generated as follows. For each starting point along the numerical interval from zero to one, inclusive, a different Halton sequence is generated. Defining the pseudo-sum x⊕py for any x and y over the interval from zero to one, inclusive, for any integer “p” having a value greater than two, the pseudo-sum is formed by adding the digits representing “x” and “y” in reverse order, from the most-significant digit to the least-significant digit, and for each addition also adding in the carry generated from the sum of next more significant digits. Thus, if “x” in base “p” is represented by 0.X1X2 . . . XM−1XM, where each “Xm” is a digit in base “p,” and if “y” in base “p” is represented by 0.Y1Y2 . . . YN−1YN, where each “Yn” is a digit in base “p” (and where “M,” the number of digits in the representation of “x” in base “p”, and “N,” the number of digits in the representation of “y” in base “p”, may differ), then the pseudo-sum “z” is represented by 0.Z1Z2 . . . ZL−1ZL, where each “Z1” is a digit in base “p” given by Z1=(X1+Y1+C1) mod p, where “mod” represents the modulo function, and




  
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is a carry value from the “l-1st” digit position, with C1 being set to zero.

Using the pseudo-sum function as described above, the generalized Halton sequence that is used in the system described in the Grabenstein application is generated as follows. If “p” is an integer, and x0 is an arbitrary value on the interval from zero to one, inclusive, then the “p”-adic von Neumann-Kakutani transformation Tp(x) is given by




  
    
      
        
          
            
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and the generalized Halton sequence x0, x1, x2, . . . is defined recursively as

xn+1=Tp(xn)  (5)

From equations (4) and (5), it is clear that, for any value for “p,” the generalized Halton sequence can provide that a different sequence will be generated for each starting value of “x,” that is, for each x0. It will be appreciated that the Halton sequence Hpk as described above is a special case of the generalized Halton sequence (equations (4) and (5)) for x0=0.

The use of a strictly deterministic low-discrepancy sequence can provide a number of advantages over the random or pseudo-random numbers that are used in connection with the Monte Carlo technique. Unlike the random numbers used in connection with the Monte Carlo technique, the low discrepancy sequences ensure that the sample points are more evenly distributed over a respective region or time interval, thereby reducing error in the image which can result from clumping of such sample points which can occur in the Monte Carlo technique. That can facilitate the generation of images of improved quality when using the same number of sample points at the same computational cost as in the Monte Carlo technique.
SUMMARY OF THE INVENTION
The invention provides a new and improved computer graphics system and method for generating pixel values for pixels in the image using a strictly deterministic methodology for generating sample points for use in evaluating integrals defining aspects of the image.
In brief summary, the invention provides, in one aspect, a computer graphics system for generating a pixel value for a pixel in an image, the pixel being representative of a point in a scene as recorded on an image plane of a simulated camera. The computer graphics system comprises a sample point generator and a function evaluator. The sample point generator is configured to generate a set of sample points representing at least one simulated element of the simulated camera, the sample points representing elements of, illustratively, for sample points on the image plane, during time interval during which the shutter is open, and on the lens, a Hammersley sequence, and, for use in global illumination, a scrambled Halton sequence. The function evaluator configured to generate at least one value representing an evaluation of said selected function at one of the sample points generated by said sample point generator, the value generated by the function evaluator corresponding to the pixel value.



BRIEF DESCRIPTION OF THE DRAWINGS
This invention is pointed out with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:
FIG. 1 depicts an illustrative computer graphics system constructed in accordance with the invention
FIG. 2 is a flowchart illustrating an overall method in accord with the invention.



DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
The invention provides an computer graphic system and method for generating pixel values for pixels in an image of a scene, which makes use of a strictly-deterministic methodology for generating sample points for use in generating sample values for evaluating the integral or integrals whose function(s) represent the contributions of the light reflected from the various points in the scene to the respective pixel value, rather than the random or pseudo-random Monte Carlo methodology which has been used in the past. The strictly-deterministic methodology ensures a priori that the sample points will be generally more evenly distributed over the interval or region over which the integral(s) is (are) to be evaluated in a low-discrepancy manner.
FIG. 1 attached hereto depicts an illustrative computer system 10 that makes use of such a strictly deterministic methodology. With reference to FIG. 1, the computer system 10 in one embodiment includes a processor module 11 and operator interface elements comprising operator input components such as a keyboard 12A and/or a mouse 12B (generally identified as operator input element(s) 12) and an operator output element such as a video display device 13. The illustrative computer system 10 is of the conventional stored-program computer architecture. The processor module 11 includes, for example, one or more processor, memory and mass storage devices, such as disk and/or tape storage elements (not separately shown), which perform processing and storage operations in connection with digital data provided thereto. Disk and/or tape storage elements are examples of computer-readable physical media on which computer programs and computer program products can be stored. The operator input element(s) 12 are provided to permit an operator to input information for processing. The video display device 13 is provided to display output information generated by the processor module 11 on a screen 14 to the operator, including data that the operator may input for processing, information that the operator may input to control processing, as well as information generated during processing. The processor module 11 generates information for display by the video display device 13 using a so-called “graphical user interface” (“GUI”), in which information for various applications programs is displayed using various “windows.” Although the computer system 10 is shown as comprising particular components, such as the keyboard 12A and mouse 12B for receiving input information from an operator, and a video display device 13 for displaying output information to the operator, it will be appreciated that the computer system 10 may include a variety of components in addition to or instead of those depicted in FIG. 1.
In addition, the processor module 11 includes one or more network ports, generally identified by reference numeral 14, which are connected to communication links which connect the computer system 10 in a computer network. The network ports enable the computer system 10 to transmit information to, and receive information from, other computer systems and other devices in the network. In a typical network organized according to, for example, the client-server paradigm, certain computer systems in the network are designated as servers, which store data and programs (generally, “information”) for processing by the other, client computer systems, thereby to enable the client computer systems to conveniently share the information. A client computer system which needs access to information maintained by a particular server will enable the server to download the information to it over the network. After processing the data, the client computer system may also return the processed data to the server for storage. In addition to computer systems (including the above-described servers and clients), a network may also include, for example, printers and facsimile devices, digital audio or video storage and distribution devices, and the like, which may be shared among the various computer systems connected in the network. The communication links interconnecting the computer systems in the network may, as is conventional, comprise any convenient information-carrying medium, including wires, optical fibers or other media for carrying signals among the computer systems. Computer systems transfer information over the network by means of messages transferred over the communication links, with each message including information and an identifier identifying the device to receive the message.
It will be helpful to initially provide some background on operations performed by the computer graphics system in generating an image. Generally, the computer graphic system generates an image attempts to simulate an image of a scene that would be generated by a camera. The camera includes a shutter that will be open for a predetermined time T starting at a time to allow light from the scene to be directed to an image plane. The camera may also include a lens that serves to focus light from the scene onto the image plane. The average radiance flux Lm,n through a pixel at position (m,n) on an image plane P, which represents the plane of the camera's recording medium, is determined by




  
    
      
        
          
            
              
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where “AP” refers to the area of the pixel, AL refers to the area of the portion of the lens through which rays of light pass from the scene to the pixel, and fm,n, represents a filtering kernel associated with the pixel. An examination of the integral in equation (6) will reveal that the variables of integration, “x,” “y” and “t,” are essentially dummy variables with the variable “y” referring to integration over the lens area (AL), the variable “t” referring to integration over time (the time interval from t0 to t0+T) and the variable “x” referring to integration over the pixel area (AP).

The value of the integral in equation (6) is approximated by identifying Np sample points xi in the pixel area, and, for each sample point, shooting NT rays at times ti,j in the time interval to t0+T through the focus into the scene, with each ray spanning NL sample points yi,j,k on the lens area AL. The manner in which subpixel jitter positions xi, points in time ti,j and positions on the lens Yi,j,k are determined will be described below. These three parameters determine the primary ray hitting the scene geometry in h(xi,ti,j,yi,j,k) with the ray direction ω(xi,ti,j,yi,j,k). In this manner, the value of the integral in equation (6) can be approximated as




  
    
      
        
          
            
              
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where “N” is the total number of rays directed at the pixel.

