Fix rendering of SVGs with viewBoxes

The presence of the viewBox attribute in the svg tag implies the
presence of the preserveAspectRatio attribute as well. It defaults to
scaling the view box to the view port while preserving the aspect ratio
and the box is also centered.

This patch does not implement the full preserveAspectRatio attribute,
but it does implement the default of aspect ratio preservation and
centering, as that is what caused the rendering error in the linked
task.

This causes some tests in the SVG Tiny 1.2 test suite to differ. This is
visible using the rendertestsuite helper program (see other change).

In particular it corrects the rendering of struct-svg-203-t.svg to
correctly center the text and avoid scaling it.

For animate-elem-38-t.svg the rendering improved by making the entire
text visible, as in the reference. The same applies to
media-video-216-t.svg. In both rendering the scaling changes slightly.

Done-With: Rainer Keller <rainer.keller@qt.io>
Fixes: QTBUG-70256
Change-Id: I9142c511890765993ee3e316f0383550919fdcd4
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
 

2 files changed, 34 insertions, 9 deletions

diff --git a/src/svg/qsvgtinydocument.cpp b/src/svg/qsvgtinydocument.cpp
index da464cc..77aafb4 100644
--- a/src/svg/qsvgtinydocument.cpp
+++ b/src/svg/qsvgtinydocument.cpp
@@ -339,6 +339,7 @@ void QSvgTinyDocument::setHeight(int len, bool percent)
 void QSvgTinyDocument::setViewBox(const QRectF &rect)
 {
     m_viewBox = rect;
+    m_implicitViewBox = false;
 }
 
 void QSvgTinyDocument::addSvgFont(QSvgFont *font)
@@ -420,14 +421,35 @@ void QSvgTinyDocument::mapSourceToTarget(QPainter *p, const QRectF &targetRect,
         source = viewBox();
 
     if (source != target && !source.isNull()) {
-        QTransform transform;
-        transform.scale(target.width() / source.width(),
-                  target.height() / source.height());
-        QRectF c2 = transform.mapRect(source);
-        p->translate(target.x() - c2.x(),
-                     target.y() - c2.y());
-        p->scale(target.width() / source.width(),
-                 target.height() / source.height());
+        if (m_implicitViewBox) {
+            QTransform transform;
+            transform.scale(target.width() / source.width(),
+                            target.height() / source.height());
+            QRectF c2 = transform.mapRect(source);
+            p->translate(target.x() - c2.x(),
+                         target.y() - c2.y());
+            p->scale(target.width() / source.width(),
+                     target.height() / source.height());
+        } else {
+            // Code path used when a view box is specified and we're not rendering a specific element by id
+            // but the entire document. This attempts to emulate the default values of the <preserveAspectRatio>
+            // tag that's implicitly defined when <viewbox> is used.
+
+            // Apply the view box translation if specified.
+            p->translate(target.x() - source.x(),
+                         target.y() - source.y());
+
+            // Scale the view box into the view port (target) by preserve the aspect ratio.
+            QSizeF viewBoxSize = source.size();
+            viewBoxSize.scale(target.width(), target.height(), Qt::KeepAspectRatio);
+
+            // Center the view box in the view port
+            p->translate((target.width() - viewBoxSize.width()) / 2,
+                         (target.height() - viewBoxSize.height()) / 2);
+
+            p->scale(viewBoxSize.width() / source.width(),
+                     viewBoxSize.height() / source.height());
+        }
     }
 }
 
diff --git a/src/svg/qsvgtinydocument_p.h b/src/svg/qsvgtinydocument_p.h
index aa51751..5f5d06b 100644
--- a/src/svg/qsvgtinydocument_p.h
+++ b/src/svg/qsvgtinydocument_p.h
@@ -125,6 +125,7 @@ private:
     bool   m_widthPercent;
     bool   m_heightPercent;
 
+    mutable bool m_implicitViewBox = true;
     mutable QRectF m_viewBox;
 
     QHash<QString, QSvgRefCounter<QSvgFont> > m_fonts;
@@ -173,8 +174,10 @@ inline bool QSvgTinyDocument::heightPercent() const
 
 inline QRectF QSvgTinyDocument::viewBox() const
 {
-    if (m_viewBox.isNull())
+    if (m_viewBox.isNull()) {
         m_viewBox = transformedBounds();
+        m_implicitViewBox = true;
+    }
 
     return m_viewBox;
 }