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use double to compute root to avoid overflow

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Bug: 850350
Change-Id: Iac04fc62e69f51b68c5fc7f55ac1be930133cc74
Reviewed-on: https://skia-review.googlesource.com/136597
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
This commit is contained in:
Mike Reed 2018-06-21 10:51:35 -04:00 committed by Skia Commit-Bot
parent f7dcdb0283
commit 1e259cda4f
1 changed files with 20 additions and 12 deletions
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32
src/core/SkGeometry.cpp
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@ -60,6 +60,15 @@ static int valid_unit_divide(SkScalar numer, SkScalar denom, SkScalar* ratio) {
return 1;
}
// Just returns its argument, but makes it easy to set a break-point to know when
// SkFindUnitQuadRoots is going to return 0 (an error).
static int return_check_zero(int value) {
if (value == 0) {
return 0;
}
return value;
}
/** From Numerical Recipes in C.
Q = -1/2 (B + sign(B) sqrt[B*B - 4*A*C])
@ -70,22 +79,21 @@ int SkFindUnitQuadRoots(SkScalar A, SkScalar B, SkScalar C, SkScalar roots[2]) {
SkASSERT(roots);
if (A == 0) {
return valid_unit_divide(-C, B, roots);
return return_check_zero(valid_unit_divide(-C, B, roots));
}
SkScalar* r = roots;
SkScalar R = B*B - 4*A*C;
if (R < 0 || !SkScalarIsFinite(R)) { // complex roots
// if R is infinite, it's possible that it may still produce
// useful results if the operation was repeated in doubles
// the flipside is determining if the more precise answer
// isn't useful because surrounding machinery (e.g., subtracting
// the axis offset from C) already discards the extra precision
// more investigation and unit tests required...
return 0;
// use doubles so we don't overflow temporarily trying to compute R
double dr = (double)B * B - 4 * (double)A * C;
if (dr < 0) {
return return_check_zero(0);
}
dr = sqrt(dr);
SkScalar R = SkDoubleToScalar(dr);
if (!SkScalarIsFinite(R)) {
return return_check_zero(0);
}
R = SkScalarSqrt(R);
SkScalar Q = (B < 0) ? -(B-R)/2 : -(B+R)/2;
r += valid_unit_divide(Q, A, r);
@ -98,7 +106,7 @@ int SkFindUnitQuadRoots(SkScalar A, SkScalar B, SkScalar C, SkScalar roots[2]) {
r -= 1; // skip the double root
}
}
return (int)(r - roots);
return return_check_zero((int)(r - roots));
}
///////////////////////////////////////////////////////////////////////////////