# How can I keep the clicked point under the mouse when rotating? (GLM)

I have a spherical mesh of radius 1, centered at (0,0,0) in world coordinates. I want to rotate the sphere so that the clicked point remains under the mouse at all times. However, I cannot find an algorithm that does this. The clicked point always drifts away from the mouse. My current algorithm does the following:

Cast a ray to a sphere of radius 1. Find intersection point in world coordinates (Pa).

After the mouse is moved, cast another ray and find a new intersection point. (Pb) since both those 3D points are on a sphere of radius one, centered at the origin, they should have length = 1 but just in case I normalize them both.

Find the rotation axis by doing Pa x Pb. Normalize the resulting axis

axis = glm::cross(Pa,Pb);
axis = glm::normalize(axis);


Find the angle by doing arccosine on the dot product

float angle = glm::acos(glm::dot(Pa,Pb));


Build a rotation matrix from the axis and angle

mat4 rotation = glm::rotate(glm::degrees(angle),axis); <-- This was the mistake glm::degrees is not needed


Multiply the existing model matrix by that calculated matrix:

modelMatrix = rotation*modelMatrix;


The rotations are correct but the clicked point does not stay under the mouse (I am checking by drawing a black square point on the sphere texture when the drag starts). If I click and drag to the right, the initially clicked point drifts to the left (lags). • Little confused - so the glm::rotate function actually takes radians, not degrees? Not sure how the last paragraph relates to the final answer. – Chaosed0 Jun 10 '15 at 17:53