It will be appreciated that rays directed from the scene toward the image can comprise rays directly from one or more light sources in the scene, as well as rays reflected off surfaces of objects in the scene. In addition, it will be appreciated that a ray that is reflected off a surface may have been directed to the surface directly from a light source, or a ray that was reflected off another surface. For a surface that reflects light rays, a reflection operator Tfr is defined that includes a diffuse portion Tfd, a glossy portion Tfg and a specular portion Tfs, or

Tfr=Tfd+Tfg+Tfs  (8).

In that case, the Fredholm integral equation L=Le+TfrL governing light transport can be represented as

L=Le+Tfr−fsLe+Tfs(L−Le)+TfsL+TfdTfg+fsL+TfdTfdL  (9),

where transparency has been ignored for the sake of simplicity; transparency is treated in an analogous manner. The individual terms in equation (9) are

(i) Le represents flux due to a light source;
(ii) Tfr−fsLe (where Tfr−fs=Tfr−Tfs) represents direct illumination, that is, flux reflected off a surface that was provided thereto directly by a light source; the specular component, associated with the specular portion Tfs of the reflection operator, will be treated separately since it is modeled as a δ-distribution;
(iii) Tfg(L−Le) represents glossy illumination, which is handled by recursive distribution ray tracing, where, in the recursion, the source illumination has already been accounted for by the direct illumination (item (ii) above);
(iv) TfsL represents a specular component, which is handled by recursively using “L” for the reflected ray;
(v) TfdTfg+fsL (where Tfg+fs=Tfg+Tfs) represents a caustic component, which is a ray that has been reflected off a glossy or specular surface (reference the Tfg+fs operator) before hitting a diffuse surface (reference the Tfd operator); this contribution can be approximated by a high resolution caustic photon map; and
(vi) TfdTfdL represents ambient light, which is very smooth and is therefore approximated using a low resolution global photon map.
As noted above, the value integral (equation (6)) is approximated by solving equation (7) making use of sample points xi, ti,j and Yi,j,k, where “xi” refers to sample points within area AL of the respective pixel at location (m,n) in the image plane, “ti,j” refers to sample points within the time interval t0 to t0+T during which the shutter is open, and “yi,j,k” refers to sample points on the lens AL. In accordance with one aspect of the invention, the sample points xi comprise two-dimensional Hammersley points, which are defined as




  
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where 0≦i<N:=(2n)2 for a selected integer parameter “n,” and Φ2(i) refers to the radical inverse of “i” in base “two.” Generally, the “s” dimensional Hammersley point set is defined as





  
    
      
        
          
            
              
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where Is is the s-dimensional unit cube [0,1) (that is, an s-dimensional cube that includes “zero,” and excludes “one”), the number of points “N” in the set is fixed and b1, . . . ,bs−1 are bases. The bases do not need to be prime numbers, but they are preferably relatively prime to provide a relatively uniform distribution. The radical inverse function Φb, in turn, is generally defined as





  
    
      
        
          
            
              
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                      0
                    
                    ∞
                  
                  ⁢
                  
                    
                  
                  ⁢
                  
                    
                      
                        a
                        j
                      
                      ⁡
                      
                        (
                        i
                        )
                      
                    
                    ⁢
                    
                      b
                      j
                    
                  
                
                ↦
                
                  
                    ∑
                    
                      j
                      =
                      0
                    
                    ∞
                  
                  ⁢
                  
                    
                  
                  ⁢
                  
                    
                      
                        a
                        j
                      
                      ⁡
                      
                        (
                        i
                        )
                      
                    
                    ⁢
                    
                      b
                      
                        
                          -
                          j
                        
                        -
                        1
                      
                    
                  
                
              
            
          
        
      
    
    
      
        (
        11
        )
      
    
  




where





  
    (
    
      a
      j
    
    )
  
  
    j
    =
    0
  
  ∞




is the representation of “i” in integer base “b.” The two-dimensional Hammersley points form a stratified pattern on a 2n by 2n grid. Considering the grid as subpixels, the complete subpixel grid underlying the image plane can be filled by simply abutting copies of the grid to each other.

Given integer subpixel coordinates (sx,sy) the instance “i” and coordinates (x,y) for the sample point xi in the image plane can be determined as follows. Preliminarily, examining




  
    
      (
      
        
          i
          N
        
        ,
        
          
            Φ
            2
          
          ⁡
          
            (
            i
            )
          
        
      
      )
    
    
      i
      =
      0
    
    
      N
      -
      1
    
  
  ,




one observes that

(a) each line in the stratified pattern is a shifted copy of another, and
(b) the pattern is symmetric to the line y=x, that is, each column is a shifted copy of another column.
Accordingly, given the integer permutation σ(k):=2nΦ2(k) for 0≦k<2n, subpixel coordinates (sx,sy) are mapped onto strata coordinates (j,k):=sx mod 2n, sy mod 2n), an instance number “i” is computed as

i=j2n+σ(k)  (12)

and the positions of the jittered subpixel sample points are determined according to





  
    
      
        
          
            
              
                x
                i
              
              =
              
              ⁢
              
                (
                
                  
                    
                      s
                      x
                    
                    +
                    
                      
                        Φ
                        2
                      
                      ⁡
                      
                        (
                        k
                        )
                      
                    
                  
                  ,
                  
                    
                      s
                      y
                    
                    +
                    
                      
                        Φ
                        2
                      
                      ⁡
                      
                        (
                        j
                        )
                      
                    
                  
                
                )
              
            
          
        
        
          
            
              =
               
              ⁢
              
                
                  (
                  
                    
                      
                        s
                        x
                      
                      +
                      
                        
                          σ
                          ⁡
                          
                            (
                            k
                            )
                          
                        
                        
                          2
                          n
                        
                      
                    
                    ,
                    
                      
                        s
                        y
                      
                      +
                      
                        
                          σ
                          ⁢
                          
                            
                          
                          ⁢
                          
                            (
                            j
                            )
                          
                        
                        
                          2
                          n
                        
                      
                    
                  
                  )
                
                .
              
            
          
        
      
    
    
      
        (
        13
        )
      
    
  




An efficient algorithm for generating the positions of the jittered subpixel sample points xi will be provided below in connection with Code Segment 1. A pattern of sample points whose positions are determined as described above in connection with equations (12) and (13) has an advantage of having much reduced discrepancy over a pattern determined using a Halton sequence or windowed Halton sequence, as described in the aforementioned Grabenstein application, and therefore the approximation described above in connection with equation (7) gives in general a better estimation to the value of the integral described above in connection with equation (6). In addition, if “n” is sufficiently large, sample points in adjacent pixels will have different patterns, reducing the likelihood that undesirable artifacts will be generated in the image.

A ray tree is a collection of paths of light rays that are traced from a point on the simulated camera's image plane into the scene. The computer graphics system 10 generates a ray tree by recursively following transmission, subsequent reflection and shadow rays using trajectory splitting. In accordance with another aspect of the invention, a path is determined by the components of one vector of a global generalized scrambled Hammersley point set. Generally, a scrambled Hammersley point set reduces or eliminates a problem that can arise in connection with higher-dimensioned low-discrepancy sequences since the radical inverse function Φb typically has subsequences of b−1 equidistant values spaced by 1/b. Although these correlation patterns are merely noticeable in the full s-dimensional space, they are undesirable since they are prone to aliasing. The computer graphics system 10 attenuates this effect by scrambling, which corresponds to application of a permutation to the digits of the b-ary representation used in the radical inversion. For a permutation a from a symmetric group Sb over integers 0, . . . ,b−1, the scrambled radical inverse is defined as




  
    
      
        
          
            
              
                Φ
                b
              
              :
              
                
                  
                    N
                    0
                  
                  ×
                  
                    S
                    b
                  
                
                →
                I
              
            
          
        
        
          
            
              
                
                  (
                  
                    i
                    ,
                    σ
                  
                  )
                
                ↦
                
                  
                    ∑
                    
                      j
                      =
                      0
                    
                    ∞
                  
                  ⁢
                  
                    
                  
                  ⁢
                  
                    σ
                    ⁢
                    
                      
                    
                    ⁢
                    
                      (
                      
                        
                          a
                          j
                        
                        ⁡
                        
                          (
                          i
                          )
                        
                      
                      )
                    
                    ⁢
                    
                      b
                      
                        
                          -
                          j
                        
                        -
                        1
                      
                    
                  
                
                ⇔
                i
              
              =
              
                
                  ∑
                  
                    j
                    =
                    0
                  
                  ∞
                
                ⁢
                
                  
                    
                      a
                      j
                    
                    ⁡
                    
                      (
                      i
                      )
                    
                  
                  ⁢
                  
                    
                      b
                      j
                    
                    .
                  
