Trouble converting matrix to quaternion and back

Question

I am trying to store my rotation matrix as a quaternion and then when I want to use it for transformation, convert it back. I am using the glm library which provides mat4_cast and quat_cast for these. However, when I execute the following code:

glm::mat4 origTest = glm::lookAt(position, lookAtPt, up);
glm::quat quatTest = glm::quat_cast(origTest);
glm::mat4 mat4Test = glm::mat4_cast(quatTest);

I get different values for origTest and mat4Test. Am I missing something here? position, lookAtPt and up are glm::vec3.

Answer 1

I was able to figure it out. A quaternion stores the rotation of an object in local space. Hence in order to derive the full view matrix that you can get from glm::lookAt(), you would first need to convert the quaternion in local space to a matrix. You would then translate an identity matrix by the desired position and you would perform an SRT multiplication to derive the final view matrix. In this case you need to invert the translation matrix since we are doing an inverse translation on the camera and also you would use an appropriate "lookAt" vector which is in the same direction as (lookAtPt - position)

/* Mat4 to Quat */
glm::vec3 lookAtPt = direction;
glm::mat4 rotMatrix = glm::lookAt(glm::vec3(0), lookAtPt, up);
glm::quat rotation = glm::quat_cast(rotMatrix);

/* Quat to Mat4 */    
glm::mat4 identityMat = glm::mat4(1.0f);
glm::mat4 rotMatrix = glm::mat4_cast(rotation);   //rotation is glm::quat
glm::mat4 transMatrix = glm::translate(identityMat, position); 
glm::mat4 viewMatrix = rotMatrix * glm::inverse(transMatrix);