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c++openglrotationquaternionsglm-math

OpenGL glm rotate model around a point issue

I have a model and some helper cubes which are located on its axes, three on each axis for transformations, I used them for rotate the model around its local axes.

enter image description here

I want to make those cubes rotate around the model center with its rotation so I translate them to the model center, rotate them by the same angle on the same axis the translate them back. This is the code:

//Rotation around X axis
GLfloat theta=glm::radians(xoffset);
glm::quat Qx(glm::angleAxis(theta, glm::vec3(1.0f, 0.0f, 0.0f)));
glm::mat4 rotX = glm::mat4_cast(Qx);
pickedObject->Transform(rotX);//Multiply the model matrix by the transformation matrix
glm::vec3 op(pickedObject->getMatrix()[3]);//model position
for(TransformationHelper* h:pickedObject->GetTransformationHelpers()){//the small cubes
     glm::mat4 m,it,t;
     glm::vec3 hp(h->getMatrix()[3]);//the cube position
     t=glm::translate(m,op);//m is a unit matrix
     it=glm::translate(m,-op);
     m=t*rotX*it;
     h->Transform(m);
}

The result is unexpected

enter image description here

Update: after updating the translation matrix I got this result:

enter image description here

Solution

  • The translation is in the wrong direction; the correct offset should be hp-op, i.e. the matrix t should restore the cube's position after rotating.

     t=glm::translate(glm::mat(1.f),hp-op);

    Also there is no need to use inverse since it is costly (and numerically less stable):

     it=glm::translate(glm::mat(1.f),op-hp);

    (Note: here translate was called with an explicitly constructed identity matrix. See this post for a similar problem. See here for why this is necessary.)