minimax.qbk

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[section:minimax Minimax Approximations and the Remez Algorithm]

The directory libs/math/minimax contains a command line driven
program for the generation of minimax approximations using the Remez
algorithm.  Both polynomial and rational approximations are supported,
although the latter are tricky to converge: it is not uncommon for
convergence of rational forms to fail.  No such limitations are present
for polynomial approximations which should always converge smoothly.

It's worth stressing that developing rational approximations to functions
is often not an easy task, and one to which many books have been devoted.
To use this tool, you will need to have a reasonable grasp of what the Remez
algorithm is, and the general form of the approximation you want to achieve.

Unless you already familar with the Remez method
you should first read the [link math_toolkit.backgrounders.remez 
brief background article explaining the principals behind the 
Remez algorithm].

The program consists of two parts:

[variablelist
[[main.cpp][Contains the command line parser, and all the calls to the Remez code.]]
[[f.cpp][Contains the function to approximate.]]
]

Therefore to use this tool, you must modify f.cpp to return the function to
approximate.  The tools supports multiple function approximations within
the same compiled program: each as a separate variant:

   NTL::RR f(const NTL::RR& x, int variant);
   
Returns the value of the function /variant/ at point /x/.  So if you
wish you can just add the function to approximate as a new variant
after the existing examples.

In addition to those two files, the program needs to be linked to
a [link math_toolkit.using_udt.use_ntl patched NTL library to compile].

Note that the function /f/ must return the rational part of the
approximation: for example if you are approximating a function
/f(x)/ then it is quite common to use:
   
   f(x) = g(x)(Y + R(x))

where /g(x)/ is the dominant part of /f(x)/, /Y/ is some constant, and
/R(x)/ is the rational approximation part, usually optimised for a low
absolute error compared to |Y|.

In this case you would define /f/ to return ['f(x)/g(x)] and then set the
y-offset of the approximation to /Y/ (see command line options below).

Many other forms are possible, but in all cases the objective is to 
split /f(x)/ into a dominant part that you can evaluate easily using
standard math functions, and a smooth and slowly changing rational approximation
part.  Refer to your favourite textbook for more examples.

Command line options for the program are as follows:

[variablelist
[[variant N][Sets the current function variant to N.  This allows multiple functions
             that are to be approximated to be compiled into the same executable.
             Defaults to 0.]]
[[range a b][Sets the domain for the approximation to the range \[a,b\], defaults
             to \[0,1\].]] 
[[relative][Sets the Remez code to optimise for relative error.  This is the default
            at program startup.  Note that relative error can only be used
            if f(x) has no roots over the range being optimised.]]
[[absolute][Sets the Remez code to optimise for absolute error.]]
[[pin \[true|false\]]["Pins" the code so that the rational approximation
                     passes through the origin.  Obviously only set this to
                     /true/ if R(0) must be zero.  This is typically used when
                     trying to preserve a root at \[0,0\] while also optimising 
                     for relative error.]]
[[order N D][Sets the order of the approximation to /N/ in the numerator and /D/
            in the denominator.  If /D/ is zero then the result will be a polynomial
            approximation.  There will be N+D+2 coefficients in total, the first
            coefficient of the numerator is zero if /pin/ was set to true, and the
            first coefficient of the denominator is always one.]]
[[working-precision N][Sets the working precision of NTL::RR to /N/ binary digits.  Defaults to 250.]]
[[target-precision N][Sets the precision of printed output to /N/ binary digits: 
               set to the same number of digits as the type that will be used to
               evaluate the approximation.  Defaults to 53 (for double precision).]]
[[skew val]["Skews" the initial interpolated control points towards one
            end or the other of the range.  Positive values skew the
            initial control points towards the left hand side of the
            range, and negative values towards the right hand side.
            If an approximation won't converge (a common situation)
            try adjusting the skew parameter until the first step yields
            the smallest possible error.  /val/ should be in the range
            \[-100,+100\], the default is zero.]]
[[brake val][Sets a brake on each step so that the change in the 
            control points is braked by /val%/.  Defaults to 50,
            try a higher value if an approximation won't converge,
            or a lower value to get speedier convergence.]]
[[x-offset val][Sets the x-offset to /val/: the approximation will
            be generated for `f(S * (x + X)) + Y` where /X/ is the 
            x-offset, /S/ is the x-scale
            and /Y/ is the y-offset.  Defaults to zero.  To avoid
            rounding errors, take care to specify a value that can
            be exactly represented as a floating point number.]]
[[x-scale val][Sets the x-scale to /val/: the approximation will
            be generated for `f(S * (x + X)) + Y` where /S/ is the 
            x-scale, /X/ is the x-offset
            and /Y/ is the y-offset.  Defaults to one.  To avoid
            rounding errors, take care to specify a value that can
            be exactly represented as a floating point number.]]
[[y-offset val][Sets the y-offset to /val/: the approximation will
            be generated for `f(S * (x + X)) + Y` where /X/ 
            is the x-offset, /S/ is the x-scale
            and /Y/ is the y-offset.  Defaults to zero.  To avoid
            rounding errors, take care to specify a value that can
            be exactly represented as a floating point number.]]
[[y-offset auto][Sets the y-offset to the average value of f(x)
            evaluated at the two endpoints of the range plus the midpoint
            of the range.  The calculated value is deliberately truncated 
            to /float/ precision (and should be stored as a /float/ 
            in your code).  The approximation will
            be generated for `f(x + X) + Y` where /X/ is the x-offset
            and /Y/ is the y-offset.  Defaults to zero.]]
[[graph N][Prints N evaluations of f(x) at evenly spaced points over the
            range being optimised.  If unspecified then /N/ defaults
            to 3.  Use to check that f(x) is indeed smooth over the range
            of interest.]]
[[step N][Performs /N/ steps, or one step if /N/ is unspecified.
         After each step prints: the peek error at the extrema of
         the error function of the approximation, 
         the theoretical error term solved for on the last step,
         and the maximum relative change in the location of the
         Chebyshev control points.  The approximation is converged on the
         minimax solution when the two error terms are (approximately)
         equal, and the change in the control points has decreased to
         a suitably small value.]]
[[test \[float|double|long\]][Tests the current approximation at float,
         double, or long double precision.  Useful to check for rounding
         errors in evaluating the approximation at fixed precision.
         Tests are conducted at the extrema of the error function of the
         approximation, and at the zeros of the error function.]]
[[test \[float|double|long\] N] [Tests the current approximation at float,
         double, or long double precision.  Useful to check for rounding
         errors in evaluating the approximation at fixed precision.
         Tests are conducted at N evenly spaced points over the range
         of the approximation.  If none of \[float|double|long\] are specified
         then tests using NTL::RR, this can be used to obtain the error
         function of the approximation.]]
[[rescale a b][Takes the current Chebeshev control points, and rescales them
         over a new interval \[a,b\].  Sometimes this can be used to obtain
         starting control points for an approximation that can not otherwise be
         converged.]]
[[rotate][Moves one term from the numerator to the denominator, but keeps the
         Chebyshev control points the same.  Sometimes this can be used to obtain
         starting control points for an approximation that can not otherwise be
         converged.]]
[[info][Prints out the current approximation: the location of the zeros of the
         error function, the location of the Chebyshev control points, the
         x and y offsets, and of course the coefficients of the polynomials.]]
] 


[endsect][/section:minimax Minimax Approximations and the Remez Algorithm]

[/ 
  Copyright 2006 John Maddock and Paul A. Bristow.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]