QPainter: fix assert when drawing bitmaps at very near to .5 coord

The code assumed that the rounding of a floating point value, and the
rounding of the sum of that value and an integer, would always snap in
the same direction. But because of accuracy limits (independently
of the rounding function employed), that is not always the case for
fractions very near to .5.

Fixes: QTBUG-122451
Pick-to: 6.7 6.6 6.5 6.2 5.15
Change-Id: I0825d42e6be7f6e3397760a5e9be5dddca42dcdc
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

2 files changed, 19 insertions, 6 deletions

diff --git a/src/gui/painting/qpaintengine_raster.cpp b/src/gui/painting/qpaintengine_raster.cpp
index 0ff52a8730..582ad500f8 100644
--- a/src/gui/painting/qpaintengine_raster.cpp
+++ b/src/gui/painting/qpaintengine_raster.cpp
@@ -3397,16 +3397,18 @@ void QRasterPaintEngine::drawBitmap(const QPointF &pos, const QImage &image, QSp
     // Boundaries
     int w = image.width();
     int h = image.height();
-    int ymax = qMin(qRound(pos.y() + h), d->rasterBuffer->height());
-    int ymin = qMax(qRound(pos.y()), 0);
-    int xmax = qMin(qRound(pos.x() + w), d->rasterBuffer->width());
-    int xmin = qMax(qRound(pos.x()), 0);
+    int px = qRound(pos.x());
+    int py = qRound(pos.y());
+    int ymax = qMin(py + h, d->rasterBuffer->height());
+    int ymin = qMax(py, 0);
+    int xmax = qMin(px + w, d->rasterBuffer->width());
+    int xmin = qMax(px, 0);
 
-    int x_offset = xmin - qRound(pos.x());
+    int x_offset = xmin - px;
 
     QImage::Format format = image.format();
     for (int y = ymin; y < ymax; ++y) {
-        const uchar *src = image.scanLine(y - qRound(pos.y()));
+        const uchar *src = image.scanLine(y - py);
         if (format == QImage::Format_MonoLSB) {
             for (int x = 0; x < xmax - xmin; ++x) {
                 int src_x = x + x_offset;
diff --git a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
index 7c361fd994..15735aecf7 100644
--- a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
+++ b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
@@ -61,6 +61,7 @@ private slots:
 #endif
     void drawPixmapFragments();
     void drawPixmapNegativeScale();
+    void drawPixmapRounding();
 
     void drawLine_data();
     void drawLine();
@@ -747,6 +748,16 @@ void tst_QPainter::drawPixmapNegativeScale()
     QVERIFY(resultImage.pixel(12, 8) == qRgba(0, 0, 0, 255)); // and right strip is now black
 }
 
+void tst_QPainter::drawPixmapRounding()
+{
+    // Just test that we don't assert
+    QBitmap bm(8, 8);
+    QImage out(64, 64, QImage::Format_RGB32);
+    QPainter p(&out);
+    qreal y = 26.499999999999996;
+    p.drawPixmap(QPointF(0, y), bm);
+}
+
 void tst_QPainter::drawLine_data()
 {
     QTest::addColumn<QLine>("line");