Fix aliased painting with non-uniform scaling

The full stroker does not produce good results for aliased lines
thinner than 1 pixel. Avoid it by making sure that such thin lines
are painted by the cosmetic stroker, even when they have
non-uniform transformation.

Fixes: QTBUG-73866
Change-Id: I7b5f0fa555903246e0c3fd92cd435cc8c0b15a24
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

2 files changed, 80 insertions, 1 deletions

diff --git a/src/gui/painting/qpaintengine_raster.cpp b/src/gui/painting/qpaintengine_raster.cpp
index 90b6d16551..ab22a71134 100644
--- a/src/gui/painting/qpaintengine_raster.cpp
+++ b/src/gui/painting/qpaintengine_raster.cpp
@@ -796,7 +796,7 @@ void QRasterPaintEngine::updatePen(const QPen &pen)
     s->flags.fast_pen = pen_style > Qt::NoPen
             && s->penData.blend
             && ((cosmetic && penWidth <= 1)
-                || (!cosmetic && s->flags.tx_noshear && penWidth * s->txscale <= 1));
+                || (!cosmetic && (s->flags.tx_noshear || !s->flags.antialiased) && penWidth * s->txscale <= 1));
 
     s->flags.non_complex_pen = qpen_capStyle(s->lastPen) <= Qt::SquareCap && s->flags.tx_noshear;
 
diff --git a/tests/auto/other/lancelot/scripts/thinlines.qps b/tests/auto/other/lancelot/scripts/thinlines.qps
new file mode 100644
index 0000000000..dddfff4538
--- /dev/null
+++ b/tests/auto/other/lancelot/scripts/thinlines.qps
@@ -0,0 +1,79 @@
+# Version: 1
+# CheckVsReference: 5%
+
+drawRect 0 0 800 800
+
+path_addRect p 0 0 75 75
+path_addEllipse p 25 25 75 75
+
+translate -500 -500
+
+begin_block drawing
+  save
+    drawLine 0 0 100 100
+
+    translate 0 100
+    drawPath p
+
+    translate 0 110
+    drawRect 0 0 100 100
+
+    translate 0 110
+    drawPolyline [0 0 100 0 50 50]
+
+    drawPoint 40 40
+    drawPoint 41 40
+    drawPoint 42 40
+    drawPoint 43 40
+    drawPoint 44 40
+    drawPoint 45 40
+    drawPoint 46 40
+    drawPoint 47 40
+    drawPoint 48 40
+    drawPoint 49 40
+    drawPoint 50 40
+
+  restore
+end_block
+
+begin_block univsnonuni
+  save
+
+    save
+      scale 0.7 0.7
+      repeat_block drawing
+    restore
+
+    translate 100 0
+    save
+      scale 0.7 0.8
+      repeat_block drawing
+    restore
+
+  restore
+end_block
+
+resetMatrix
+translate 20.5 20.5
+
+begin_block row
+save
+  repeat_block univsnonuni
+
+  translate 240 0
+  save
+    rotate 10
+    repeat_block univsnonuni
+  restore
+
+  translate 220 0
+  save
+    rotate_y 30
+    repeat_block univsnonuni
+  restore
+restore
+end_block
+
+translate 0 320
+setRenderHint AntiAliasing
+repeat_block row