LookAt (glm) returns wrong translate z-value - c++

Question

Hello i have an issue with a returning value of the glm lookAt function. When i am executing in debug mode, i get at this point
... Result[3][2] = dot(f, eye); ... of the glm function a wrong value in the translate z-position of the matrix. The value is -2, that shows me that the forward and eye vector are in the opposite position. My eye, center and up vectors are eye(0,0,2), center(0,0,-1) and up(0,1,0). The cam coodinate vectors are: f(0,0,-1), s(1,0,0) and u(0,1,0). And the vantage point the user looks at is (0,0,0). So the right view matrix should be this one:
1 0 0 0
0 1 0 0
0 0 1 0
0 0 0 1
but i get this one:
1 -0 0 -0
-0 1 -0 -0
0 0 1 -2
0 0 0 1
My code is:
struct camera {
vec3 position = vec3(0.0f); // position of the camera
vec3 view_direction = vec3(0.0f); // forward vector (orientation)
vec3 side = vec3(0.0f); // right vector (side)
vec3 up = vec3(0.0f, 1.0f, 0.0f); // up vector
float speed = 0.1;
float yaw = 0.0f; // y-rotation
float cam_yaw_speed = 10.0f; // 10 degrees per second
float pitch = 0.0f; // x-rotation
float roll = 0.0f;
...
// calculate the orientation vector (forward)
vec3 getOrientation(vec3 vantage_point) {
// calc the difference and normalize the resulting vector
vec3 result = vantage_point - position;
result = normalize(result);
return result;
}
// calculate the right (side) vector of the camera, by given orientation(forward) and up vectors
mat4 look_at_point(vec3 vantage_point) {
view_direction = getOrientation(vantage_point);
// calculate the lookat matrix
return lookAt(position, position + view_direction, up);
}
};
I have tryied to figure out how to manage this problem but i still have no idea. Can someone help me?
The main function where i am executing the main_cam.look_at_point(vantage_point) function is showed below:
...
GLfloat points[] = {
0.0f, 0.5f, 0.0f,
0.5f, 0.0f, 0.0f,
-0.5f, 0.0f, 0.0f };
float speed = 1.0f; // move at 1 unit per second
float last_position = 0.0f;
// init camera
main_cam.position = vec3(0.0f, 0.0f, 2.0f); // don't start at zero, or will be too close
main_cam.speed = 1.0f; // 1 unit per second
main_cam.cam_yaw_speed = 10.0f; // 10 degrees per second
vec3 vantage_point = vec3(0.0f, 0.0f, 0.0f);
mat4 T = translate(mat4(1.0), main_cam.position);
//mat4 R = rotate(mat4(), -main_cam.yaw, vec3(0.0, 1.0, 0.0));
mat4 R = main_cam.look_at_point(vantage_point);
mat4 view_matrix = R * T;
// input variables
float near = 0.1f; // clipping plane
float far = 100.0f; // clipping plane
float fov = 67.0f * ONE_DEG_IN_RAD; // convert 67 degrees to radians
float aspect = (float)g_gl_width / (float)g_gl_height; // aspect ratio
mat4 proj_matrix = perspective(fov, aspect, near, far);
use_shader_program(shader_program);
set_uniform_matrix4fv(shader_program, "view", 1, GL_FALSE, &view_matrix[0][0]);
set_uniform_matrix4fv(shader_program, "proj", 1, GL_FALSE, &proj_matrix[0][0]);
...
Testing with the rotate function of glm the triangle is shown right.
Triangle shown with the rotate function of glm

Answer 1

I suspect that the problem is here:
mat4 view_matrix = R * T; // <---
The matrix returned by lookAt already does the translation.
Try manually applying the transformation on the (0,0,0) point that is inside your triangle. T will translate it to (0,0,2), but now it coincides with the camera, so R will send it back into (0,0,0). Now you get a division by zero accident in the projective divide.
So remove the multiplication by T:
mat4 view_matrix = R;
Now (0,0,0) will be mapped to (0,0,-2), which is in the direction camera is looking. (In camera space the center-of-projection is at (0,0,0) and the camera is looking towards the negative Z direction).
EDIT: I want to point out that calculating the view_direction from vantage_point and then feeding position + view_direction back into lookAt is a rather contrived way of achieving your goals. What you do in getOrientation function is what lookAt already does inside. Instead you can get the view_direction from the result of lookAt:
mat4 look_at_point(vec3 vantage_point) {
// calculate the lookat matrix
mat4 M = lookAt(position, vantage_point, up);
view_direction = -vec3(M[2][0], M[2][1], M[2][2]);
return M;
}
However, considering that ultimately you're trying to implement yaw/pitch/roll camera controls, you are better off to not using lookAt at all.