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/*
* Copyright 2012 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#include "SkDQuadImplicit.h"
/* from http://tom.cs.byu.edu/~tom/papers/cvgip84.pdf 4.1
*
* This paper proves that Syvester's method can compute the implicit form of
* the quadratic from the parameterized form.
*
* Given x = a*t*t + b*t + c (the parameterized form)
* y = d*t*t + e*t + f
*
* we want to find an equation of the implicit form:
*
* A*x*x + B*x*y + C*y*y + D*x + E*y + F = 0
*
* The implicit form can be expressed as a 4x4 determinant, as shown.
*
* The resultant obtained by Syvester's method is
*
* | a b (c - x) 0 |
* | 0 a b (c - x) |
* | d e (f - y) 0 |
* | 0 d e (f - y) |
*
* which expands to
*
* d*d*x*x + -2*a*d*x*y + a*a*y*y
* + (-2*c*d*d + b*e*d - a*e*e + 2*a*f*d)*x
* + (-2*f*a*a + e*b*a - d*b*b + 2*d*c*a)*y
* +
* | a b c 0 |
* | 0 a b c | == 0.
* | d e f 0 |
* | 0 d e f |
*
* Expanding the constant determinant results in
*
* | a b c | | b c 0 |
* a*| e f 0 | + d*| a b c | ==
* | d e f | | d e f |
*
* a*(a*f*f + c*e*e - c*f*d - b*e*f) + d*(b*b*f + c*c*d - c*a*f - c*e*b)
*
*/
// OPTIMIZATION: test, verify tricky arithmetic
static bool straight_forward = true;
SkDQuadImplicit::SkDQuadImplicit(const SkDQuad& q) {
double a, b, c;
SkDQuad::SetABC(&q[0].fX, &a, &b, &c);
double d, e, f;
SkDQuad::SetABC(&q[0].fY, &d, &e, &f);
// compute the implicit coefficients
if (straight_forward) { // 42 muls, 13 adds
fP[kXx_Coeff] = d * d;
fP[kXy_Coeff] = -2 * a * d;
fP[kYy_Coeff] = a * a;
fP[kX_Coeff] = -2*c*d*d + b*e*d - a*e*e + 2*a*f*d;
fP[kY_Coeff] = -2*f*a*a + e*b*a - d*b*b + 2*d*c*a;
fP[kC_Coeff] = a*(a*f*f + c*e*e - c*f*d - b*e*f)
+ d*(b*b*f + c*c*d - c*a*f - c*e*b);
} else { // 26 muls, 11 adds
double aa = a * a;
double ad = a * d;
double dd = d * d;
fP[kXx_Coeff] = dd;
fP[kXy_Coeff] = -2 * ad;
fP[kYy_Coeff] = aa;
double be = b * e;
double bde = be * d;
double cdd = c * dd;
double ee = e * e;
fP[kX_Coeff] = -2*cdd + bde - a*ee + 2*ad*f;
double aaf = aa * f;
double abe = a * be;
double ac = a * c;
double bb_2ac = b*b - 2*ac;
fP[kY_Coeff] = -2*aaf + abe - d*bb_2ac;
fP[kC_Coeff] = aaf*f + ac*ee + d*f*bb_2ac - abe*f + c*cdd - c*bde;
}
}
/* Given a pair of quadratics, determine their parametric coefficients.
* If the scaled coefficients are nearly equal, then the part of the quadratics
* may be coincident.
* FIXME: optimization -- since comparison short-circuits on no match,
* lazily compute the coefficients, comparing the easiest to compute first.
* xx and yy first; then xy; and so on.
*/
bool SkDQuadImplicit::match(const SkDQuadImplicit& p2) const {
int first = 0;
for (int index = 0; index <= kC_Coeff; ++index) {
if (approximately_zero(fP[index]) && approximately_zero(p2.fP[index])) {
first += first == index;
continue;
}
if (first == index) {
continue;
}
if (!AlmostEqualUlps(fP[index] * p2.fP[first], fP[first] * p2.fP[index])) {
return false;
}
}
return true;
}
bool SkDQuadImplicit::Match(const SkDQuad& quad1, const SkDQuad& quad2) {
SkDQuadImplicit i1(quad1); // a'xx , b'xy , c'yy , d'x , e'y , f
SkDQuadImplicit i2(quad2);
return i1.match(i2);
}
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