Fixed inconsistent QPainter fill rules for aliased painting.

Qt 5 is the time to get rid of all the old inconsistencies in the raster
paint engine caused by trying to preserve the old X11 based
coordinate system where (0, 0) is in the center of the top-left pixel
instead of the upper left corner of said pixel. However, this was only
adhered for line drawing and path / rect filling, and not for image or
pixmap drawing and not at all when doing antialiased painting. By
defining the antialiased coordinate system as being the right one and
letting the aliased fill rules follow from that we finally end up with
some consistent behavior that doesn't lead to surprises and workarounds
in application code.

It is still possible for applications to get the old behavior by
setting the QPainter::Qt4CompatiblePainting render hint. This should
make porting easier for the few cases where an application relies on the
aliased fill rules we used to have in Qt 4.

Task-number: QTBUG-27500
Change-Id: If86b95e77d838ec83033d64af86632b9a73c74a9
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

1 files changed, 34 insertions, 16 deletions

diff --git a/src/gui/painting/qrasterizer.cpp b/src/gui/painting/qrasterizer.cpp
index fca037b964..e418f2c326 100644
--- a/src/gui/painting/qrasterizer.cpp
+++ b/src/gui/painting/qrasterizer.cpp
@@ -129,7 +129,7 @@ public:
     ~QScanConverter();
 
     void begin(int top, int bottom, int left, int right,
-               Qt::FillRule fillRule, QSpanBuffer *spanBuffer);
+               Qt::FillRule fillRule, bool legacyRounding, QSpanBuffer *spanBuffer);
     void end();
 
     void mergeCurve(const QT_FT_Vector &a, const QT_FT_Vector &b,
@@ -177,6 +177,7 @@ private:
     Q16Dot16 m_rightFP;
 
     int m_fillRuleMask;
+    bool m_legacyRounding;
 
     int m_x;
     int m_y;
@@ -196,6 +197,7 @@ class QRasterizerPrivate
 {
 public:
     bool antialiased;
+    bool legacyRounding;
     ProcessSpans blend;
     void *data;
     QRect clipRect;
@@ -219,7 +221,8 @@ QScanConverter::~QScanConverter()
 }
 
 void QScanConverter::begin(int top, int bottom, int left, int right,
-                           Qt::FillRule fillRule, QSpanBuffer *spanBuffer)
+                           Qt::FillRule fillRule, bool legacyRounding,
+                           QSpanBuffer *spanBuffer)
 {
     m_top = top;
     m_bottom = bottom;
@@ -229,6 +232,7 @@ void QScanConverter::begin(int top, int bottom, int left, int right,
     m_lines.reset();
 
     m_fillRuleMask = fillRule == Qt::WindingFill ? ~0x0 : 0x1;
+    m_legacyRounding = legacyRounding;
     m_spanBuffer = spanBuffer;
 }
 
@@ -595,16 +599,20 @@ void QScanConverter::mergeLine(QT_FT_Vector a, QT_FT_Vector b)
         winding = -1;
     }
 
-    a.x += COORD_OFFSET;
-    a.y += COORD_OFFSET;
-    b.x += COORD_OFFSET;
-    b.y += COORD_OFFSET;
+    if (m_legacyRounding) {
+        a.x += COORD_OFFSET;
+        a.y += COORD_OFFSET;
+        b.x += COORD_OFFSET;
+        b.y += COORD_OFFSET;
+    }
+
+    int rounding = m_legacyRounding ? COORD_ROUNDING : 0;
 
-    int iTop = qMax(m_top, int((a.y + 32 - COORD_ROUNDING) >> 6));
-    int iBottom = qMin(m_bottom, int((b.y - 32 - COORD_ROUNDING) >> 6));
+    int iTop = qMax(m_top, int((a.y + 32 - rounding) >> 6));
+    int iBottom = qMin(m_bottom, int((b.y - 32 - rounding) >> 6));
 
     if (iTop <= iBottom) {
-        Q16Dot16 aFP = Q16Dot16Factor/2 + (a.x << 10) - COORD_ROUNDING;
+        Q16Dot16 aFP = Q16Dot16Factor/2 + (a.x << 10) - rounding;
 
         if (b.x == a.x) {
             Line line = { qBound(m_leftFP, aFP, m_rightFP), 0, iTop, iBottom, winding };
@@ -635,6 +643,7 @@ void QScanConverter::mergeLine(QT_FT_Vector a, QT_FT_Vector b)
 QRasterizer::QRasterizer()
     : d(new QRasterizerPrivate)
 {
+    d->legacyRounding = false;
 }
 
