Add an extra overload for QMatrix4x4::toTransform()

Change 100afe8d fixed a bug in QGraphicsRotation related to when
the "distance to plane" projection needed to be performed.  As a
side effect it made the toTransform() API not do the expected
thing when the function is called with no argument.

This change makes the default no-argument version of toTransform()
do the simple "drop row 3 and column 3" orthographic transformation
that normal users of the class expect, and adds a new overload for
the "distance to plane" projection case for the special case.

Reviewed-by: trustme

4 files changed, 24 insertions, 5 deletions

diff --git a/src/gui/graphicsview/qgraphicsitem_p.h b/src/gui/graphicsview/qgraphicsitem_p.h
index 046f5dc..7c3c4f0 100644
--- a/src/gui/graphicsview/qgraphicsitem_p.h
+++ b/src/gui/graphicsview/qgraphicsitem_p.h
@@ -542,7 +542,7 @@ struct QGraphicsItemPrivate::TransformData
             QMatrix4x4 m;
             for (int i = 0; i < graphicsTransforms.size(); ++i)
                 graphicsTransforms.at(i)->applyTo(&m);
-            x *= m.toTransform(0);
+            x *= m.toTransform();
         }
         x.translate(xOrigin, yOrigin);
         x.rotate(rotation);
diff --git a/src/gui/graphicsview/qgraphicstransform.cpp b/src/gui/graphicsview/qgraphicstransform.cpp
index 49d8999..a0b5493 100644
--- a/src/gui/graphicsview/qgraphicstransform.cpp
+++ b/src/gui/graphicsview/qgraphicstransform.cpp
@@ -549,7 +549,7 @@ void QGraphicsRotation::applyTo(QMatrix4x4 *matrix) const
     matrix->translate(d->origin);
     QMatrix4x4 m;
     m.rotate(d->angle, d->axis.x(), d->axis.y(), d->axis.z());
-    *matrix *= m.toTransform();
+    *matrix *= m.toTransform(1024.0f);  // Project back to 2D.
     matrix->translate(-d->origin);
 }
 
diff --git a/src/gui/math3d/qmatrix4x4.cpp b/src/gui/math3d/qmatrix4x4.cpp
index ed1b13d..00e8f15 100644
--- a/src/gui/math3d/qmatrix4x4.cpp
+++ b/src/gui/math3d/qmatrix4x4.cpp
@@ -1430,6 +1430,24 @@ QMatrix QMatrix4x4::toAffine() const
                    m[3][0], m[3][1]);
 }
 
+/*!
+    Returns the conventional Qt 2D transformation matrix that
+    corresponds to this matrix.
+
+    The returned QTransform is formed by simply dropping the
+    third row and third column of the QMatrix4x4.  This is suitable
+    for implementing orthographic projections where the z co-ordinate
+    should be dropped rather than projected.
+
+    \sa toAffine()
+*/
+QTransform QMatrix4x4::toTransform() const
+{
+    return QTransform(m[0][0], m[0][1], m[0][3],
+                      m[1][0], m[1][1], m[1][3],
+                      m[3][0], m[3][1], m[3][3]);
+}
+
 static const qreal inv_dist_to_plane = 1. / 1024.;
 
 /*!
@@ -1437,8 +1455,8 @@ static const qreal inv_dist_to_plane = 1. / 1024.;
     corresponds to this matrix.
 
     If \a distanceToPlane is non-zero, it indicates a projection
-    factor to use to adjust for the z co-ordinate.  The default
-    value of 1024 corresponds to the projection factor used
+    factor to use to adjust for the z co-ordinate.  The value of
+    1024 corresponds to the projection factor used
     by QTransform::rotate() for the x and y axes.
 
     If \a distanceToPlane is zero, then the returned QTransform
diff --git a/src/gui/math3d/qmatrix4x4.h b/src/gui/math3d/qmatrix4x4.h
index b32e00a..42d992e 100644
--- a/src/gui/math3d/qmatrix4x4.h
+++ b/src/gui/math3d/qmatrix4x4.h
@@ -159,7 +159,8 @@ public:
     void toValueArray(qreal *values) const;
 
     QMatrix toAffine() const;
-    QTransform toTransform(qreal distanceToPlane = 1024.0f) const;
+    QTransform toTransform() const;
+    QTransform toTransform(qreal distanceToPlane) const;
 
     QPoint map(const QPoint& point) const;
     QPointF map(const QPointF& point) const;