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(* kernel.sml * * COPYRIGHT (c) 2010 The Diderot Project (http://diderot.cs.uchicago.edu) * All rights reserved. * * QUESTION: should we *) structure Kernel : sig type coefficient = Rational.rat (* polynomial represented as list of coefficients, where ith element is * coefficient for x^i. *) type polynomial = coefficient list type kernel (* kernel name *) val name : kernel -> string val toString : kernel -> string (* are two kernels the same *) val same : kernel * kernel -> bool (* hash value *) val hash : kernel -> word (* kernel support *) val support : kernel -> int (* representation of i'th derivative of the kernel *) val curve : kernel * int -> { isOdd : bool, isCont : bool, segs : polynomial list (* piece-wise polynomial that defines *) (* the curve over the positive support *) } val evaluate : polynomial * int -> Rational.rat (* some standard kernels *) val tent : kernel (* linear interpolation *) val ctmr : kernel (* Catmull-Rom interpolation *) val bspln3 : kernel (* cubic bspline reconstruction, doesn't interpolate *) val bspln5 : kernel (* quintic bspline reconstruction, doesn't interpolate *) end = struct structure R = Rational structure A = Array val maxDiffLevels = 15 (* support upto 15 levels of differentiation *) type coefficient = R.rat val zero = R.zero val one = R.fromInt 1 (* polynomial represented as list of coefficients, where ith element is * coefficient for x^i. *) type polynomial = coefficient list fun differentiate [] = raise Fail "invalid polynomial" | differentiate [_] = [zero] | differentiate (_::coeffs) = let fun lp (_, []) = [] | lp (i, c::r) = R.*(R.fromInt i, c) :: lp(i+1, r) in lp (1, coeffs) end (* evaluate a polynomial at an integer coordinate (used to test continuity) *) fun evaluate (poly, x) = let val x = R.fromInt x fun eval (sum, [], xn) = sum | eval (sum, c::r, xn) = eval(R.+(sum, R.*(c, xn)), r, R.*(x, xn)) in eval (zero, poly, one) end type curve = { isOdd : bool, isCont : bool, segs : polynomial list (* piece-wise polynomial that defines *) (* the curve over the positive support *) } datatype kernel = K of { name : string, id : Stamp.stamp, (* unique ID *) support : int, (* number of samples to left/right *) curves : curve option array (* cache of curves indexed by differentiation level *) } (* determine if a list of polynomials represents a continuous piece-wise polynomial *) fun isContinuous [_] = true | isContinuous (f0::r) = let fun chk (i, f_i, []) = (R.zero = evaluate(f_i, i)) | chk (i, f_i, f_i1::r) = let val y_i = evaluate(f_i, i) val y_i1 = evaluate(f_i1, i) in (y_i = y_i1) andalso chk(i+1, f_i1, r) end in chk (1, f0, r) end (* kernel name *) fun name (K{name, ...}) = name val toString = name (* kernel support *) fun support (K{support, ...}) = support fun hash (K{id, ...}) = Stamp.hash id fun same (K{id=a, ...}, K{id=b, ...}) = Stamp.same (a, b) (* representation of i'th derivative of the kernel *) fun curve (K{curves, ...}, k) = (case A.sub(curves, k) of SOME curve => curve | NONE => let (* compute the (k+1)'th derivative, given the k'th *) fun diff (k, {isOdd, isCont, segs}) = let val segs' = List.map differentiate segs val isOdd' = not isOdd val isCont' = if isCont andalso isContinuous segs' then (not isOdd') orelse (evaluate(List.hd segs', 0) = R.zero) else false in { isOdd = isOdd', isCont = isCont', segs = segs' } end fun lp (j, curve) = if (j < k) then (case A.sub(curves, j+1) of NONE => let val curve' = diff(j+1, curve) in A.update(curves, j+1, SOME curve'); lp (j+1, curve') end | SOME curve' => lp(j+1, curve') (* end case *)) else curve in lp (0, valOf(A.sub(curves, 0))) end (* end case *)) (* some standard kernels *) local val op / = R./ fun r i = R.fromInt i fun mkKernel {name, support, segs} = let val curves = Array.array(maxDiffLevels+1, NONE) val curve0 = { isOdd = false, isCont = isContinuous segs, segs = segs } in A.update (curves, 0, SOME curve0); K{name=name, id=Stamp.new(), support=support, curves=curves} end in val tent : kernel = mkKernel{ (* linear interpolation *) name = "tent", support = 1, segs = [[r 1, r ~1]] } val ctmr : kernel = mkKernel{ (* Catmull-Rom interpolation *) name = "ctmr", support = 2, segs = [ [r 1, r 0, ~5/2, 3/2], [r 2, r ~4, 5/2, ~1/2] ] } val bspln3 : kernel = mkKernel{ (* cubic bspline reconstruction, doesn't interpolate *) name = "bspln3", support = 2, segs = [ [ 2/3, r 0, r ~1, 1/2 ], [ 4/3, r ~2, r 1, ~1/6 ] ] } val bspln5 : kernel = mkKernel{ (* quintic bspline reconstruction, doesn't interpolate *) name = "bspln5", support = 3, segs = [ [ 11/20, r 0, ~1/2, r 0, 1/4, ~1/12 ], [ 17/40, 5/8, ~7/4, 5/4, ~3/8, 1/24 ], [ 81/40, ~27/8, 9/4, ~3/4, 1/8, ~1/120 ] ] } end end

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