# Drawn frustum does not match with glm::perspective

I am working on a personal project and I got a 3d viewport with moving camera working nicely. Now I am working on being able to freeze the time and switch to a debug camera and move in the game freely. In order to do so I also wish to see where the original camera is and what the frustum is. I started by creating at the origin the camera shape and compute the proper frustum. The camera frustum is the result of a vertical angle of view of 30 degree, near plane 100, far plane 5000. (no particular reason behind those value).

Those values are the exact same of the flying camera so I would expect if i put the camera at the origin, pointing in the same direction (-z), the frustum should match the screen size.

Unluckily that doesnt happen here is the result I get: If in the frustum point calculation I scale the fov by 2.75~ I get the right size, no idea if that number has a specific meaning. I am quite sure the frustum is correct, I say that because if I do atan(plane-width/2 / near) I get half the fov*window ratio. Same for far plane, which is correct. That lead me to think it might be a problem just in the visualization.

I use glm to compute both look at and perspective:

mat4 Camera3dPivot::get_world_view_matrix() const
{
//just computing and returning the camera
mat4 look = lookAt(m_pos , m_pivot, vec3(0.0,1.0,0.0));
return look;
}

mat4 Camera3dPivot::get_projection_matrix() const
{
//computing perspective

mat4 temp44= perspective(m_fov,((float)m_width/(float)m_height),m_near,m_far);
//flipping the perspective to avoid upside down effect
temp44 *=  -1.0f ;
temp44 *=  -1.0f ;
return temp44;
}


After that the MVP matrix is nothing more that proj * lookM.

view_to_proj_matrix = projection_matrix* model_to_view_matrix;


So far the camera worked really nicely, no glitches or artefact I am aware of, so I am quite confidant in the code both cpu and shader. (shaders are the most basic one you can imagine, i can post them if needed).

This is how I compute the frustum:

Camera3dPivot* cam = (Camera3dPivot*)m_cam;
float angle = cam->get_fov()* (M_PI / 180.0);

float near =cam->get_near_plane();
float hnear = 2.0f*tanf(angle / 2) *near;
float wnear = hnear * cam->get_ratio();

float far = cam->get_far_plane()*0.9;
float hfar = 2.0f*tanf(angle / 2) * far;
float wfar= hfar * cam->get_ratio();

float hnearhalf = hnear / 2.0;
float wnearhalf = wnear / 2.0;

float hfarhalf = hfar / 2.0;
float wfarhalf = wfar / 2.0;


There is only one camera in the scene. The flying one, the drawn camera is just a not transformed drawn of the flying camera.

I went through the code several time checking the data and everything is fine and makes sense, from the angles, to the aspect ratio of the viewport etc.

So I am starting to think I am actually missing something, also because it has to work, since if I replicate the same exact scenario in maya the frustum matches the viewport. Although I might try to see if maya gives me the points of the frustum and compare them with mine.

Any help on the matter is appreciated. PS: this is the tuorial i looked at for the frustum calculation: View Frustum Culling Tutorial PPS: I am also sure i am inputting the right angle, glm::perspective I feed degrees , tanf I feed radians, you can see the conversion in the frustum code.