Fixed scaled point drawing with square cap in raster paint engine.

With a large pen width and a small scale, due to the hacky way we draw
points (stroking a line from (x, y) to (x + tiny_amount, y)), we some
times end up snapping these two points to the same in rasterizeLine().
If we instead apply the SquareCap before we do clipping / snapping we
don't get this problem.

Task-number: QTBUG-13429
Reviewed-by: Trond
(cherry picked from commit 7c673a4cf64ba043bb27f90287517bdcdd7a21db)

2 files changed, 39 insertions, 24 deletions

diff --git a/src/gui/painting/qrasterizer.cpp b/src/gui/painting/qrasterizer.cpp
index aed9291..fae3e94 100644
--- a/src/gui/painting/qrasterizer.cpp
+++ b/src/gui/painting/qrasterizer.cpp
@@ -712,17 +712,21 @@ void QRasterizer::rasterizeLine(const QPointF &a, const QPointF &b, qreal width,
     QPointF pa = a;
     QPointF pb = b;
 
-    QPointF offs = QPointF(qAbs(b.y() - a.y()), qAbs(b.x() - a.x())) * width * 0.5;    
-    if (squareCap)
-        offs += QPointF(offs.y(), offs.x());
+    if (squareCap) {
+        QPointF delta = pb - pa;
+        pa -= (0.5f * width) * delta;
+        pb += (0.5f * width) * delta;
+    }
+
+    QPointF offs = QPointF(qAbs(b.y() - a.y()), qAbs(b.x() - a.x())) * width * 0.5;
     const QRectF clip(d->clipRect.topLeft() - offs, d->clipRect.bottomRight() + QPoint(1, 1) + offs);
 
-    if (!clip.contains(a) || !clip.contains(b)) {
+    if (!clip.contains(pa) || !clip.contains(pb)) {
         qreal t1 = 0;
         qreal t2 = 1;
 
-        const qreal o[2] = { a.x(), a.y() };
-        const qreal d[2] = { b.x() - a.x(), b.y() - a.y() };
+        const qreal o[2] = { pa.x(), pa.y() };
+        const qreal d[2] = { pb.x() - pa.x(), pb.y() - pa.y() };
 
         const qreal low[2] = { clip.left(), clip.top() };
         const qreal high[2] = { clip.right(), clip.bottom() };
@@ -745,8 +749,12 @@ void QRasterizer::rasterizeLine(const QPointF &a, const QPointF &b, qreal width,
             if (t1 >= t2)
                 return;
         }
-        pa = a + (b - a) * t1;
-        pb = a + (b - a) * t2;
+
+        QPointF npa = pa + (pb - pa) * t1;
+        QPointF npb = pa + (pb - pa) * t2;
+
+        pa = npa;
+        pb = npb;
     }
 
     if (!d->antialiased) {
@@ -793,12 +801,7 @@ void QRasterizer::rasterizeLine(const QPointF &a, const QPointF &b, qreal width,
         pa = QPointF(x, y - dy);
         pb = QPointF(x, y + dy);
 
-        if (squareCap)
-            width = 1 / width + 1.0f;
-        else
-            width = 1 / width;
-
-        squareCap = false;
+        width = 1 / width;
     }
 
     if (q16Dot16Compare(pa.x(), pb.x())) {
@@ -808,11 +811,6 @@ void QRasterizer::rasterizeLine(const QPointF &a, const QPointF &b, qreal width,
         const qreal dy = pb.y() - pa.y();
         const qreal halfWidth = 0.5f * width * dy;
 
-        if (squareCap) {
-            pa.ry() -= halfWidth;
-            pb.ry() += halfWidth;
-        }
-
         qreal left = pa.x() - halfWidth;
         qreal right = pa.x() + halfWidth;
 
@@ -893,11 +891,6 @@ void QRasterizer::rasterizeLine(const QPointF &a, const QPointF &b, qreal width,
         delta *= 0.5f * width;
         const QPointF perp(delta.y(), -delta.x());
 
-        if (squareCap) {
-            pa -= delta;
-            pb += delta;
-        }
-
         QPointF top;
         QPointF left;
         QPointF right;
diff --git a/tests/auto/qpainter/tst_qpainter.cpp b/tests/auto/qpainter/tst_qpainter.cpp
index d453b50..729dd5e 100644
--- a/tests/auto/qpainter/tst_qpainter.cpp
+++ b/tests/auto/qpainter/tst_qpainter.cpp
@@ -251,6 +251,8 @@ private slots:
 
     void QTBUG5939_attachPainterPrivate();
 
+    void drawPointScaled();
+
 private:
     void fillData();
     void setPenColor(QPainter& p);
@@ -4458,6 +4460,26 @@ void tst_QPainter::QTBUG5939_attachPainterPrivate()
     QCOMPARE(widget->deviceTransform, proxy->deviceTransform);
 }
 
+void tst_QPainter::drawPointScaled()
+{
+    QImage image(32, 32, QImage::Format_RGB32);
+    image.fill(0xffffffff);
+
+    QPainter p(&image);
+
+    p.scale(0.1, 0.1);
+
+    QPen pen;
+    pen.setWidth(1000);
+    pen.setColor(Qt::red);
+
+    p.setPen(pen);
+    p.drawPoint(0, 0);
+    p.end();
+
+    QCOMPARE(image.pixel(16, 16), 0xffff0000);
+}
+
 QTEST_MAIN(tst_QPainter)
 
 #include "tst_qpainter.moc"