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>

src/gui/kernel/qhighdpiscaling_p.h

@@ -219,10 +219,9 @@ inline QRegion scale(const QRegion &region, qreal scaleFactor, QPoint origin = Q
     if (!QHighDpiScaling::isActive())
         return region;
 
-    QRegion scaled;
+    QRegion scaled = region.translated(-origin);
-    for (const QRect &rect : region)
+    scaled = QTransform::fromScale(scaleFactor, scaleFactor).map(scaled);
-        scaled += scale(QRectF(rect), scaleFactor, origin).toRect();
+    return scaled.translated(origin);
-    return scaled;
 }
 
 template <typename T>

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

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;
     }

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);
             }

src/opengl/qopenglpaintengine.cpp

@@ -2526,7 +2526,7 @@ void QOpenGL2PaintEngineEx::clip(const QVectorPath &path, Qt::ClipOperation op)
                 && qFuzzyIsNull(state()->matrix.m11())
                 && qFuzzyIsNull(state()->matrix.m22())))
         {
-            state()->rectangleClip = state()->rectangleClip.intersected(state()->matrix.mapRect(rect).toAlignedRect());
+            state()->rectangleClip &= qt_mapFillRect(rect, state()->matrix);
             d->updateClipScissorTest();
             return;
         }

tests/auto/gui/painting/qpainter/tst_qpainter.cpp

@@ -69,6 +69,9 @@ Q_OBJECT
 public:
     tst_QPainter();
 
+    enum ClipType { ClipRect, ClipRectF, ClipRegionSingle, ClipRegionMulti, ClipPathR, ClipPath };
+    Q_ENUM(ClipType);
+
 private slots:
     void cleanupTestCase();
     void getSetCheck();
@@ -155,6 +158,8 @@ private slots:
 
     void clipBoundingRect();
     void transformedClip();
+    void scaledClipConsistency_data();
+    void scaledClipConsistency();
 
     void setOpacity_data();
     void setOpacity();
@@ -1752,10 +1757,11 @@ void tst_QPainter::setClipRect()
 
 /*
     Verify that the clipping works correctly.
-    The red outline should be covered by the blue rect on top and left,
+    Just like fillRect, cliprect should snap rightwards and downwards in case of .5 coordinates.
-    while it should be clipped on the right and bottom and thus the red outline be visible
+    The red outline should be covered by the blue rect on top,
+    while it should be clipped on the other edges and thus the red outline be visible
 
-    See: QTBUG-83229
+    See: QTBUG-83229, modified by QTBUG-100329
 */
 void tst_QPainter::clipRect()
 {
@@ -1781,7 +1787,7 @@ void tst_QPainter::clipRect()
     p.end();
 
     QCOMPARE(image.pixelColor(clipRect.left() + 1, clipRect.top()), QColor(Qt::blue));
-    QCOMPARE(image.pixelColor(clipRect.left(), clipRect.top() + 1), QColor(Qt::blue));
+    QCOMPARE(image.pixelColor(clipRect.left(), clipRect.top() + 1), QColor(Qt::red));
     QCOMPARE(image.pixelColor(clipRect.left() + 1, clipRect.bottom()), QColor(Qt::red));
     QCOMPARE(image.pixelColor(clipRect.right(), clipRect.top() + 1), QColor(Qt::red));
 }
@@ -4583,6 +4589,96 @@ void tst_QPainter::transformedClip()
     }
 }
 
