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Details

Type: Bug
Resolution: Done
Priority: P4: Low
Fix Version/s: Some future release
Affects Version/s: 4.8.5, 5.1.0
Component/s: GUI: Painting
Labels:
- path
Environment:
Mac OS X 10.6
gcc 4.2.1
qt-everywhere-opensource-src-4.7.0 (Built from sources)

Description

QpainterPath::intersected function does not function properly in some rare cases. The experiencing behavior is not well defined: in the application I'm working on the function returns an empty QPainterPath, while in the standalone test case it just returns a wrong value. This discrepancy might be caused by the rounding errors.

The test case consists of two QPainterPaths: p1 and p2. They are intersected like this:
p1.intersected(p2) and p2.intersected(p1). The results should be equal, while they are not.

#include <QtGui>

#if (QT_VERSION >= QT_VERSION_CHECK(5, 0, 0))
#include <QtWidgets>
#endif

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QGraphicsView view;
    view.resize(1000, 800);
    view.setGeometry(QStyle::alignedRect(Qt::LeftToRight, Qt::AlignCenter, view.size(), qApp->desktop()->availableGeometry()));
    view.setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
    QGraphicsScene *scene = new QGraphicsScene;
    view.setScene(scene);
    view.setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    view.setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);

//#define SHOW_BUG
#ifdef SHOW_BUG
    // Doesn't work
    QPainterPath p1;
    p1.moveTo(256.0000000000000000, 135.8384137532701743);
    p1.lineTo(50.9999999999999715, 107.9999999999999857);
    p1.lineTo(233.5425474228109123, 205.3560252921671462);
    p1.lineTo(191.7771366877784373, 318.0257074407572304);
    p1.lineTo(-48.2616272048215151, 229.0459803737862216);
    p1.lineTo(0.0000000000000000, 98.8515898136580801);
    p1.lineTo(0.0000000000000000, 0.0000000000000000);
    p1.lineTo(256.0000000000000000, 0.0000000000000000);
    p1.lineTo(256.0000000000000000, 135.8384137532701743);

    QPainterPath p2;
    p2.moveTo(1516.2703263523442274, 306.9795200262722119);
    p2.lineTo(-1296.8426224886295585, -75.0331736542986931);
    p2.lineTo(-1678.8553161692004778, 2738.0797751866753060);
    p2.lineTo(1134.2576326717733081, 3120.0924688672457705);
    p2.lineTo(1516.2703263523442274, 306.9795200262722119);
#else
    // Works
    QPainterPath p1 = QPainterPath();
    p1.moveTo(256.00000000, 135.83841375);
    p1.lineTo(50.99999999, 107.99999999);
    p1.lineTo(233.54254742, 205.35602529);
    p1.lineTo(191.77713668, 318.02570744);
    p1.lineTo(-48.26162720, 229.04598037);
    p1.lineTo(0.00000000, 98.85158981);
    p1.lineTo(0.00000000, 0.00000000);
    p1.lineTo(256.00000000, 0.00000000);
    p1.lineTo(256.00000000, 135.83841375);

    QPainterPath p2 = QPainterPath();
    p2.moveTo(1516.27032635, 306.97952002);
    p2.lineTo(-1296.84262248, -75.03317365);
    p2.lineTo(-1678.85531616, 2738.07977518);
    p2.lineTo(1134.25763267, 3120.09246886);
    p2.lineTo(1516.27032635, 306.97952002);
#endif
    QPainterPath i1 = p1.intersected(p2);
    QPainterPath i2 = p2.intersected(p1);
    qDebug() << i1 << i2;

    scene->addPath(p1, QPen(Qt::red));
    scene->addPath(p2, QPen(Qt::blue));
    view.fitInView(scene->itemsBoundingRect().adjusted(600, 600, -600, -600), Qt::KeepAspectRatio);
    view.show();
    return app.exec();
}