How to apply transformations to mesh geometry

Question

On Qt3D, an entity can have these two components:

1. Qt3DRender::QGeometryRenderer component which contains the mesh geometry data like vertex positions and normals.
2. Qt3DCore::QTransform component which contains the rotations, translations and scalings.

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I intend to export the mesh data as STL with all the transformations applied to mesh geometry. I mean, all rotations and translations need to be applied to vertex positions...

When I access the mesh geometry data, like below, the transformations are not applied to geometry.

mesh->geometry()->attributes().at(i)->buffer()->data();

How can I apply the transformations to the geometry?

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The Qt3DCore::QTransform component gives me a 4x4 Matrix, but my vertex positions are 3x1, I don't know how to apply this 4x4 matrix into my 3x1 vertex positions.

Answer 1

From the math of transformations, to transform a vector with a 4x4 transformation matrix, everything has to be in homogeneous coordinates. You do this by adding a fourth component to the vector and setting it to one and then just multiply it with the (already homogeneous) 4x4 matrix. so:

QVector3D oldVec = QVector3D(x,y,z);     //this is your 3x1 vertex position
QVector4D newVec = QVector4D(oldVec.x(), oldVec.y(), oldVec.z(), 1);    //added 1 as fourth component
QVector4D transformedVec = matrix*newVec;    //matrix is your 4x4 transformation matrix
//transformedVec is now what you need but with the fourth component still being the 1, so we just leave it out:
QVector3D transformedVec3D = QVector3D(transformedVec.x(), transformedVec.y(), transformedVec.z());

To read more about the math behind it, you can check out this link.