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>