QMatrix4x4: deprecate operator*(QVector3D, QMatrix4x4)

The operation is sketchy for a number of reasons:

1) Mathematically, it doesn't make sense. The code interprets the
QVector3D as a point, extending it with w=1, and uses it as a
row vector. But similarly, the vector could be intepreted as a
directional vector, with w=0. No semantic is "better" than the
other.

2) QMatrix4x4 is not meant to be post-multiplied. Granted,
one could use a QMatrix4x4 as arbitrary storage for 16 floats,
but QMatrix4x4 builds itself to be always *pre* multiplied
(e.g. translate changes the 4th column, not the 4th row). We
can keep post multiplication for the general case if we do it
against a QVector4D, but I don't feel that we should support it
also for QVector3D.

[ChangeLog][QtGui][QMatrix4x4] The multiplication operator
(operator*) between a QVector3D and a QMatrix4x4 has been deprecated.
User code needs to extend the QVector3D to a QVector4D first
(by specifying the intended w coordinate), and then multiply
the QVector4D by the matrix.

Pick-to: 6.1
Change-Id: I41b64d8ab7eb6126dc4c49fe29cf3f1b7afc7987
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 4 insertions, 3 deletions

diff --git a/src/gui/math3d/qmatrix4x4.h b/src/gui/math3d/qmatrix4x4.h
index 05e882d7c4..7b0c790ab6 100644
--- a/src/gui/math3d/qmatrix4x4.h
+++ b/src/gui/math3d/qmatrix4x4.h
@@ -109,9 +109,9 @@ public:
 #ifndef QT_NO_VECTOR3D
 #if QT_DEPRECATED_SINCE(6, 1)
     friend QVector3D operator*(const QMatrix4x4& matrix, const QVector3D& vector);
-#endif
     friend QVector3D operator*(const QVector3D& vector, const QMatrix4x4& matrix);
 #endif
+#endif
 #ifndef QT_NO_VECTOR4D
     friend QVector4D operator*(const QVector4D& vector, const QMatrix4x4& matrix);
     friend QVector4D operator*(const QMatrix4x4& matrix, const QVector4D& vector);
@@ -720,6 +720,9 @@ inline QMatrix4x4 operator*(const QMatrix4x4& m1, const QMatrix4x4& m2)
 
 #ifndef QT_NO_VECTOR3D
 
+#if QT_DEPRECATED_SINCE(6, 1)
+
+QT_DEPRECATED_VERSION_X_6_1("Extend the QVector3D to a QVector4D before multiplying")
 inline QVector3D operator*(const QVector3D& vector, const QMatrix4x4& matrix)
 {
     float x, y, z, w;
@@ -745,8 +748,6 @@ inline QVector3D operator*(const QVector3D& vector, const QMatrix4x4& matrix)
         return QVector3D(x / w, y / w, z / w);
 }
 
-#if QT_DEPRECATED_SINCE(6, 1)
-
 QT_DEPRECATED_VERSION_X_6_1("Use matrix.map(vector) or matrix.mapVector(vector) instead")
 inline QVector3D operator*(const QMatrix4x4& matrix, const QVector3D& vector)
 {