                
              
            
          
        
      
    
    
      
        (
        14
        )
      
    
  




If the permutation “σ” is the identity, the scrambled radical inverse corresponds to the unscrambled radical inverse. In one embodiment, the computer graphics system generates the permutation a recursively as follows. Starting from the permutation σ2=(0,2) for base b=2, the sequence of permutations is defined as follows:

    
    
        (i) if the base “b” is even, the permutation σb is generated by first taking the values of
    
    






  2
  ⁢
  
    
  
  ⁢
  
    σ
    
      b
      2
    
  




and appending the values of





  
    
      2
      ⁢
      
        
      
      ⁢
      
        σ
        
          b
          2
        
      
    
    +
    1
  
  ,




and

    
    
        (ii) if the base “b” is odd, the permutation σb is generated by taking the values of σb−1, incrementing each value that is greater than or equal
    
    






  
    b
    -
    1
  
  2




by one, and inserting the value b−1 in the middle.

This recursive procedure results in

σ2=(0,1)
σ3=(0,1,2)
σ4=(0,2,1,3)
σ5=(0,3,2,1,4)
σ6=(0,2,4,1,3,5)
σ7=(0,2,5,3,1,4,6)
σ8=(0,4,2,6,1,5,3,7). . .
The computer graphics system 10 can generate a generalized low-discrepancy point set as follows. It is possible to obtain a low-discrepancy sequence by taking any rational s-dimensional point “x” as a starting point and determine a successor by applying the corresponding incremental radical inverse function to the components of “x.” The result is referred to as the generalized low-discrepancy point set. This can be applied to both the Halton sequence and the Hammersley sequence. In the case of the generalized Halton sequence, this can be formalized as

xi=(Φb1(i+i1), Φb1(i+i2), . . . , Φ(+is))  (15),

where the integer vector (i1, i2, . . . , is) represents the offsets per component and is fixed in advance for a generalized sequence. The integer vector can be determined by applying the inverse of the radical inversion to the starting point “x.” A generalized Hammersley sequence can be generated in an analogous manner.

Returning to trajectory splitting, generally trajectory splitting is the evaluation of a local integral, which is of small dimension and which makes the actual integrand smoother, which improves overall convergence. Applying replication, positions of low-discrepancy sample points are determined that can be used in evaluating the local integral. The low-discrepancy sample points are shifted by the corresponding elements of the global scrambled Hammersley point set. Since trajectory splitting can occur multiple times on the same level in the ray tree, branches of the ray tree are decorrelated in order to avoid artifacts, the decorrelation being accomplished by generalizing the global scrambled Hammersley point set.
An efficient algorithm for generating a ray tree will be provided below in connection with Code Segment 2. Generally, in that algorithm, the instance number “i” of the low-discrepancy vector, as determined above in connection with equation (12), and the number “d” of used components, which corresponds to the current integral dimension, are added to the data structure that is maintained for the respective ray in the ray tree. The ray tree of a subpixel sample is completely specified by the instance number “i.” After the dimension has been set to “two,” which determines the component of the global Hammersley point set that is to be used next, the primary ray is cast into the scene to span its ray tree. In determining the deterministic splitting by the components of low discrepancy sample points, the computer graphics system 10 initially allocates the required number of dimensions “Δd.” For example, in simulating glossy scattering, the required number of dimensions will correspond to “two.” Thereafter, the computer graphics system 10 generates scattering directions from the offset given by the scrambled radical inverses Φbd(i, σbd), . . . , Φbd+Δd−1(i,σbd+Δd−1), yielding the instances




  
    
      
        
          
            
              
                
                  (
                  
                    y
                    
                      i
                      ,
                      j
                    
                  
                  )
                
                
                  j
                  =
                  0
                
                
                  M
                  -
                  1
                
              
              =
              
              ⁢
              
                (
                
                  
                    
                      
                        Φ
                        
                          b
                          d
                        
                      
                      ⁡
                      
                        (
                        
                          i
                          ,
                          
                            σ
                            
                              b
                              d
                            
                          
                        
                        )
                      
                    
                    ⊕
                    
                      j
                      M
                    
                  
                  ,
                  ...
                  ⁢
                  
                    
                  
                  ,
                  
                    
                      
                        Φ
                        
                          b
                          
                            d
                            +
                            
                              Δ
                              ⁢
                              
                                
                              
                              ⁢
                              d
                            
                            -
                            1
                          
                        
                      
                      ⁡
                      
                        (
                        
                          i
                          ,
                          
                            σ
                            
                              b
                              
                                d
                                +
                                
                                  Δ
                                  ⁢
                                  
                                    
                                  
                                  ⁢
                                  d
                                
                                -
                                1
                              
                            
                          
                        
                        )
                      
                    
                    ⊕
                  
                
              
            
          
        
        
          
            
              
                
                ⁢
                
                  
                    Φ
                    
                      b
                      
                        d
                        +
                        
                          Δ
                          ⁢
                          
                            
                          
                          ⁢
                          d
                        
                        -
                        2
                      
                    
                  
                  ⁡
                  
                    (
                    
                      i
                      ,
                      
                        σ
                        
                          b
                          
                            d
                            +
                            
                              Δ
                              ⁢
                              
                                
                              
                              ⁢
                              d
                            
                            -
                            2
                          
                        
                      
                    
                    )
                  
                
                )
              
              ,
            
          
        
      
    
    
      
        (
        16
        )
      
    
  




where “⊕” refers to addition modulo “one.” Each direction of the “M” replicated rays is determined by yi,j and enters the next level of the ray tree with d′:=d+Δd as the new integral dimension in order to use the next elements of the low-discrepancy vector, and i′=i+j in order to decorrelate subsequent trajectories. Using an infinite sequence of low-discrepancy sample points, the replication heuristic is turned into an adaptive consistent sampling arrangement. That is, computer graphics system 10 can fix the sampling rate ΔM, compare current and previous estimates every ΔM samples, and, if the estimates differ by less than a predetermined threshold value, terminate sampling. The computer graphics system 10 can, in turn, determine the threshold value, by importance information, that is, how much the local integral contributes to the global integral.

As noted above, the integral described above in connection with equation (6) is over a finite time period T from t0 to t0+T, during which time the shutter of the simulated camera is open. During the time period, if an object in the scene moves, the moving object may preferably be depicted in the image as blurred, with the extent of blurring being a function of the object's motion and the time interval t0+T. Generally, motion during the time an image is recorded is linearly approximated by motion vectors, in which case the integrand (equation (6)) is relatively smooth over the time the shutter is open and is suited for correlated sampling. For a ray instance “i,” started at the subpixel position xi, the offset Φ3(i) into the time interval is generated and the NT−1 subsequent samples




  
    
      
        
          
            
              Φ
              3
            
            ⁡
            
              (
              i
              )
            
          
          +
          
            
              j
              
                N
                T
              
            
            ⁢
            
              
            
            ⁢
            mod
            ⁢
            
              
            
            ⁢
            1
            ⁢
            
              
            
            ⁢
            for
            ⁢
            
              
            
            ⁢
            0
          
        
        <
        j
        <
        
          
            N
            T
          
          ⁢
          
            
          
          ⁢
          that
          ⁢
          
            
          
          ⁢
          is
        
      
    
    
      
        
      
    
  
  
    
      
        
          t
          
            i
            ,
            j
          
        
        :=
        
          
            t
            0
          
          +
          
            
              (
              
                
                  
                    Φ
                    3
                  
                  ⁡
                  
                    (
                    i
                    )
                  
                
                ⊕
                
                  j
                  
                    N
                    T
                  
                
              
              )
            
            ·
            
              T
              .
            
