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## Camera rotation - First Person Camera

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### #1tempvar  Members

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Posted 19 June 2012 - 12:59 PM

Hi

I've just switched from deprecated opengl functions to using shaders and GLM math library and i'm having a few problems setting up my camera rotations (first person camera). I'll show what i've got setup so far.

I'm setting up my ViewMatrix using the glm::lookAt function which takes an eye position, target and up vector

// arbitrary pos and target values
pos = glm::vec3(0.0f, 0.0f, 10.0f);
target = glm::vec3(0.0f, 0.0f, 0.0f);
up = glm::vec3(0.0f, 1.0f, 0.0f);

m_view = glm::lookAt(pos, target, up);


i'm using glm::perspective for my projection and the model matrix is just identity

m_projection = glm::perspective(m_fov, m_aspectRatio, m_near, m_far);

model = glm::mat4(1.0);


I send the MVP matrix to my shader to multiply the vertex position

glm::mat4 MVP = camera->getProjection() * camera->getView() * model;
gl_Position = MVP * vec4(vertexPos, 1.0);


My camera class has standard rotate and translate functions which call glm::rotate and glm::translate respectively

void camera::rotate(float amount, glm::vec3 axis)
{
m_view = glm::rotate(m_view, amount, axis);
}
void camera::translate(glm::vec3 dir)
{
m_view = glm::translate(m_view, dir);
}


and i usually just use the mouse delta position as the amount for rotation

Now normally in my previous opengl applications i'd just setup the yaw and pitch angles and have a sin and cos to change the direction vector using (gluLookAt) but i'd like to be able to do this using GLM and matrices.

So at the moment i have my camera set 10 units away from the origin facing that direction. I can see my geometry fine, it renders perfectly. When i use my rotation function...

camera->rotate(mouseDeltaX, glm::vec3(0, 1, 0));


What i want is for me to look to the right and left (like i would with manipulating the lookAt vector with gluLookAt) but what's happening is
It just rotates the model i'm looking at around the origin, like im just doing a full circle around it. Because i've translated my view matrix, shouldn't i need to translate it to the centre, do the rotation then translate back away for it to be rotating around the origin? Also, i've tried using the rotate function around the x axis to get pitch working, but as soon as i rotate the model about 90 degrees, it starts to roll instead of pitch (gimbal lock?).

and if i've not explained it well, basically i'm trying to get a first person camera working with matrix multiplication and rotating my view matrix is just rotating the model around the origin.

### #2diegzumillo  Members

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Posted 01 July 2012 - 11:27 AM

I never really paid much attention to these opengl camera matrices. You might have more help in an opengl forum.

### #3clb  Members

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Posted 01 July 2012 - 11:56 AM

If I followed your snippets correctly, I think your camera::rotate doesn't look like it produces proper yaw/pitch rotation. I'm not familiar with the GLM api, but I think that rotation function would rotate the camera about the world origin point, and not about the camera point in world space. This however depends on the order glm::rotate concatenates orientations, and I was not immediately able to google the glm specs for it, so try to double-check that. MathGeoLib has a float3x4::RotateAxisAngle, which takes in a reference point that specifies the off-center axis to rotate about. You can see the source code there, and adapt something similar for GLM.
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### #4Martin Perry  Members

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Posted 01 July 2012 - 12:05 PM

I am solving my camera with its own coordinate system in camera space (similar to object space).

My camera has forward, up and right vectors. If I rotate camera, i rotate those 3 vectors around axis by angle.

For example, rotation around Y axis
MyMath::Matrix4x4 rotation = MyMath::Matrix4x4::Rotation(this->up, angle);
this->right = MyMath::Vector3::TransformNormal(this->right, rotation);
this->forward = MyMath::Vector3::TransformNormal(this->forward, rotation);

TransformNormal takes only 3x3 part of 4x4 matrix. No translation and homogenous part is involved.

Than if I build view matrix
this->lookAt = this->cameraPos + this->forward;
this->viewMatrix = MyMath::Matrix4x4::LookAtLH(this->cameraPos, this->lookAt, MyMath::Vector3::UnitY());


Final step is to update camera system from view matrix.
this->right.X = this->viewMatrix.M[0][0];
this->right.Y = this->viewMatrix.M[1][0];
this->right.Z = this->viewMatrix.M[2][0];
this->up.X = this->viewMatrix.M[0][1];
this->up.Y = this->viewMatrix.M[1][1];
this->up.Z = this->viewMatrix.M[2][1];
this->forward.X = this->viewMatrix.M[0][2];
this->forward.Y = this->viewMatrix.M[1][2];
this->forward.Z = this->viewMatrix.M[2][2];


And thats basicly my whole camera. Of course, I have some utils function around it, but idea is to have independent coordinate system and all transformation do on this system.
P.S: Used notification is DirectX (row-base), so for OpenGL matrices should be transposed (col-based)

Edited by Martin Perry, 01 July 2012 - 12:06 PM.

### #5BornToCode  Members

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Posted 11 July 2012 - 04:10 PM

Open has no meaning of row or column major. People needs to stop confusing others with that row/column major nonsense. All Open Gl expects is that the translation values be in the last three elements of the array.

### #6ZBethel  Members

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Posted 21 July 2012 - 05:03 PM

You're intuition about your current code rotating the camera around the origin is correct. You want to use the inverse of the rotation matrix when you apply it to the view, because the view matrix is a glorified inverse world matrix for the camera. Try swapping the order of the matrices when you multiply them in rotate(...). Remember, the inverse of a rotation matrix is its transpose.

Right now you have:

view = view * rotation.

I believe you want:

view = rotation * view.

The reason this works is that flipping the multiplication is equivalent to transposing the rotation matrix before multiplying like you were before. As it turns out, the transpose of a rotation matrix is its inverse.

EDIT: After thinking about it, I don't think this is correct either. I think the issue is that you you're trying to rotate a matrix that has already been translated. You're probably better off doing what Martin Perry suggested.

Edited by ZBethel, 21 July 2012 - 06:11 PM.

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