# Need help with Model View Projection with Quaternions

• 01-07-2013, 08:33 AM
rvc.pt
Need help with Model View Projection with Quaternions
Hi,

I'm new to OpenGL and quaternions, but after doing some tutorials from opengl-tutorial, I'm trying to do the MVP with quats.

With the MVP I get the expected rotated triangle. But with quats I get a different rotation (I would link to an image, but seems I can't).

This is what I have in my C++ loop:
Code :

``` // Send our transformation to the currently bound shader, // in the "MVP" uniform glUniformMatrix4fv(MatrixID, 1, GL_FALSE, &MVP[0][0]);   // Let's try the same Matrix in Quat glm::quat qModel = glm::gtx::quaternion::toQuat(MVP); glm::gtc::quaternion::normalize(qModel); glUniform4f(qModelID, qModel.x, qModel.y, qModel.z, qModel.w);```

This is what I have in the shader:
Code :

```// Values that stay constant for the whole mesh. uniform mat4 MVP;   uniform vec4 qModel;   //rotate vector, taken from kri engine vec3 qrot(vec4 q, vec3 v) { return v + 2.0*cross(q.xyz, cross(q.xyz,v) + q.w*v); }     void main(){   // Output position of the vertex, in clip space : MVP * position //gl_Position = MVP * vec4(vertexPosition_modelspace, 1);   gl_Position = vec4(qrot(qModel, vertexPosition_modelspace),1); }```

Like you can see, for now, I just want to converted the MVP matrix to a Quat and use it in the shader, but I'm messing something up.

Any help would be appreciated.

Many thanks.
• 01-08-2013, 02:55 AM
mbentrup
The MVP transformation is not a rotation, so you can't represent it with a quaternion.