+void tst_QPainter::scaledClipConsistency_data()
+{
+    QTest::addColumn<ClipType>("clipType");
+
+    QTest::newRow("clipRect") << ClipRect;
+    QTest::newRow("clipRectF") << ClipRectF;
+    QTest::newRow("clipRegionSingle") << ClipRegionSingle;
+    QTest::newRow("clipRegionMulti") << ClipRegionMulti;
+    QTest::newRow("clipPathR") << ClipPathR;
+    QTest::newRow("clipPath") << ClipPath;
+}
+
+void tst_QPainter::scaledClipConsistency()
+{
+    QFETCH(ClipType, clipType);
+
+    const QList<QRect> clipRects = {
+        // Varying odd and even coordinates and width/height
+        QRect(1, 1, 7, 8),
+        QRect(8, 0, 8, 9),
+        QRect(0, 9, 8, 7),
+        QRect(8, 9, 8, 7),
+    };
+    // Assert that these are edge to edge:
+    QPointF center = QRectF(clipRects[0]).bottomRight();
+    Q_ASSERT(QRectF(clipRects[1]).bottomLeft() == center);
+    Q_ASSERT(QRectF(clipRects[2]).topRight() == center);
+    Q_ASSERT(QRectF(clipRects[3]).topLeft() == center);
+
+    QRegion multiRegion;
+    for (const QRect &clipRect : clipRects)
+        multiRegion += clipRect;
+
+    QColor fillColor(Qt::black);
+    fillColor.setAlphaF(0.5);
+
+    for (int i = 100; i <= 300; i++) {
+        qreal dpr = qreal(i) / 100.0;
+        QImage img(QSize(16, 16) * dpr, QImage::Format_RGB32);
+        img.fill(Qt::white);
+        img.setDevicePixelRatio(dpr);
+
+        for (const QRect &clipRect : clipRects) {
+            QPainter p(&img);
+            switch (clipType) {
+            case ClipRect:
+                p.setClipRect(clipRect);
+                break;
+            case ClipRectF:
+                p.setClipRect(QRectF(clipRect));
+                break;
+            case ClipRegionSingle:
+                p.setClipRegion(QRegion(clipRect));
+                break;
+            case ClipRegionMulti:
+                p.setClipRegion(multiRegion);
+                break;
+            case ClipPath:
+                p.rotate(0.001); // Avoid the path being optimized to a rectf
+                Q_FALLTHROUGH();
+            case ClipPathR: {
+                QPainterPath path;
+                path.addRect(clipRect); // Will be recognized and converted back to a rectf
+                p.setClipPath(path);
+                break;
+            }
+            default:
+                Q_ASSERT(false);
+                break;
+            }
+            p.fillRect(p.window(), fillColor);
+            if (clipType == ClipRegionMulti)
+                break; // once is enough, we're not using the clipRect anyway
+        }
+
+        int qtWidth = img.width() / 4;
+        int qtHeight = img.height() / 4;
+        QPoint imgCenter = img.rect().center();
+        const QRgb targetColor = img.pixel(qtWidth, qtHeight);
+
+        // Test that there are no gaps or overlaps where the cliprects meet
+        for (int offset = -2; offset <= 2; offset++) {
+            QCOMPARE(img.pixel(imgCenter.x() + offset, qtHeight), targetColor);
+            QCOMPARE(img.pixel(imgCenter.x() + offset, img.height() - qtHeight), targetColor);
+            QCOMPARE(img.pixel(qtWidth, imgCenter.y() + offset), targetColor);
+            QCOMPARE(img.pixel(img.width() - qtWidth, imgCenter.y() + offset), targetColor);
+        }
+    }
+}
+
 #if defined(Q_OS_MAC)
 // Only Mac supports sub pixel positions in raster engine currently
 void tst_QPainter::drawText_subPixelPositionsInRaster_qtbug5053()

tests/auto/gui/qopengl/tst_qopengl.cpp

@@ -1695,10 +1695,11 @@ void tst_QOpenGL::nullTextureInitializtion()
 
 /*
     Verify that the clipping works correctly.
-    The red outline should be covered by the blue rect on top and left,
+    Just like fillRect, cliprect should snap rightwards and downwards in case of .5 coordinates.
-    while it should be clipped on the right and bottom and thus the red outline be visible
+    The red outline should be covered by the blue rect on top,
+    while it should be clipped on the other edges and thus the red outline be visible
 
-    See: QTBUG-83229
+    See: QTBUG-83229, modified by QTBUG-100329
 */
 void tst_QOpenGL::clipRect()
 {
@@ -1754,7 +1755,7 @@ void tst_QOpenGL::clipRect()
     QCOMPARE(fb.size(), size);
 
     QCOMPARE(fb.pixelColor(clipRect.left() + 1, clipRect.top()), QColor(Qt::blue));
-    QCOMPARE(fb.pixelColor(clipRect.left(), clipRect.top() + 1), QColor(Qt::blue));
+    QCOMPARE(fb.pixelColor(clipRect.left(), clipRect.top() + 1), QColor(Qt::red));
     QCOMPARE(fb.pixelColor(clipRect.left() + 1, clipRect.bottom()), QColor(Qt::red));
 
     // Enable this once QTBUG-85286 is fixed