QSvgGenerator: add support for clip paths

SVG 1.1 allows to specify clipping paths. Before they were silently
discarded, but now we can support them.

The SVG generator code is very simple at its core -- at *any* state
change of the painter, a new <g> tag is emitted with the new state
(brush, pen, transform, ...).

Clipping is slightly more complicated because:

1) it needs its own element (<clipPath>), which needs to be referenced
by a shape/group by using a clip-path attribute (specifying a url).

2) in QPainter clipping happens in the logical coordinates when the clip
was set. Then the coordinates may get transformed again, but the drawn
shapes still have to honor the original clipping. In SVG, if one
specifies both the clip-path and the transform attributes on a shape,
the transformation also affects the clip-path (!). This is the
'clipPathUnits' attribute [1], that however doesn't match QPainter
semantics. As a workaround:
a) store clip paths already transformed (using the transform existing
when the clip path got set)
b) when clipping is active, emit an untransformed group, clip that
group, then open another inner group with the current painter
transformation. This ensures that the clip path is unaffected by any
further modification of the painter's transform.

Add a manual test.

[1] https://www.w3.org/TR/SVG11/masking.html#EstablishingANewClippingPath

Change-Id: I78161091925dc09c86e35ed042e31cece2618b9d
Reviewed-by: Albert Astals Cid <aacid@kde.org>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

3 files changed, 143 insertions, 0 deletions

diff --git a/tests/manual/CMakeLists.txt b/tests/manual/CMakeLists.txt
new file mode 100644
index 0000000..2a050a0
--- /dev/null
+++ b/tests/manual/CMakeLists.txt
@@ -0,0 +1,2 @@
+add_subdirectory(cliptests)
+
diff --git a/tests/manual/cliptests/CMakeLists.txt b/tests/manual/cliptests/CMakeLists.txt
new file mode 100644
index 0000000..0d568d0
--- /dev/null
+++ b/tests/manual/cliptests/CMakeLists.txt
@@ -0,0 +1,9 @@
+qt_internal_add_manual_test(svgcliptest
+    SOURCES
+        svgcliptest.cpp
+    INCLUDE_DIRECTORIES
+        .
+    LIBRARIES
+        Qt::Svg
+)
+
diff --git a/tests/manual/cliptests/svgcliptest.cpp b/tests/manual/cliptests/svgcliptest.cpp
new file mode 100644
index 0000000..b150a18
--- /dev/null
+++ b/tests/manual/cliptests/svgcliptest.cpp
@@ -0,0 +1,132 @@
+// Copyright (C) 2015 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
+
+#include <QGuiApplication>
+
+#include <QFile>
+
+#include <QSvgGenerator>
+#include <QPainter>
+#include <QBrush>
+#include <QPen>
+#include <QPainterPath>
+#include <QRect>
+#include <QSize>
+
+int main(int argc, char **argv)
+{
+    QGuiApplication app(argc, argv);
+
+    const QStringList arguments = app.arguments();
+    if (arguments.size() < 2) {
+        qWarning("Missing file name");
+        return 0;
+    }
+
+    QFile output(arguments[1]);
+    if (!output.open(QIODevice::WriteOnly))
+        qFatal("Cannot open output file name");
+
+    QSvgGenerator generator(QSvgGenerator::SvgVersion::Svg11);
+    generator.setOutputDevice(&output);
+    generator.setSize(QSize(1000, 500));
+    generator.setViewBox(QRect(0, 0, 1000, 500));
+
+    {
+        QPainter painter(&generator);
+        QFont f = painter.font();
+        f.setPointSize(48);
+        painter.setFont(f);
+
+        {
+            painter.save();
+            // clipped rectangle
+            painter.setClipRect(QRect(100, 100, 250, 200));
+
+            painter.setBrush(QColorConstants::Blue);
+            painter.drawEllipse(QRect(0, 100, 400, 200));
+
+            {
+                painter.save();
+
+                // transformed element within clip
+                painter.setBrush(QColorConstants::Green);
+                painter.translate(300, 150);
+                painter.rotate(45);
+                painter.drawEllipse(QPointF(0, 0), 100, 50);
+
+                painter.restore();
+            }
+
+            painter.drawText(200, 200, "A very long clipped text");
+
+            painter.restore();
+        }
+        {
+            // unclipped
+            painter.setBrush(QColorConstants::Red);
+            painter.drawEllipse(0, 0, 200, 150);
+        }
+        {
+            painter.save();
+
+            // transformed clip (by transforming the painter before setting the clip);
+            painter.translate(500, 0);
+            painter.rotate(45);
+
+            painter.setClipRect(QRect(50, 50, 150, 200));
+
+            painter.setBrush(QColorConstants::Green);
+            QPainterPath path;
+            path.addRect(50, 50, 100, 100);
+            path.moveTo(0, 0);
+            path.cubicTo(300, 0,  150, 150,  300, 300);
+            path.cubicTo(0, 300,  150, 150,  0, 0);
+            painter.drawPath(path);
+
+            painter.setBrush(QColorConstants::Blue);
+            painter.drawEllipse(QPointF(125, 125), 100, 50);
+
+            painter.restore();
+        }
+
+        {
+            painter.save();
+
+            // transformed clip
+            painter.translate(700, 50);
+
+            // clip by path
+            QPainterPath path;
+            path.moveTo(0, 0);
+            path.cubicTo(300, 0,  150, 150,  300, 300);
+            path.cubicTo(0, 300,  150, 150,  0, 0);
+
+            painter.setBrush(QColorConstants::Svg::red);
+            painter.drawPath(path);
+
+            painter.setClipPath(path);
+
+            painter.setBrush(QColorConstants::Svg::purple);
+            painter.drawEllipse(QPointF(150, 50), 150, 50);
+
+            // transform and remove clipping
+            painter.translate(0, 100);
+            painter.setClipping(false);
+            painter.setBrush(QColorConstants::Svg::darkblue);
+            painter.drawEllipse(QPointF(150, 50), 150, 50);
+
+            // transform and clip again
+            painter.translate(0, 100);
+            painter.setClipping(true);
+            painter.setBrush(QColorConstants::Svg::green);
+            painter.drawEllipse(QPointF(150, 50), 150, 50);
+
+
+            painter.restore();
+        }
+
+    }
+
+    output.close();
+}