Improve rounding in QCosmeticStroker

Fixes the drawn position of end points in drawLine.

Based on a patch by Jørgen Lind, and modified so that
it caused no test regressions.

Task-number: QTBUG-38144
Change-Id: I24aa28480cc6ae09abf91d80378970565a29b254
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

2 files changed, 53 insertions, 4 deletions

diff --git a/src/gui/painting/qcosmeticstroker.cpp b/src/gui/painting/qcosmeticstroker.cpp
index 93c95e4a86..f82b098012 100644
--- a/src/gui/painting/qcosmeticstroker.cpp
+++ b/src/gui/painting/qcosmeticstroker.cpp
@@ -437,8 +437,9 @@ void QCosmeticStroker::calculateLastPoint(qreal rx1, qreal ry1, qreal rx2, qreal
         int y = (y1 + 32) >> 6;
         int ys = (y2 + 32) >> 6;
 
+        int round = (xinc > 0) ? 32 : 0;
         if (y != ys) {
-            x += ( ((((y << 6) + 32 - y1)))  * xinc ) >> 6;
+            x += ( ((((y << 6) + round - y1)))  * xinc ) >> 6;
 
             if (swapped) {
                 lastPixel.x = x >> 16;
@@ -468,8 +469,9 @@ void QCosmeticStroker::calculateLastPoint(qreal rx1, qreal ry1, qreal rx2, qreal
         int x = (x1 + 32) >> 6;
         int xs = (x2 + 32) >> 6;
 
+        int round = (yinc > 0) ? 32 : 0;
         if (x != xs) {
-            y += ( ((((x << 6) + 32 - x1)))  * yinc ) >> 6;
+            y += ( ((((x << 6) + round - x1)))  * yinc ) >> 6;
 
             if (swapped) {
                 lastPixel.x = x;
@@ -762,9 +764,10 @@ static bool drawLine(QCosmeticStroker *stroker, qreal rx1, qreal ry1, qreal rx2,
 
         int y = (y1 + 32) >> 6;
         int ys = (y2 + 32) >> 6;
+        int round = (xinc > 0) ? 32 : 0;
 
         if (y != ys) {
-            x += ( ((((y << 6) + 32 - y1)))  * xinc ) >> 6;
+            x += ( ((((y << 6) + round - y1)))  * xinc ) >> 6;
 
             // calculate first and last pixel and perform dropout control
             QCosmeticStroker::Point first;
@@ -837,9 +840,10 @@ static bool drawLine(QCosmeticStroker *stroker, qreal rx1, qreal ry1, qreal rx2,
 
         int x = (x1 + 32) >> 6;
         int xs = (x2 + 32) >> 6;
+        int round = (yinc > 0) ? 32 : 0;
 
         if (x != xs) {
-            y += ( ((((x << 6) + 32 - x1)))  * yinc ) >> 6;
+            y += ( ((((x << 6) + round - x1)))  * yinc ) >> 6;
 
             // calculate first and last pixel to perform dropout control
             QCosmeticStroker::Point first;
diff --git a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
index 159bb6a041..f6167262a9 100644
--- a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
+++ b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
@@ -111,6 +111,7 @@ private slots:
     void drawLine_task190634();
     void drawLine_task229459();
     void drawLine_task234891();
+    void drawLineEndPoints();
 
     void drawRect_data() { fillData(); }
     void drawRect();
@@ -1036,6 +1037,50 @@ void tst_QPainter::drawLine_task216948()
         QCOMPARE(img.pixel(0, i), QColor(Qt::black).rgba());
 }
 
+void tst_QPainter::drawLineEndPoints()
+{
+    QImage img(256, 256, QImage::Format_ARGB32_Premultiplied);
+    img.fill(0x0);
+
+    QPainter p;
+    for (int x = 0; x < img.width(); ++x) {
+        QRgb color = qRgb(x, 0, 0);
+        p.begin(&img);
+        p.setPen(QPen(color));
+        p.drawLine(x, 0, 255 - x, 255);
+        p.end();
+        QCOMPARE(img.pixel(x, 0), color);
+        QCOMPARE(img.pixel(255 - x, 255), color);
+    }
+    for (int y = 0; y < img.height(); ++y) {
+        QRgb color = qRgb(0, y, 0);
+        p.begin(&img);
+        p.setPen(QPen(color));
+        p.drawLine(0, y, 255, 255 - y);
+        p.end();
+        QCOMPARE(img.pixel(0, y), color);
+        QCOMPARE(img.pixel(255, 255 - y), color);
+    }
+    for (int x = 0; x < img.width(); ++x) {
+        QRgb color = qRgb(x, 0, x);
+        p.begin(&img);
+        p.setPen(QPen(color));
+        p.drawLine(x, 255, 255 - x, 0);
+        p.end();
+        QCOMPARE(img.pixel(x, 255), color);
+        QCOMPARE(img.pixel(255 - x, 0), color);
+    }
+    for (int y = 0; y < img.height(); ++y) {
+        QRgb color = qRgb(0, y, y);
+        p.begin(&img);
+        p.setPen(QPen(color));
+        p.drawLine(255, y, 0, 255 - y);
+        p.end();
+        QCOMPARE(img.pixel(255, y), color);
+        QCOMPARE(img.pixel(0, 255 - y), color);
+    }
+}
+
 void tst_QPainter::drawRect()
 {
     QFETCH(QRect, rect);