          
        
      
    
    
      
        (
        17
        )
      
    
  




Determining sample points in this manner fills the sampling space, resulting in a more rapid convergence to the value of the integral (equation (6)). For subsequent trajectory splitting, rays are decorrelated by setting the instance i′=i+j.

In addition to determining the position of the jittered subpixel sample point xi, and adjusting the camera and scene according to the sample point ti,j for the time, the computer graphics system also simulates depth of field. In simulating depth of field, the camera to be simulated is assumed to be provided with a lens having known optical characteristics and, using geometrical optics, the subpixel sample point xi is mapped through the lens to yield a mapped point xi′. The lens is sampled by mapping the dependent samples




  
    
      
        
          y
          
            i
            ,
            j
            ,
            k
          
        
        =
        
          (
          
            
              (
              
                
                  
                    Φ
                    5
                  
                  ⁡
                  
                    (
                    
                      
                        i
                        +
                        j
                      
                      ,
                      
                        σ
                        5
                      
                    
                    )
                  
                
                ⊕
                
                  k
                  
                    N
                    L
                  
                
              
              )
            
            ,
            
              (
              
                
                  
                    Φ
                    7
                  
                  ⁡
                  
                    (
                    
                      
                        i
                        +
                        j
                      
                      ,
                      
                        σ
                        7
                      
                    
                    )
                  
                
                ⊕
                
                  
                    Φ
                    2
                  
                  ⁡
                  
                    (
                    k
                    )
                  
                
              
              )
            
          
          )
        
      
    
    
      
        (
        18
        )
      
    
  




onto the lens area AL using a suitable one of a plurality of known transformations. Thereafter, a ray is shot from the sample point on the lens specified by yi,j,k through the point xi′ into the scene. The offset (Φ5(i+j,σ5), Φ7(i+j, σ7)) in equation (18) comprise the next components taken from the generalized scrambled Hammersley point set, which, for trajectory splitting, is displaced by the elements





  (
  
    
      k
      
        N
        L
      
    
    ,
    
      
        Φ
        2
      
      ⁡
      
        (
        k
        )
      
    
  
  )




of the two-dimensional Hammersley point set. The instance of the ray originating from sample point yi,j,k is set to “i+j+k” in order to decorrelate further splitting down the ray tree. In equation (18), the scrambled samples (Φ5(i+j,σ5),Φ7(i+j,σ7)) are used instead of the unscrambled samples of (Φ5(i+j), Φ7(i+j)) since in bases “five” and “seven” the up to five unscrambled samples will lie on a straight line, which will not be the case for the scrambled samples.

In connection with determination of a value for the direct illumination (Tfr−fsLe above), direct illumination is represented as an integral over the surface of the scene ∂V, which integral is decomposed into a sum of integrals, each over one of the “L” single area light sources in the scene. The individual integrals in turn are evaluated by dependent sampling, that is




   
  
    
      
        
          
            
              
                
                  
                    (
                    
                      
                        T
                        
                          
                            f
                            r
                          
                          -
                          
                            f
                            s
                          
                        
                      
                      ⁢
                      
                        L
                        e
                      
                    
                    )
                  
                  ⁢
                  
                    (
                    
                      y
                      ,
                      z
                    
                    )
                  
                
                =
                
                ⁢
                
                  
                    ∫
                    
                      ∂
                      V
                    
                  
                  ⁢
                  
                    
                      
                        L
                        e
                      
                      ⁡
                      
                        (
                        
                          x
                          ,
                          y
                        
                        )
                      
                    
                    ⁢
                    
                      
                        f
                        r
                      
                      ⁡
                      
                        (
                        
                          x
                          ,
                          y
                          ,
                          z
                        
                        )
                      
                    
                    ⁢
                    
                      G
                      ⁡
                      
                        (
                        
                          x
                          ,
                          y
                        
                        )
                      
                    
                    ⁢
                    
                      
                    
                    ⁢
                    
                      d
                      x
                    
                  
                
              
            
          
          
            
              
                =
                
                ⁢
                
                  
                    ∑
                    
                      k
                      =
                      1
                    
                    L
                  
                  ⁢
                  
                    
                  
                  ⁢
                  
                    
                      ∫
                      
                        supp
                        ⁢
                        
                          
                        
                        ⁢
                        
                          L
                          
                            e
                            ,
                            k
                          
                        
                      
                    
                    ⁢
                    
                      
                        
                          L
                          e
                        
                        ⁡
                        
                          (
                          
                            x
                            ,
                            y
                          
                          )
                        
                      
                      ⁢
                      
                        
                          f
                          r
                        
                        ⁡
                        
                          (
                          
                            x
                            ,
                            y
                            ,
                            z
                          
                          )
                        
                      
                      ⁢
                      
                        G
                        ⁡
                        
                          (
                          
                            x
                            ,
                            y
                          
                          )
                        
                      
                      ⁢
                      
                        
                      
                      ⁢
                      
                        d
                        x
                      
                    
                  
                
              
            
          
          
            
              
                ≈
                
                ⁢
                
                  
                    ∑
                    
                      k
                      =
                      1
                    
                    L
                  
                  ⁢
                  
                    
                      1
                      
                        M
                        K
                      
                    
                    ⁢
                    
                      
                        ∑
                        
                          j
                          =
                          0
                        
                        
                          
                            M
                            k
                          
                          -
                          1
                        
                      
                      ⁢
                      
                        
                      
                      ⁢
                      
                        
                          
                            L
                            e
                          
                          ⁡
                          
                            (
                            
                              
                                x
                                j
                              
                              ,
                              y
                            
                            )
                          
                        
                        ⁢
                        
                          
                            f
                            r
                          
                          ⁡
                          
                            (
                            
                              
                                x
                                j
                              
                              ,
                              y
                              ,
                              z
                            
                            )
                          
                        
                        ⁢
                        
                          
                            G
                            ⁡
                            
                              (
                              
                                
                                  x
                                  j
                                
                                ,
                                y
                              
                              )
                            
                          
                          .
                        
                      
                    
                  
                
              
            
          
        
      
      
        
          (
          19
          )
        
      
    
  




where suppLe,k refers to the surface of the respective “k-th” light source. In evaluating the estimate of the integral for the “k-th” light source, for the Mk-th query ray, shadow rays determine the fraction of visibility of the area light source, since the point visibility varies much more than the smooth shadow effect. For each light source, the emission Le is attenuated by a geometric term G, which includes the visibility, and the surface properties are given by a bidirectional distribution function fr-fs. These integrals are local integrals in the ray tree yielding the value of one node in the ray tree, and can be efficiently evaluated using dependent sampling. In dependent sampling, the query ray comes with the next free integral dimension “d” and the instance “i,” from which the dependent samples are determined in accordance with





  
    
      
        
          x
          j
        
        =
        
          
            (
            
              
                
                  
                    Φ
                    
                      b
                      d
                    
                  
                  ⁡
                  
                    (
                    
                      i
                      ,
                      
                        σ
                        
                          b
                          d
                        
                      
                    
                    )
                  
                
                ⊕
                
                  j
                  
                    M
                    k
                  
                
              
              ,
              
                
                  
                    Φ
                    
                      b
                      
                        d
                        +
                        1
                      
                    
                  
                  ⁡
                  
                    (
                    
                      i
                      ,
                      
                        σ
                        
                          b
                          
                            d
                            +
                            1
                          
                        
                      
                    
                    )
                  
                
                ⊕
                
                  
                    Φ
                    2
                  
                  ⁡
                  
                    (
                    j
                    )
                  
                
              
            
            )
          
          .
        
      
    
    
      
        (
        20
        )
      
    
  




The offset (Φbd(i,σbd),Φbd+1(i,σbd+1)) again is taken from the corresponding generalized scrambled Hammersley point set, which shifts the two-dimensional Hammersley point set





  (
  
    
      j
      
        M
        k
      
    
    ,
    
      
        Φ
        2
      
      ⁡
      
        (
        j
        )
      
    
  
  )




on the light source. Selecting the sample rate Mk=2nk as a power of two, the local minima is obtained for the discrepancy of the Hammersley point set that perfectly stratifies the light source. As an alternative, the light source can be sampled using an arbitrarily-chosen number Mk of sample points using





  
    (
    
      
        j
        
          M
          k
        
      
      ,
      
        
          Φ
          
            M
            k
          
        
        ⁡
        
          (
          
            j
            ,
            
              σ
              
                M
                k
              
            
          
          )
        
      
    
    )
  
  
    j
    =
    0
  
  
    
      M
      k
    
    -
    1
  




as a replication rule. Due to the implicit stratification of the positions of the sample points as described above, the local convergence will be very smooth.

