Fixed inconsistent rounding of square cap pens.

A horizontal line should round up at the same time as a vertical line
with square cap, when rendering at subpixel coordinates. Thus, the
special casing in the cosmetic stroker of offsetting by half a pixel
should be for flat caps instead of for square caps.

Task-number: QTBUG-26013
(cherry picked from commit 72aaba336c7afe6d79d59995bfb31a8effca4e9e)

Change-Id: If7e7c6b6e37c5b1ce4685012a11fd29ff0b1df5a
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

2 files changed, 40 insertions, 10 deletions

diff --git a/src/gui/painting/qcosmeticstroker.cpp b/src/gui/painting/qcosmeticstroker.cpp
index 7517cc8..9ed8e85 100644
--- a/src/gui/painting/qcosmeticstroker.cpp
+++ b/src/gui/painting/qcosmeticstroker.cpp
@@ -409,7 +409,7 @@ void QCosmeticStroker::calculateLastPoint(qreal rx1, qreal ry1, qreal rx2, qreal
     if (clipLine(rx1, ry1, rx2, ry2))
         return;
 
-    const int half = 32;
+    const int half = 31;
     int x1 = toF26Dot6(rx1) + half;
     int y1 = toF26Dot6(ry1) + half;
     int x2 = toF26Dot6(rx2) + half;
@@ -429,8 +429,8 @@ void QCosmeticStroker::calculateLastPoint(qreal rx1, qreal ry1, qreal rx2, qreal
         int xinc = F16Dot16FixedDiv(x2 - x1, y2 - y1);
         int x = x1 << 10;
 
-        int y = y1 >> 6;
-        int ys = y2 >> 6;
+        int y = (y1 + 32) >> 6;
+        int ys = (y2 + 32) >> 6;
 
         if (y != ys) {
             x += ( ((((y << 6) + 32 - y1)))  * xinc ) >> 6;
@@ -460,8 +460,8 @@ void QCosmeticStroker::calculateLastPoint(qreal rx1, qreal ry1, qreal rx2, qreal
         int yinc = F16Dot16FixedDiv(y2 - y1, x2 - x1);
         int y = y1 << 10;
 
-        int x = x1 >> 6;
-        int xs = x2 >> 6;
+        int x = (x1 + 32) >> 6;
+        int xs = (x2 + 32) >> 6;
 
         if (x != xs) {
             y += ( ((((x << 6) + 32 - x1)))  * yinc ) >> 6;
@@ -703,7 +703,7 @@ static void drawLine(QCosmeticStroker *stroker, qreal rx1, qreal ry1, qreal rx2,
     if (stroker->clipLine(rx1, ry1, rx2, ry2))
         return;
 
-    static const int half = 32;
+    static const int half = 31;
     int x1 = toF26Dot6(rx1) + half;
     int y1 = toF26Dot6(ry1) + half;
     int x2 = toF26Dot6(rx2) + half;
@@ -736,8 +736,8 @@ static void drawLine(QCosmeticStroker *stroker, qreal rx1, qreal ry1, qreal rx2,
 
         capAdjust(caps, y1, y2, x, xinc);
 
-        int y = y1 >> 6;
-        int ys = y2 >> 6;
+        int y = (y1 + 32) >> 6;
+        int ys = (y2 + 32) >> 6;
 
         if (y != ys) {
             x += ( ((((y << 6) + 32 - y1)))  * xinc ) >> 6;
@@ -811,8 +811,8 @@ static void drawLine(QCosmeticStroker *stroker, qreal rx1, qreal ry1, qreal rx2,
 
         capAdjust(caps, x1, x2, y, yinc);
 
-        int x = x1 >> 6;
-        int xs = x2 >> 6;
+        int x = (x1 + 32) >> 6;
+        int xs = (x2 + 32) >> 6;
 
         if (x != xs) {
             y += ( ((((x << 6) + 32 - x1)))  * yinc ) >> 6;
diff --git a/tests/auto/qpainter/tst_qpainter.cpp b/tests/auto/qpainter/tst_qpainter.cpp
index 9e98013..5d5f9c6 100644
--- a/tests/auto/qpainter/tst_qpainter.cpp
+++ b/tests/auto/qpainter/tst_qpainter.cpp
@@ -269,6 +269,8 @@ private slots:
 
     void drawTextOutsideGuiThread();
 
+    void QTBUG26013_squareCapStroke();
+
 private:
     void fillData();
     void setPenColor(QPainter& p);
@@ -4780,6 +4782,34 @@ void tst_QPainter::drawTextOutsideGuiThread()
     QCOMPARE(referenceRendering, t.rendering);
 }
 
+void tst_QPainter::QTBUG26013_squareCapStroke()
+{
+    QImage image(4, 4, QImage::Format_RGB32);
+
+    QPainter p(&image);
+    p.setPen(QPen(Qt::black, 0, Qt::SolidLine, Qt::SquareCap));
+
+    for (int i = 0; i < 3; ++i) {
+        qreal d = i / 3.0;
+
+        image.fill(0xffffffff);
+
+        p.drawLine(QLineF(0, d, 0, d + 2));
+        p.drawLine(QLineF(1, d, 3, d));
+
+        // ensure that a horizontal line and a vertical line with square cap round up (downwards) at the same time
+        QCOMPARE(image.pixel(0, 0), image.pixel(1, 0));
+
+        image.fill(0xffffffff);
+
+        p.drawLine(QLineF(d, 0, d + 2, 0));
+        p.drawLine(QLineF(d, 1, d, 3));
+
+        // ensure that a vertical line and a horizontal line with square cap round up (to the right) at the same time
+        QCOMPARE(image.pixel(0, 0), image.pixel(0, 1));
+    }
+}
+
 QTEST_MAIN(tst_QPainter)
 
 #include "tst_qpainter.moc"