From 824d2936010bb46322d29eeb708bee0cee87d081 Mon Sep 17 00:00:00 2001
From: Eirik Aavitsland <eirik.aavitsland@qt.io>
Date: Tue, 20 Feb 2024 08:07:38 +0100
Subject: [PATCH] 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: 6.6 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>
(cherry picked from commit a43d86fe1c0bc9d352f67c134a9ee5f754aea5e6)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
---
 src/gui/painting/qpaintengine_raster.cpp          | 14 ++++++++------
 tests/auto/gui/painting/qpainter/tst_qpainter.cpp | 11 +++++++++++
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/src/gui/painting/qpaintengine_raster.cpp b/src/gui/painting/qpaintengine_raster.cpp
index 0ff52a87303..582ad500f8d 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 7c361fd994f..15735aecf78 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");