The glossy contribution Tfe(L−Le) is determined in a manner similar to that described above in connection with area light sources (equations (19) and (20), except that a model fg used to simulate glossy scattering will be used instead of the bidirectional distribution function fr. In determining the glossy contribution, two-dimensional Hammersley points are generated for a fixed splitting rate M and shifted modulo “one” by the offset (Φbd(i,σbd,Φbd+1(i,σbd+1)) taken from the current ray tree depth given by the dimension field “d” of the incoming ray. The ray trees X spanned into the scattering direction are decorrelated by assigning the instance fields i′=i+j in a manner similar to that done for simulation of motion blur and depth of field, as described above. The estimates generated for all rays are averaged by the splitting rate “M” and propagated up the ray tree.
Volumetric effects are typically provided by performing a line integration along respective rays from their origins to the nearest surface point in the scene. In providing for a volumetric effect, the computer graphics system 10 generates from the ray data a corresponding offset Φbd(i) which it then uses to shift the M equidistant samples on the unit interval seen as a one-dimensional torus. In doing so, the rendering time is reduced in comparison to use of an uncorrelated jittering methodology.
Global illumination includes a class of optical effects, such as indirect illumination, diffuse and glossy inter-reflections, caustics and color bleeding, that the computer graphics system 10 simulates in generating an image of objects in a scene. Simulation of global illumination typically involves the evaluation of a rendering equation. For the general form of an illustrative rendering equation useful in global illumination simulation, namely:

L({right arrow over (x)},{right arrow over (w)})=Le({right arrow over (x)},{right arrow over (w)})+∫|Sf({right arrow over (x)},{right arrow over (w)}′→{right arrow over (w)})G({right arrow over (x)},{right arrow over (x)}′)V({right arrow over (x)},{right arrow over (x)}′)L({right arrow over (i)},{right arrow over (w)}′)dA′  (21)

it is recognized that the light radiated at a particular point {right arrow over (x)} in a scene is generally the sum of two components, namely, the amount of light (if any) that is emitted from the point and the amount of light (if any) that originates from all other points and which is reflected or otherwise scattered from the point {right arrow over (x)}. In equation (21), L({right arrow over (x)},{right arrow over (w)}) represents the radiance at the point x in the direction {right arrow over (w)}=(θ,φ) (where “0” represents the angle of direction {right arrow over (w)} relative to a direction orthogonal of the surface of the object in the scene containing the point {right arrow over (x)}, and “φ” represents the angle of the it component of direction {right arrow over (w)} in a plane tangential to the point {right arrow over (x)}). Similarly, L({right arrow over (x)}′, {right arrow over (w)}′) in the integral represents the radiance at the point {right arrow over (x)}′ in the direction {right arrow over (w)}′=(θ′,φ′) (where “θ′” represents the angle of direction {right arrow over (w)}′ relative to a direction orthogonal of the surface of the object in the scene containing the point {right arrow over (x)}′, and “φ′” represents the angle of the component of direction {right arrow over (w)}′ in a plane tangential to the point {right arrow over (x)}′), and represents the light, if any, that is emitted from point {right arrow over (x)}′ which may be reflected or otherwise scattered from point {right arrow over (x)}.

Continuing with equation (21), Le({right arrow over (x)},{right arrow over (w)}) represents the first component of the sum, namely, the radiance due to emission from the point {right arrow over (x)} in the direction {right arrow over (w)}, and the integral over the sphere S′ represents the second component, namely, the radiance due to scattering of light at point {right arrow over (x)}. f({right arrow over (x)},{right arrow over (w)}′→{right arrow over (w)}) is a bidirectional scattering distribution function which describes how much of the light coming from direction {right arrow over (w)}′ is reflected, refracted or otherwise scattered in the direction {right arrow over (w)}, and is generally the sum of a diffuse component, a glossy component and a specular component. In equation (21), the function G({right arrow over (x)}, {right arrow over (x)}′) is a geometric term




  
    
      
        
          G
          ⁡
          
            (
            
              
                x
                ->
              
              ,
              
                
                  x
                  ->
                
                ′
              
            
            )
          
        
        =
        
          
            cos
            ⁢
            
              
            
            ⁢
            θ
            ⁢
            
              
            
            ⁢
            cos
            ⁢
            
              
            
            ⁢
            
              θ
              ′
            
          
          
            
              
              
                
                  x
                  ->
                
                -
                
                  
                    x
                    ->
                  
                  ′
                
              
              
            
            2
          
        
      
    
    
      
        (
        22
        )
      
    
  




where θ and θ′ are angles relative to the normals of the respective surfaces at points {right arrow over (x)} and {right arrow over (x)}′, respectively. Further in equation (21), V({right arrow over (x)},{right arrow over (x)}′) is a visibility function which equals the value one if the point {right arrow over (x)}′ is visible from the point x and zero if the point {right arrow over (x)}′ is not visible from the point {right arrow over (x)}′.

The computer graphics system 10 makes use of global illumination specifically in connection with determination of the diffuse component TfdTfdL and the caustic component TfdTfg+fsL using a photon map technique. Generally, a photon map is constructed by simulating the emission of photons by light source(s) in the scene and tracing the path of each of the photons. For each simulated photon that strikes a surface of an object in the scene, information concerning the simulated photon is stored in a data structure referred to as a photon map, including, for example, the simulated photon's color, position and direction angle. Thereafter a Russian roulette operation is performed to determine the photon's state, that is, whether the simulated photon will be deemed to have been absorbed or reflected by the surface. If a simulated photon is deemed to have been reflected by the surface, the simulated photon's direction is determined using, for example, a bidirectional reflectance distribution function (“BRDF”). If the reflected simulated photon strikes another surface, these operations will be repeated (reference the aforementioned Grabenstein application). The data structure in which information for the various simulated photons is stored may have any convenient form; typically k-dimensional trees, for “k” an integer” are used. After the photon map has been generated, it can be used in rendering the respective components of the image.
In generating a photon map, the computer graphics system 10 simulates photons trajectories, thus avoiding the necessity of discretizing the kernel of the underlying integral equation. The interactions of the photons with the scene, as described above, are stored and used for density estimation. The computer graphics system 10 makes use of a scrambled Halton sequence, which has better discrepancy properties in higher dimensions than does an unscrambled Halton sequence. The scrambled Halton sequence also has the benefit, over a random sequence, that the approximation error decreases more smoothly, which will allow for use of an adaptive termination scheme during generation of the estimate of the integral. In addition, since the scrambled Halton sequence is strictly deterministic, generation of estimates can readily be parallelized by assigning certain segments of the low-discrepancy sequence to ones of a plurality of processors, which can operate on portions of the computation independently and in parallel. Since usually the space in which photons will be shot by selecting directions will be much larger than the area of the light sources from which the photons were initially shot, it is advantageous to make use of components of smaller discrepancy, for example, Φ2 or Φ3 (where, as above, Φb refers to the radical inverse function for base “b”), for angular scattering and components of higher discrepancy, for example, Φ5 or Φ7 for area sampling, which will facilitate filling the space more uniformly.
The computer graphics system 10 estimates the radiance from the photons in accordance with




  
    
      
        
          
            
              L
              _
            
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          ⁡
          
            (
            
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                  (
                  
                    
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                i
              
            
          
        
      
    
    
      
        (
        23
        )
      
    
  




where, in equation (23), Φi represents the energy of the respective “i-th” photon, ωi is the direction of incidence of the “i-th photon,” Bk(x) represents the set of the “k” nearest photons around the point “x,” and “A” represents an area around point “x” that includes the photons in the set Bk(x). The computer graphics system 10 makes use of an unbiased but consistent estimator is used for **, which is determined as follows. Taking the radius r(Bk(x)) of the query ball (**what is this?**) a tangential disk D of radius r(Bk(x)) centered on the point x is divided into M equal-sized subdomains Di, that is





  
    
      
         
        
          
            
              
                
                  
                    
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                        ⁢
                        
                          
                        
                        ⁢
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                        ⁢
                        
                          
                        
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                      ⁢
                      
                        
                      
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                    j
                  
                
                ,
              
            
            
              
                
              
            
          
        
      
    
    
      
        (
        24
        )
      
    
  
  
    
      
        
          where
          ⁢
          
            
          
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              D
              i
            
            
          
        
        =
        
          
            
              
              D
              
            
            M
          
          =
          
            
              
                π
                ⁢
                
                  
                
                ⁢
                
                  
                    r
                    2
                  
                  ⁡
                  
                    (
                    
                      
                        B
                        k
                      
                      ⁡
                      
                        (
                        x
                        )
                      
                    
                    )
                  
                
              
              M
            
            .
          
