Fix painting clipping glitches with fractional scaling

In QPainter, clipping can only be done on whole pixels. The various
ways of specifying a clipping rectangle to the QPainter API have been
inconsistent in how fractional rectangles (either specified directly,
or as a result of fractional scaling) are mapped (rounded) to integer
coordinates.

Also, the mappings have not made sure to keep the edge-to-edge
property of clip rects under scaling. This is particularly important
when scaling QRegions with multiple rects, as QRegion is designed on
the assumption that an area can be described as a set of edge-to-edge
rects.

The fix rounds a clip rect identically with a fill rect. (Indeed, a
followup plan would be to merge QRasterPaintEngine's
toNormalizedFillRect() with the rectangle rounding function in this
commit).

Notably, a QRectF clip is now interpreted the same as a QPainterPath
clip describing the same area.

This modifies d9cc1499954829faf9486fb72056e29f1bad58e3

Task-number: QTBUG-100329
Fixes: QTBUG-95957
Task-number: QTBUG-100343
Pick-to: 6.3
Change-Id: Iaae6464b9b17f8bf3adc69007f6ef8d623bf2c80
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

3 files changed, 21 insertions, 6 deletions

diff --git a/src/gui/painting/qmath_p.h b/src/gui/painting/qmath_p.h
index 0191f8381f..6691665f41 100644
--- a/src/gui/painting/qmath_p.h
+++ b/src/gui/painting/qmath_p.h
@@ -53,12 +53,27 @@
 
 #include <qmath.h>
 #include <private/qglobal_p.h>
+#include <qtransform.h>
 
 QT_BEGIN_NAMESPACE
 
 static const qreal Q_PI   = qreal(M_PI);     // pi
 static const qreal Q_MM_PER_INCH = 25.4;
 
+inline QRect qt_mapFillRect(const QRectF &rect, const QTransform &xf)
+{
+    // Only for xf <= scaling or 90 degree rotations
+    Q_ASSERT(xf.type() <= QTransform::TxScale
+             || (xf.type() == QTransform::TxRotate && qFuzzyIsNull(xf.m11()) && qFuzzyIsNull(xf.m22())));
+    // Transform the corners instead of the rect to avoid hitting numerical accuracy limit
+    // when transforming topleft and size separately and adding afterwards,
+    // as that can sometimes be slightly off around the .5 point, leading to wrong rounding
+    QPoint pt1 = xf.map(rect.topLeft()).toPoint();
+    QPoint pt2 = xf.map(rect.bottomRight()).toPoint();
+    // Normalize and adjust for the QRect vs. QRectF bottomright
+    return QRect::span(pt1, pt2).adjusted(0, 0, -1, -1);
+}
+
 QT_END_NAMESPACE
 
 #endif // QMATH_P_H
diff --git a/src/gui/painting/qpaintengine_raster.cpp b/src/gui/painting/qpaintengine_raster.cpp
index b044b485df..6fbedabdbd 100644
--- a/src/gui/painting/qpaintengine_raster.cpp
+++ b/src/gui/painting/qpaintengine_raster.cpp
@@ -49,7 +49,7 @@
 #include <qpainterpath.h>
 #include <qdebug.h>
 #include <qbitmap.h>
-#include <qmath.h>
+#include "qmath_p.h"
 #include <qrandom.h>
 
 //   #include <private/qdatabuffer_p.h>
@@ -1195,7 +1195,7 @@ void QRasterPaintEngine::clip(const QVectorPath &path, Qt::ClipOperation op)
 #endif
             const qreal *points = path.points();
             QRectF r(points[0], points[1], points[4]-points[0], points[5]-points[1]);
-            if (setClipRectInDeviceCoords(s->matrix.mapRect(r).toAlignedRect(), op))
+            if (setClipRectInDeviceCoords(qt_mapFillRect(r, s->matrix), op))
                 return;
         }
     }
@@ -1255,7 +1255,7 @@ void QRasterPaintEngine::clip(const QRect &rect, Qt::ClipOperation op)
         QPaintEngineEx::clip(rect, op);
         return;
 
-    } else if (!setClipRectInDeviceCoords(s->matrix.mapRect(QRectF(rect)).toRect(), op)) {
+    } else if (!setClipRectInDeviceCoords(qt_mapFillRect(rect, s->matrix), op)) {
         QPaintEngineEx::clip(rect, op);
         return;
     }
diff --git a/src/gui/painting/qtransform.cpp b/src/gui/painting/qtransform.cpp
index 9d4f28c895..c318958207 100644
--- a/src/gui/painting/qtransform.cpp
+++ b/src/gui/painting/qtransform.cpp
@@ -45,7 +45,7 @@
 #include "qpainterpath.h"
 #include "qpainterpath_p.h"
 #include "qvariant.h"
-#include <qmath.h>
+#include "qmath_p.h"
 #include <qnumeric.h>
 
 #include <private/qbezier_p.h>
@@ -1486,12 +1486,12 @@ QRegion QTransform::map(const QRegion &r) const
         QRegion res;
         if (m11() < 0 || m22() < 0) {
             for (const QRect &rect : r)
-                res += mapRect(QRectF(rect)).toRect();
+                res += qt_mapFillRect(QRectF(rect), *this);
         } else {
             QVarLengthArray<QRect, 32> rects;
             rects.reserve(r.rectCount());
             for (const QRect &rect : r) {
-                QRect nr = mapRect(QRectF(rect)).toRect();
+                QRect nr = qt_mapFillRect(QRectF(rect), *this);
                 if (!nr.isEmpty())
                     rects.append(nr);
             }