qFuzzyCompare() requires that neither argument is numerically zero. This cannot be guaranteed for the parameters of a "TCB" spline; indeed, Wikipedia says t = b = c = 0 is "the default": https://en.wikipedia.org/wiki/Kochanek%E2%80%93Bartels_spline#Parameters_and_Effects Fix by using the new QtPrivate::fuzzyCompare() function, which does things in the correct way. As a drive-by, put the operators at the beginning of continued lines, as requested by https://wiki.qt.io/Qt_Coding_Style#Line_breaks Item 2, make the operator a hidden friend instead of a member function, and explicitly use qFuzzyCompare() instead of operator== for the QPointF member. The latter drive-by change will need to be dropped from the 6.5 pick-to, because the overload didn't exist in 6.5, yet (added by fa0d77e290f5ccb5afa7d02716f8726aa6b810e6 for 6.8). Amends b9f0bde16e85161666d5090952955bebacc40f89. Pick-to: 6.11 6.10 6.8 6.5 Task-number: QTBUG-142020 Change-Id: I76d1adefd39154d7a39454d676dbfd5dfcaf9c1d Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>