        
      
    
    
      
        
      
    
  




The set

P={Di|Di∩{xi|D|iεBk(x)}≠0}  (25)

contains all the subdomains Di that contain a point xi|D on the disk, which is the position of the “i-th” photon projected onto the plane defined by the disk D along its direction of incidence ωi. Preferably, the number M of subdomains will be on the order of √{square root over (k)} and the angular subdivision will be finer than the radial subdivision in order to capture geometry borders. The actual area A is then determined by





  
    
      
        A
        =
        
          π
          ⁢
          
            
          
          ⁢
          
            
              r
              2
            
            ⁡
            
              (
              
                
                  B
                  k
                
                ⁡
                
                  (
                  x
                  )
                
              
              )
            
          
          ⁢
          
            
              
                
                P
                
              
              M
            
            .
          
        
      
    
    
      
        (
        26
        )
      
    
  




Determining the actual coverage of the disk D by photons significantly improves the radiance estimate (equation (23)) in corners and on borders, where the area obtained by the standard estimate πr2(Bk(x)) would be too small, which would be the case at corners, or too large, which would be the case at borders. In order to avoid blurring of sharp contours of caustics and shadows, the computer graphics system 10 sets the radiance estimate L to black if all domains Di that touch x do not contain any photons.

It will be appreciated that, in regions of high photon density, the “k” nearest photons may lead to a radius r(Bk(x) that is nearly zero, which can cause an over-modulation of the estimate. Over-modulation can be avoided by selecting a minimum radius rmin, which will be used if r(Bk(x)) is less than rmin. In that case, instead of equation (23), the estimate is generated in accordance with




  
    
      
        
          
            
              L
              _
            
            r
          
          ⁡
          
            (
            
              x
              ,
              ω
            
            )
          
        
        =
        
          
            N
            k
          
          ⁢
          
            
              ∑
              
                i
                ∈
                
                  
                    B
                    k
                  
                  ⁡
                  
                    (
                    x
                    )
                  
                
              
            
            ⁢
            
              
            
            ⁢
            
              
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                i
              
              ⁢
              
                
                  f
                  r
                
                ⁡
                
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                      r
                    
                  
                  )
                
              
            
          
        
      
    
    
      
        (
        27
        )
      
    
  




assuming each photon is started with 1/N of the total flux Φ. The estimator in equation (27) provides an estimate for the mean flux of the “k” photons if r(Bk(x))<rmin.

The global photon map is generally rather coarse and, as a result, subpixel samples can result in identical photon map queries. As a result, the direct visualization of the global photon map is blurry and it is advantageous to perform a smoothing operation in connection therewith In performing such an operation, the computer graphics system 10 performs a local pass integration that removes artifacts of the direct visualization. Accordingly, the computer graphics system 10 generates an approximation for the diffuse illumination term TfdTfdL as




  
    
      
        
          
            
              
                
                  
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                          ⁡
                          
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                                ⁡
                                
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                                      ⁢
                                      
                                        
                                          u
                                          
                                            i
                                            ,
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                                    ,
                                    
                                      2
                                      ⁢
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                                  )
                                
                              
                            
                            )
                          
                        
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              ,
            
          
        
      
    
    
      
        (
        28
        )
      
    
  




with the integral over the hemisphere S2(x) of incident directions aligned by the surface normal in “x” being evaluated using importance sampling. The computer graphics system 10 stratifies the sample points on a two-dimensional grid by applying dependent trajectory splitting with the Hammersley sequence and thereafter applies irradiance interpolation. Instead of storing the incident flux Φi of the respective photons, the computer graphics system 10 stores their reflected diffuse power fd(xi)Φi with the respective photons in the photon map, which allows for a more exact approximation than can be obtained by only sampling the diffuse BRDF in the hit points of the final gather rays. In addition, the BRDF evaluation is needed only once per photon, saving the evaluations during the final gathering. Instead of sampling the full grid, the computer graphics system 10 uses adaptive sampling, in which refinement is triggered by contrast, distance traveled by the final gather rays in order to more evenly sample the projected solid angle, and the number of photons that are incident form the portion of the projected hemisphere. The computer graphics system fills in positions in the grid that are not sampled by interpolation. The resulting image matrix of the projected hemisphere is median filtered in order to remove weak singularities, after which the approximation is generated. The computer graphics system 10 performs the same operation in connection with hemispherical sky illumination.

The computer graphics system 10 processes final gather rays that strike objects that do not cause caustics, such as plane glass windows, by recursive ray tracing. If the hit point of a final gather ray is closer to its origin than a predetermined threshold, the computer graphics system 10 also performs recursive ray tracing. This reduces the likelihood that blurry artifacts will appear in corners, which might otherwise occur since for close hit points the same photons would be collected, which can indirectly expose the blurry structure of the global photon map.
Generally, photon maps have been taken as a snapshot at one point in time, and thus were unsuitable in connection with rendering of motion blur. Following the observation that averaging the result of a plurality of photon maps is generally similar to querying one photon map with the total number of photons from all of the plurality of photon maps, the computer graphics system 10 generates NT photon maps, where NT is determined as described above, at points in time




  
    
      
        
          
            t
            b
          
          =
          
            
              t
              0
            
            +
            
              
                
                  b
                  +
                  
                    1
                    2
                  
                
                
                  N
                  T
                
              
              ⁢
              T
            
          
        
        ,
      
    
    
      
        (
        29
        )
      
    
  




for 0≦b<NT. During rendering, the computer graphics system 10 uses the photon map with the smallest time difference |ti,j−tb| in connection with rendering for the time sample point ti,j.

The invention provides a number of advantages. In particular, the invention provides a computer graphics system that makes use of strictly deterministic distributed ray tracing based on low-discrepancy sampling and dependent trajectory splitting in connection with rendering of an image of a scene. Generally, strictly deterministic distributed ray tracing based on low-discrepancy sampling and dependent trajectory splitting is simpler to implement than an implementation based on random or pseudo-random numbers. Due to the properties of the radical inversion function, stratification of sample points is intrinsic and does not need to be considered independently of the generation of the positions of the sample points. In addition, since the methodology is strictly deterministic, it can be readily parallelized by dividing the image into a plurality of tasks, which can be executed by a plurality of processors in parallel. There is no need to take a step of ensuring that positions of sample points are not correlated, which is generally necessary if a methodology based on random or pseudo-random numbers is to be implemented for processing in parallel.
Generally, a computer graphics system that makes use of low-discrepancy sampling in determination of sample points will perform better than a computer graphics system that makes use of random or pseudo-random sampling, but the performance may degrade to that of a system that makes use of random or pseudo-random sampling in higher dimensions. By providing that the computer graphics system performs dependent splitting by replication, the superior convergence of low-dimensional low-discrepancy sampling can be exploited with the effect that the overall integrand becomes smoother, resulting in better convergence than with stratified random or pseudo-random sampling. Since the computer graphics system also makes use of dependent trajectory sampling by means of infinite low discrepancy sequences, consistent adaptive sampling of, for example, light sources, can also be performed.
In addition, it will be appreciated that, although the computer graphics system has been described as making use of sample points generated using generalized scrambled and/or unscrambled Hammersley and Halton sequences, it will be appreciated that generally any (t,m,s)-net or (t,s)-sequence can be used.
Code Segment 1
The following is a code fragment in the C++ programming language for generating the positions of the jittered subpixel sample points xi












unsigned short Period, *Sigma;


void InitSigma(int n)


{


unsigned short Inverse, Digit, Bits;


Period = 1 << n;


Sigma = new unsigned short [Period];


for (unsigned short i = 0; i < Period; i++)


{









Digit = Period



Inverse = 0;



for (bits = i; bits; bits >>= 1)



{









Digit >>= 1;



if (Bits & 1)









inverse += Digit;









}



Sigma[i] = Inverse;







 }


}


void SampleSubpixel(unsigned int *i, double *x, double *y, int sx, int sy)


{









int j = sx & (Period − 1);



int k = sy & (Period − 1);



*i = j * Period + Sigma[k]



*x = (double) sx + (double) Sigma[k] / (double) Period;



*y = (double) sy + (double) Sigma[j] / (double) Period;







}









Code Segment 2
The following is a code fragment in the C++ programming language for generating a ray tree












class Ray


{









int i; //current instance of low discrepancy vector



int d; //current integral dimension in ray tree



...







