Very rough veresion of turn table camera.

cam.cpp

void Cam::init(const vec3& p, const vec3& t, const vec3& u, float winWidth, float winHeight) {
  win_width = winWidth;
  win_height = winHeight;
  dir = t - p;
  pos = p;
  target = t;
  up = u;
  pm.perspective(60.0f, winWidth / winHeight, 0.1f, 100.0f);

  lm.lookat(pos, target, up);
}

void Cam::update() {

  lm.lookat(target - (dir * zoom), target, up);
  
  mat4 trans;
  trans.translate(pan.x, pan.y, 0.0);
  
  mat4 rot;
  rot.rotateY(yaw);
  rot.rotateX(pitch);

  vm = lm * trans * rot;
}

void Cam::onMouseDown(float x, float y) {

  screen_to_world = pm * vm;
  screen_to_world.invert();

  mouse_down_pos = screenToWorld(x, y);
  is_down = true;
}

void Cam::onMouseMove(float x, float y) {

  if (false == is_down) {
    return;
  }

  vec4 mouse_move = screenToWorld(x, y);
  pan.x = mouse_move.x - mouse_down_pos.x;
  pan.y = mouse_move.y - mouse_down_pos.y;
}

void Cam::onMouseUp() {
  is_down = false;
}

vec4 Cam::screenToWorld(float x, float y) {
  float nx =  2.0 * x / win_width - 1.0;
  float ny = -2.0 * y / win_height + 1.0;
  vec4 world_pos(nx, ny, -1.0f, 1.0f);
  return screen_to_world * world_pos;
}

cam.h

class Cam {
public:
  void init(const vec3& pos, const vec3& target, const vec3& up, float winWidth, float winHeight);
  void update();
  void onMouseDown(float x, float y);
  void onMouseMove(float x, float y);
  void onMouseUp();
  vec4 screenToWorld(float x, float y);
  
public:
  float win_width = 0.0f;
  float win_height = 0.0f;
  float zoom = 1.0f;
  vec3 pos;
  vec3 target;
  vec3 up;
  vec3 dir;
  vec2 pan;
  mat4 pm; /* Projection Matrix. */

  mat4 lm; /* Lookat matrix */
  mat4 vm; /* View matrix. */
  float yaw = 0.0f; /* Around Y-axis. */
  float pitch = 0.0f; /* Around X-axis. */

  mat4 screen_to_world; /* Inverted projection matrix. */
  vec4 mouse_down_pos;
  bool is_down = false;
};