More fuzzy evaluation when calculating roots

If the discriminant is close to zero, evaluate it as being zero.

Change-Id: Ief2624dd512d703e44d8a0c63431baef3cee7c45
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

2 files changed, 14 insertions, 1 deletions

diff --git a/src/animation/backend/bezierevaluator.cpp b/src/animation/backend/bezierevaluator.cpp
index 8a7f0139b..d211c584b 100644
--- a/src/animation/backend/bezierevaluator.cpp
+++ b/src/animation/backend/bezierevaluator.cpp
@@ -230,7 +230,7 @@ int BezierEvaluator::findCubicRoots(const float coeffs[4], float roots[3])
     const double pCubed = p * p * p;
     const double discriminant = q * q + pCubed;
 
-    if (qIsNull(discriminant)) {
+    if (almostZero(discriminant, 1e-6f)) {
         if (qIsNull(q)) {
             // One repeated triple root
             roots[0] = 0.0;
diff --git a/tests/auto/animation/bezierevaluator/tst_bezierevaluator.cpp b/tests/auto/animation/bezierevaluator/tst_bezierevaluator.cpp
index c61ce2d7e..06389bd36 100644
--- a/tests/auto/animation/bezierevaluator/tst_bezierevaluator.cpp
+++ b/tests/auto/animation/bezierevaluator/tst_bezierevaluator.cpp
@@ -187,6 +187,19 @@ private Q_SLOTS:
         roots[2] = -1.39297f;
         QTest::newRow("a=-0.75, b=0.75, c=2.5, d=0") << a << b << c << d << roots.size() << roots;
         roots.clear();
+
+        // Case that produces a discriminant that is close enough to zero that it should be
+        // evaluated as zero.
+        // Expected roots = 0.0, ~1.5
+        a = -3.998f;
+        b = 5.997f;
+        c = 0.0f;
+        d = 0.0f;
+        roots.resize(2);
+        roots[0] = 1.5f;
+        roots[1] = 0.0f;
+        QTest::newRow("a=-3.998, b=5.997, c=0, d=0") << a << b << c << d << roots.size() << roots;
+        roots.clear();
     }
 
     void checkFindCubicRoots()