}


void Shade (Ray& Ray)


{









Ray next_ray;



int i = ray.i;



int d = ray.d



...



for (int j = 0;j < M;j++)



{









...



// ray set up for recursion



 







  
    
      
        
          y
          =
          
            (
            
              
                
                  
                    Φ
                    
                      b
                      d
                    
                  
                  ⁡
                  
                    (
                    
                      i
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                        σ
                        
                          b
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                    )
                  
                
                ⊕
                
                  j
                  M
                
              
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              ...
              ⁢
              
                
              
              ,
            
          
        
      
    
    
      
        
          
            
              
                Φ
                
                  b
                  
                    d
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                      ⁢
                      
                        
                      
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                    1
                  
                
              
              ⁡
              
                (
                
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              ⁡
              
                (
                
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next_ray = SetUpRay(y); // determine ray parameters by y



next_ray.i = i + j; // decorrelation by generalization



next_ray.d = d + Δd; //dimension allocation



Shade(next_ray);



...









}







}









It will be appreciated that a system in accordance with the invention can be constructed in whole or in part from special purpose hardware or a general purpose computer system, or any combination thereof, any portion of which may be controlled by a suitable program. Any program may in whole or in part comprise part of or be stored on the system in a conventional manner, or it may in whole or in part be provided in to the system over a network or other mechanism for transferring information in a conventional manner. In addition, it will be appreciated that the system may be operated and/or otherwise controlled by means of information provided by an operator using operator input elements (not shown) which may be connected directly to the system or which may transfer the information to the system over a network or other mechanism for transferring information in a conventional manner.
The foregoing description has been limited to a specific embodiment of this invention. It will be apparent, however, that various variations and modifications may be made to the invention, with the attainment of some or all of the advantages of the invention. It is the object of the appended claims to cover these and such other variations and modifications as come within the true spirit and scope of the invention.

## Claims

1. A computer program product, comprising computer program instructions executable in a computer graphics system operable to generate one or more pixel values for a pixel in an image, the pixel being representative of a point in a scene as recorded on an image plane of a simulated camera, the image being displayable to a user via a display element in electrical communication with the computer, in response to display-controlling electrical signals generated by the computing element and communicated to the display element, the display-controlling electrical signals being generated by the computer based on the pixel values, the computer program instructions being executable by the computer graphics system to enable the computer graphics system to generate the pixel values for the image, the computer program product comprising a computer-readable physical medium having stored thereon:
A. computer-readable instructions executable by the computer graphics system to enable the computer graphics system to generate a set of sample points representing at least one simulated element of the simulated camera, the sample points representing elements of a Hammersley sequence, and
B. computer-readable instructions executable by the computer graphics system to enable the computer graphics system to generate at least one value representing an evaluation of a selected function at one of the sample points, the value generated corresponding to the pixel value;
wherein the generating of sample points comprises generating sample position points xi representing jittered sample point positions on a subpixel grid for at least one pixel on the image plane for use by the computer graphics system in evaluating the selected function; and
wherein sample position points xi are generated in accordance with





  
    
      
        
          x
          i
        
        =
        
        ⁢
        
          (
          
            
              
                s
                x
              
              +
              
                
                  Φ
                  2
                
                ⁡
                
                  (
                  k
                  )
                
              
            
            ,
            
              
                s
                y
              
              +
              
                
                  Φ
                  2
                
                ⁡
                
                  (
                  j
                  )
                
              
            
          
          )
        
      
    
  
  
    
      
        =
        
        ⁢
        
          (
          
            
              
                s
                x
              
              +
              
                
                  σ
                  ⁡
                  
                    (
                    k
                    )
                  
                
                
                  2
                  n
                
              
            
            ,
            
              
                s
                y
              
              +
              
                
                  σ
                  ⁡
                  
                    (
                    j
                    )
                  
                
                
                  2
                  n
                
              
            
          
          )
        
      
    
  




where (sx,sy) are subpixel coordinates mapped onto strata coordinates


(j,k):=(sx mod 2n,sy mod 2n),


and instance number “i” corresponds to


i=j2n+σ(k)


where integer permutation


σ(k):=2nΦ2(k) for 0≦k<2n 


for “n” a selected integer, and


Φδ(q)


is the radical inverse function of “q” in base “b”.

2. A computer program product, comprising computer program instructions executable in a computer graphics system operable to generate one or more pixel values for a pixel in an image, the pixel being representative of a point in a scene as recorded on an image plane of a simulated camera, the image being displayable to a user via a display element in electrical communication with the computer, in response to display-controlling electrical signals generated by the computing element and communicated to the display element, the display-controlling electrical signals being generated by the computer based on the pixel values, the computer program instructions being executable by the computer graphics system to enable the computer graphics system to generate the pixel values for the image, the computer program product comprising a computer-readable physical medium having stored thereon:
A. computer-readable instructions executable by the computer graphics system to enable the computer graphics system to generate a set of sample points representing at least one simulated element of the simulated camera, the sample points representing elements of a Hammersley sequence; and
B. computer-readable instructions executable by the computer graphics system to enable the computer graphics system to generate at least one value representing an evaluation of a selected function at one of the sample points, the value generated corresponding to the pixel value;
wherein the generating of sample points comprises generating sample position points xi representing jittered sample point positions on a subpixel grid for at least one pixel on the image plane for use by the computer graphics system in evaluating the selected function,
wherein the simulated camera is to be provided with a shutter, and the computer-readable instructions executable by the computer graphics system to enable the computer graphics system to generate a set of sample points comprises computer-readable instructions executable by the computer graphics system to enable the computer graphics system to generate, for at least one sample point on a subpixel for at least one pixel on the image plane, sample time points ti,j representing “j” points in time during a time interval t0 to t0+T during which the shutter is to be open tot use by the computer graphics system in evaluating the selected function; and
wherein the generating of sample points comprises generating the sample time points ti,j in accordance with





  
    t
    
      i
      ,
      j
    
  
  :=
  
    
      t
      0
    
    +
    
      
        (
        
          
            
              Φ
              3
            
            ⁡
            
              (
              i
              )
            
          
          ⊕
          
            j
            
              N
              T
            
          
        
        )
      
      ·
      T
    
  




where NT corresponds to a predetermined number of times at which sample time points ti,j are to be generated and


Φδ(q)


is the radical inverse function of “q ” in base “b|”.