 QRasterizer::~QRasterizer()
@@ -658,6 +667,11 @@ void QRasterizer::setClipRect(const QRect &clipRect)
     d->clipRect = clipRect;
 }
 
+void QRasterizer::setLegacyRoundingEnabled(bool legacyRoundingEnabled)
+{
+    d->legacyRounding = legacyRoundingEnabled;
+}
+
 static Q16Dot16 intersectPixelFP(int x, Q16Dot16 top, Q16Dot16 bottom, Q16Dot16 leftIntersectX, Q16Dot16 rightIntersectX, Q16Dot16 slope, Q16Dot16 invSlope)
 {
     Q16Dot16 leftX = IntToQ16Dot16(x);
@@ -775,7 +789,7 @@ void QRasterizer::rasterizeLine(const QPointF &a, const QPointF &b, qreal width,
         pb = npb;
     }
 
-    if (!d->antialiased) {
+    if (!d->antialiased && d->legacyRounding) {
         pa.rx() += (COORD_OFFSET - COORD_ROUNDING)/64.;
         pa.ry() += (COORD_OFFSET - COORD_ROUNDING)/64.;
         pb.rx() += (COORD_OFFSET - COORD_ROUNDING)/64.;
@@ -1174,13 +1188,15 @@ void QRasterizer::rasterize(const QT_FT_Outline *outline, Qt::FillRule fillRule)
         max_y = qMax(p.y, max_y);
     }
 
-    int iTopBound = qMax(d->clipRect.top(), int((min_y + 32 + COORD_OFFSET - COORD_ROUNDING) >> 6));
-    int iBottomBound = qMin(d->clipRect.bottom(), int((max_y - 32 + COORD_OFFSET - COORD_ROUNDING) >> 6));
+    int rounding = d->legacyRounding ? COORD_OFFSET - COORD_ROUNDING : 0;
+
+    int iTopBound = qMax(d->clipRect.top(), int((min_y + 32 + rounding) >> 6));
+    int iBottomBound = qMin(d->clipRect.bottom(), int((max_y - 32 + rounding) >> 6));
 
     if (iTopBound > iBottomBound)
         return;
 
-    d->scanConverter.begin(iTopBound, iBottomBound, d->clipRect.left(), d->clipRect.right(), fillRule, &buffer);
+    d->scanConverter.begin(iTopBound, iBottomBound, d->clipRect.left(), d->clipRect.right(), fillRule, d->legacyRounding, &buffer);
 
     int first = 0;
     for (int i = 0; i < outline->n_contours; ++i) {
@@ -1210,13 +1226,15 @@ void QRasterizer::rasterize(const QPainterPath &path, Qt::FillRule fillRule)
 
     QRectF bounds = path.controlPointRect();
 
-    int iTopBound = qMax(d->clipRect.top(), int(bounds.top() + 0.5 + (COORD_OFFSET - COORD_ROUNDING)/64.));
-    int iBottomBound = qMin(d->clipRect.bottom(), int(bounds.bottom() - 0.5 + (COORD_OFFSET - COORD_ROUNDING)/64.));
+    double rounding = d->legacyRounding ? (COORD_OFFSET - COORD_ROUNDING) / 64. : 0.0;
+
+    int iTopBound = qMax(d->clipRect.top(), int(bounds.top() + 0.5 + rounding));
+    int iBottomBound = qMin(d->clipRect.bottom(), int(bounds.bottom() - 0.5 + rounding));
 
     if (iTopBound > iBottomBound)
         return;
 
-    d->scanConverter.begin(iTopBound, iBottomBound, d->clipRect.left(), d->clipRect.right(), fillRule, &buffer);
+    d->scanConverter.begin(iTopBound, iBottomBound, d->clipRect.left(), d->clipRect.right(), fillRule, d->legacyRounding, &buffer);
 
     int subpathStart = 0;
     QT_FT_Vector last = { 0, 0 };