Qt3D Transform{} skew uncompatible?

  • I'm trying to implement the following transformation in Qt3D: fit a textured rectangle in an arbitrary quaternion.

    I've found the calculation of the transformation 4x4 matrix on the internet and it works very well in 2D QtQuick. Then I tried to bring the same matrix to Qt3D and failed. From what I see Qt3D doesn't want to distort the objects same as QtQuick does. However simple affine transformations like translations, rotations and scale do work very well in Qt3D. Skew doesn't.

    I don't believe Qt3D treats transformation matrices in a different way that all other frameworks. I must have missed something. Is it possible Qt3D clears certain 'non-important' matrix values?

    Sources: https://drive.google.com/open?id=0BxC-DMpqMeoKZkkwc1ZQM3FVNHc
    The test application has all controls to dynamically play with the matrices and transformations.

    Any ideas appreciated. Thanks!

    A good guide to 4x4 transformations: https://www.w3.org/TR/css-transforms-1/#mathematical-description

  • Lifetime Qt Champion


    I'd recommend bringing this question to the interest mailing list. You'll find there Qt3D's developers/maintainers. This forum is more user oriented.

  • Finally I've found the problem root cause:
    The Qt3D's Transform { ... } is compatible with affine transformations ONLY.
    It means you cannot load an arbitrary matrix into Transform and expect it to work.
    After receiving a matrix they decompose it to translation, rotations and scale and compose a new matrix basing on these values. Sucks.

  • @sylc Interesting. How did you find out?

  • @Wieland

    Looks like I will need to implement a "RawTransform { matrix: m }" QML class that would accept only a matrix and do no calculations at all.

    How did I find out: Qt3D sources, file qtransform.cpp contains a matrix setter

    void QTransform::setMatrix(const QMatrix4x4 &m)
        if (m != matrix()) {
            d->m_matrix = m;
            d->m_matrixDirty = false;
            QVector3D s;
            QVector3D t;
            QQuaternion r;
            decomposeQMatrix4x4(m, t, r, s);
            d->m_scale = s;
            d->m_rotation = r;
            d->m_translation = t;
            d->m_eulerRotationAngles = d->m_rotation.toEulerAngles();
            emit scale3DChanged(s);
            emit rotationChanged(r);
            emit translationChanged(t);
            const bool wasBlocked = blockNotifications(true);
            emit matrixChanged();
            emit scaleChanged(d->m_scale.x());
            emit rotationXChanged(d->m_eulerRotationAngles.x());
            emit rotationYChanged(d->m_eulerRotationAngles.y());
            emit rotationZChanged(d->m_eulerRotationAngles.z());

  • Hey guys, just found this thread.
    Is this related to what I found here ? https://bugreports.qt.io/browse/QTBUG-58987

    That issue is blocking my project and apprently the Qt3D folks don't seem to be interested in fixing it.
    I definitely need to use nested meshes and be able to freely transform each submesh.

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