Graphics View Optimization: Use a simple style option by default.

QStyleOptionGraphicsItem extends QStyleOption with three values:
1) matrix, 2) levelOfDetail, 3) exposedRect, and they all
involve expensive QTranform operations when calculated. We
pass style option(s) to drawItems() and paint(), but the
extended values are usually not in use. We can therefore gain
quite some nice speedup by making them opt-in with the
QGraphicsItem::ItemUsesExtendedStyleOption flag.

Additionally, QStyleOptionGraphicsItem::levelOfDetail has been
obsoleted, and a new function QStyleOptionGraphicsItem::
levelOfDetailFromTransform(const QTransform &) has been added.

Me and Andreas don't consider this change to be too controversial
even though it changes the behavior.

Auto tests still pass.

Reviewed-by: Andreas

1 files changed, 32 insertions, 12 deletions

diff --git a/src/gui/styles/qstyleoption.cpp b/src/gui/styles/qstyleoption.cpp
index ce053ae..5b1bc61 100644
--- a/src/gui/styles/qstyleoption.cpp
+++ b/src/gui/styles/qstyleoption.cpp
@@ -48,6 +48,7 @@
 #ifndef QT_NO_DEBUG
 #include <qdebug.h>
 #endif
+#include <QtCore/qmath.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -4998,6 +4999,34 @@ QStyleOptionGraphicsItem::QStyleOptionGraphicsItem(int version)
 }
 
 /*!
+    \since 4.6
+
+    Returns the level of detail from the \a worldTransform.
+
+    Its value represents the maximum value of the height and
+    width of a unity rectangle, mapped using the \a worldTransform
+    of the painter used to draw the item. By default, if no
+    transformations are applied, its value is 1. If zoomed out 1:2, the level
+    of detail will be 0.5, and if zoomed in 2:1, its value is 2.
+
+    For more advanced level-of-detail metrics, use
+    QStyleOptionGraphicsItem::matrix directly.
+
+    \sa QStyleOptionGraphicsItem::matrix
+*/
+qreal QStyleOptionGraphicsItem::levelOfDetailFromTransform(const QTransform &worldTransform)
+{
+    if (worldTransform.type() <= QTransform::TxTranslate)
+        return 1; // Translation only? The LOD is 1.
+
+    // Two unit vectors.
+    QLineF v1(0, 0, 1, 0);
+    QLineF v2(0, 0, 0, 1);
+    // LOD is the transformed area of a 1x1 rectangle.
+    return qSqrt(worldTransform.map(v1).length() * worldTransform.map(v2).length());
+}
+
+/*!
     \fn QStyleOptionGraphicsItem::QStyleOptionGraphicsItem(const QStyleOptionGraphicsItem &other)
 
     Constructs a copy of \a other.
@@ -5029,19 +5058,10 @@ QStyleOptionGraphicsItem::QStyleOptionGraphicsItem(int version)
 
 /*!
     \variable QStyleOptionGraphicsItem::levelOfDetail
-    \brief a simple metric for determining an item's level of detail
-
-    This simple metric provides an easy way to determine the level of detail
-    for an item. Its value represents the maximum value of the height and
-    width of a unity rectangle, mapped using the complete transformation
-    matrix of the painter used to draw the item. By default, if no
-    transformations are applied, its value is 1. If zoomed out 1:2, the level
-    of detail will be 0.5, and if zoomed in 2:1, its value is 2.
-
-    For more advanced level-of-detail metrics, use
-    QStyleOptionGraphicsItem::matrix directly.
+    \obsolete
 
-    \sa QStyleOptionGraphicsItem::matrix
+    Use QStyleOptionGraphicsItem::levelOfDetailFromTransform
+    together with QPainter::worldTransform() instead.
 */
 
 /*!