Orbiting an object around another object, but it doesn't move - c++

Question

I'm trying to make a visual "simulation" of the solar system using OpenGL and am using this function to orbit a planet around the sun (in a circular orbit).
glm::vec3 application::orbit(glm::vec3 thisPlanet, glm::vec3 otherPlanet, float rotSpeed, const time &dt)
{
float radius = glm::distance(thisPlanet, otherPlanet);
float angle = acosf((thisPlanet.x - otherPlanet.x) / radius) - atanf(1.0f) * 4.0f;
angle += dt.as_seconds() * rotSpeed;
if (angle > 2 * atanf(1.0f) * 4.0f)
angle -= 2 * atanf(1.0f) * 4.0f;
float x = otherPlanet.x + cosf(angle) * radius;
float z = otherPlanet.z + sinf(angle) * radius;
return glm::vec3(x, thisPlanet.y, z);
}
The function is called every frame like this:
void application::tick(const time &dt)
{
if (m_keyboard.key_released(GLFW_KEY_ESCAPE)) {
m_running = false;
}
m_controller.update(m_keyboard, m_mouse, dt);
m_cube_rotation += dt.as_seconds();
m_mercury_position = orbit(m_mercury_position, m_sun_position, 2.0f, dt);
// glm::mat4 world = glm::translate(glm::mat4(1.0f), m_cube_position)
// * glm::rotate(glm::mat4(1.0f), m_cube_rotation, glm::normalize(glm::vec3(1.0f, 1.0f, -1.0f)));
glm::mat4 sun = glm::translate(glm::scale(glm::mat4(1.0f), glm::vec3(2.0f, 2.0f, 2.0f)), m_sun_position)
* glm::rotate(glm::mat4(1.0f), m_cube_rotation, glm::normalize(glm::vec3(1.0f, 1.0f, -1.0f)));
glm::mat4 mercury = glm::translate(glm::scale(glm::mat4(1.0f), glm::vec3(1.0f, 1.0f, 1.0f)), m_mercury_position)
* glm::rotate(glm::mat4(1.0f), m_cube_rotation, glm::normalize(glm::vec3(1.0f, 1.0f, -1.0f)));
//m_crate.set_transform(world);
m_sun.set_transform(sun);
m_mercury.set_transform(mercury);
const int frames_per_second = int(1.0f / dt.as_seconds());
const int frame_timing_ms = int(dt.as_milliseconds());
m_overlay.pre_frame(m_width, m_height);
m_overlay.push_line("FPS: %d (%dms)", frames_per_second, frame_timing_ms);
}
Why doesn't the planet move?

Answer 1

This is how I would construct the model matrix:
mat4 model_mat;
const vec3 n_forward = dir.x;
const vec3 n_up = vec3(0, 1, 0);
const vec3 n_left = dir.z;
// construct a basis and a translation
model_mat[0] = normalize(vec4(n_left, 0.0f));
model_mat[1] = normalize(vec4(n_forward, 0.0f));
model_mat[2] = normalize(vec4(n_up, 0.0f));
model_mat[3] = vec4(p, 1.0f);
The position (e.g. p) is given by the parametric circle equation p = r cos(t) + r sin(t), where t is the elapsed time and r is the circular orbit radius. Also see: https://www.mathopenref.com/coordparamcircle.html
Does this help at all?