3. A computer program product, comprising computer program instructions executable in a computer graphics system operable to generate one or more pixel values for a pixel in an image the pixel being representative of a point in a scene as recorded on an image plane of a simulated camera, the image being displayable to a user via a display element in electrical communication with the computer, in response to display-controlling electrical signals generated by the computing element and communicated to the display element, the display-controlling electrical signals being generated by the computer based on the pixel values, the computer program instructions being executable by the computer graphics system to enable the computer graphics system to generate the pixel values for the image, the computer program product comprising a computer-readable physical medium having stored thereon:
A. computer-readable instructions executable by the computer graphics system to enable the computer graphics system to generate a set of sample points representing at least one simulated element of the simulated camera, the sample points representing elements of a Hammersley sequence; and
B. computer-readable instructions executable by the computer graphics system to enable the computer graphics system to generate at least one value representing an evaluation of a selected function at one of the sample points, the value generated corresponding to the pixel value;
wherein the generating of sample points comprises generating sample position points xi representing jittered sample point positions on a subpixel grid for at least one pixel on the image plane for use by the computer graphics system in evaluating the selected function;
wherein the simulated camera is to be provided with a shutter, and the generating of sample points comprises generating, for at least one sample point position on a subpixel for at least one pixel on the image plane, sample time points ti,j representing “j” points in time during a time interval t0 to t0+T during which the shutter is to be open for use by the computer graphics system in evaluating the selected function;
wherein the simulated camera is to be provided with a lens, and the generating of sample points comprises generating, for at least one sample point position and at least one sample time point, sample lens position points yi,j,k representing “k” points on the lens for use by the computer graphics system in evaluating the selected function; and
wherein the generating of sample points comprises generating the sample lens points in accordance with





  
    y
    
      i
      ,
      j
      ,
      k
    
  
  =
  
    (
    
      
        (
        
          
            
              Φ
              5
            
            ⁡
            
              (
              
                
                  i
                  +
                  j
                
                ,
                
                  σ
                  5
                
              
              )
            
          
          ⊕
          
            k
            
              N
              6
            
          
        
        )
      
      ,
      
        (
        
          
            
              Φ
              7
            
            ⁡
            
              (
              
                
                  i
                  +
                  j
                
                ,
                
                  σ
                  7
                
              
              )
            
          
          ⊕
          
            
          
          ⁢
          
            
              Φ
              2
            
            ⁡
            
              (
              k
              )
            
          
        
        )
      
    
    )
  




where


Φδ(q,σ)


is the scrambled radical inverse function of “q” in base“b” and “⊕” refers to addition modulo a predetermined value and instance number “i” corresponds to


i=j2n+σ(k)


where integer permutation


σ(k):=2nΦ2(k) for 0≦k<2n 


for “n” a selected integer.

4. In a computer graphics system operable to generate a pixel value for a pixel in an image, the pixel being representative of a point in a scene as recorded on an image plane of a simulated camera, the image being displayable to a user via a display element in electrical communication with the computer, in response to display-controlling electrical signals generated by the computing element and communicated to the display element, the display-controlling electrical signals being generated by the computer based on the pixel values, the improvement comprising:
A. a sample point generator configured to generate a set of sample points representing at least one simulated element of the simulated camera, the sample points representing elements of Hammersley sequence; and
B. a function evaluator configured to generate at least one value representing an evaluation of a selected function at one of the sample points generated by the sample point generator, the value generated by the function evaluator corresponding to the pixel value;
wherein the sample point generator is configured to generate sample position points xi representing jittered sample point positions on a subpixel grid for at least one pixel on the image plane for use by the function evaluator in evaluating the selected function; and
wherein the sample point generator is configured to generate the sample position points xi n accordance with





  
    
      
        
          x
          i
        
        =
        
        ⁢
        
          (
          
            
              
                s
                x
              
              +
              
                
                  Φ
                  2
                
                ⁡
                
                  (
                  k
                  )
                
              
            
            ,
            
              
                s
                y
              
              +
              
                
                  Φ
                  2
                
                ⁡
                
                  (
                  j
                  )
                
              
            
          
          )
        
      
    
  
  
    
      
        =
        
        ⁢
        
          (
          
            
              
                s
                x
              
              +
              
                
                  σ
                  ⁡
                  
                    (
                    k
                    )
                  
                
                
                  2
                  n
                
              
            
            ,
            
              
                s
                y
              
              +
              
                
                  σ
                  ⁡
                  
                    (
                    j
                    )
                  
                
                
                  2
                  n
                
              
            
          
          )
        
      
    
  




where (sx, sy) are subpixel coordinates mapped onto strata coordinates


(j,k):=(sx mod 2n,sy mod 2n),


and instance number “i” corresponds to


i=j2n+σ(k)


where integer permutation


σ(k):=2nΦ2(k) for 0≦k<2n 


for “n” a selected integer, and


Φδ(q)


is the radical inverse function of “q” in base “b|”.

5. In a computer graphics system operable to generate a pixel value for a pixel in an image, the pixel being representative of a point in a scene as recorded on an image plane of a simulated camera, the image being displayable to a user via a display element in electrical communication with the computer, in response to display-controlling electrical signals generated by the computing element and communicated to the display element, the display-controlling electrical signals being generated by the computer based on the pixel values, the improvement comprising:
A. a sample point generator configured to generate a set of sample points representing at least one simulated element of the simulated camera, the sample points representing elements of a Hammerstey sequence; and
B. a function evaluator configured to generate at least one value representing an evaluation of a selected function at one of the sample points generated by the sample point generator, the value generated by the function evaluator corresponding to the pixel value;
wherein the sample point generator is configured to generate sample position points xi representing jittered sample point positions on a subpixel grid for at least one pixel on the image plane for use by the function evaluator in evaluating the selected function;
wherein the simulated camera is to he provided with a shutter, and the sample point generator is configured to generate, for at least one sample point position on a subpixel for at least one pixel on the image plane, sample time points ti,j representing “j” points in time during a time interval t0 to t0+T during which the shutter is to be open for use by the function evaluator in evaluating the selected function; and
wherein the sample point generator is configured to generate the sample time points ti,j in accordance with





  
    t
    
      i
      ,
      j
    
  
  :=
  
    
      t
      0
    
    +
    
      
        (
        
          
            
              Φ
              3
            
            ⁡
            
              (
              i
              )
            
          
          ⊕
          
            j
            
              N
              T
            
          
        
        )
      
      ·
      T
    
  




where NT corresponds to a predetermined number of times at which sample time points ti,j are to be generated and


Φδ(q)


is the radical inverse function of “q” in base “b|”.

6. In a computer graphics system operable to generate a pixel value for a pixel in an image, the pixel being representative of a point in a scene as recorded on an image plane of a simulated camera, the image being displayable to a user via a display element in electrical communication with the computer, in response to display-controlling electrical signals generated by the computing element and communicated to the display element, the display-controlling electrical signals being generated by the computer based on the pixel values, the improvement comprising:
A. a sample point generator configured to generate a set of sample points representing at least one simulated element of the simulated camera, the sample points representing elements Hammersley sequence; and
B. a function evaluator configured to generate at least one value representing an evaluation of a selected function at one of the sample points generated by the sample point generator,the value generated by the function evaluator corresponding to the pixel value;
wherein the sample point generator is configured to generate sample position points xi representing jittered sample point positions on a subpixel grid for at least one pixel on the image plane for use by the function evaluator in evaluating the selected function;
wherein the simulated camera is to be provided with a shutter, and the sample point generator is con figured to generate, for at least one sample point position on a subpixel for at least one pixel on the image plane, sample time points ti,j representing “j” points in time during a time interval t0 to t0+T during which the shutter is to be open for use by the function evaluator in evaluating the selected function;
wherein the simulated camera is to be provided with a lens, and the sample point generator is configured to generate for at least one sample point position and at least one sample time point, sample lens position points Yi,j,K representing “k” points on the lens for use by the function evaluator in evaluating the selected function; and
wherein the sample point generator is configured to generate the sample lens points in accordance with





  
    y
    
      i
      ,
      j
      ,
      k
    
  
  =
  
    (
    
      
        (
        
          
            
              Φ
              5
            
            ⁡
            
              (
              
                
                  i
                  +
                  j
                
                ,
                
                  σ
                  5
                
              
              )
            
          
          ⊕
          
            k
            
              N
              L
            
          
        
        )
      
      ,
      
        (
        
          
            
              Φ
              7
            
            ⁡
            
              (
              
                
                  i
                  +
                  j
                
                ,
                
                  σ
                  7
                
              
              )
            
          
          ⊕
          
            
              Φ
              2
            
            ⁡
            
              (
              k
              )
            
          
        
        )
      
    
    )
  




where


Φδ(q,σ)


is the scrambled radical inverse function of “q” in base “b” and “⊕” refers to addition modulo a predetermined value and instance number “i” corresponds to


i=j2n+σ(k)


where integer permutation


σ(k):=2nΦ2(k) for 0≦k<2n 


for “n” a selected integer.