Fix camera follow - maintain distance and proper orbital rotation

Problems fixed:
1. Camera now maintains fixed distance to followed body
2. Camera uses proper up vector and cross products for rotation

Technical changes:
- Added camera_offset and was_following_body to RenderState
- Store offset when follow is first enabled
- Update camera position = body_pos + offset each frame
- Use cross products for proper orbital camera rotation:
  - Calculate right vector from up x forward
  - Rotate forward vector around right axis
  - Update camera offset after each movement
- Zoom controls also update offset when following

Result: Camera maintains consistent perspective and distance
while tracking moving celestial bodies.

src/main.cpp

@@ -43,6 +43,8 @@ void run_gui_simulation(SimulationState* sim) {
     render_state.body_list_scroll = 0;     // Initial scroll position
     render_state.body_list_active = -1;    // No active item initially
     render_state.camera_follow_body = false; // Camera not following initially
+    render_state.camera_offset = (Vector3){ 0, 50, 100 }; // Default offset
+    render_state.was_following_body = false;
 
     bool paused = false;
     double speed_multiplier = 1.0;

src/renderer.cpp

@@ -37,46 +37,107 @@ void setup_camera(RenderState* render_state) {
 
 // Update camera with keyboard/mouse controls
 void update_camera(RenderState* render_state, SimulationState* sim) {
-    // If following a body, update target to body position
+    float angle_speed = 0.02f;
+
+    // Handle camera follow state transitions
+    if (render_state->camera_follow_body && !render_state->was_following_body) {
+        // Just started following - store current offset
+        if (render_state->selected_body_index >= 0 && render_state->selected_body_index < sim->body_count) {
+            CelestialBody* body = &sim->bodies[render_state->selected_body_index];
+            Vector3 body_pos = sim_to_render(body->position, render_state->distance_scale);
+            render_state->camera.target = body_pos;
+            render_state->camera_offset = Vector3Subtract(render_state->camera.position, body_pos);
+        }
+    }
+
+    // Update target position when following
     if (render_state->camera_follow_body && render_state->selected_body_index >= 0 &&
         render_state->selected_body_index < sim->body_count) {
         CelestialBody* body = &sim->bodies[render_state->selected_body_index];
         Vector3 body_pos = sim_to_render(body->position, render_state->distance_scale);
         render_state->camera.target = body_pos;
+        render_state->camera.position = Vector3Add(body_pos, render_state->camera_offset);
     }
 
-    // Orbital camera rotation with arrow keys
-    float camera_distance = Vector3Distance(render_state->camera.position, render_state->camera.target);
-    float angle_speed = 0.02f;
+    // Camera rotation with proper up vector
+    Vector3 up = (Vector3){ 0.0f, 1.0f, 0.0f };  // Global up for space
+    Vector3 to_camera = Vector3Subtract(render_state->camera.position, render_state->camera.target);
+    float camera_distance = Vector3Length(to_camera);
 
-    // Rotate around target
+    // Rotate around target using cross products
     if (IsKeyDown(KEY_LEFT)) {
-        Vector3 pos = render_state->camera.position;
-        float angle = angle_speed;
-        float x = pos.x * cosf(angle) - pos.z * sinf(angle);
-        float z = pos.x * sinf(angle) + pos.z * cosf(angle);
-        render_state->camera.position.x = x;
-        render_state->camera.position.z = z;
+        Vector3 right = Vector3Normalize(Vector3CrossProduct(up, to_camera));
+        Vector3 forward = Vector3Normalize(to_camera);
+
+        // Rotate forward vector around right axis
+        float cos_a = cosf(angle_speed);
+        float sin_a = sinf(angle_speed);
+
+        Vector3 new_forward = Vector3Add(
+            Vector3Scale(forward, cos_a),
+            Vector3Scale(Vector3CrossProduct(right, forward), sin_a)
+        );
+
+        render_state->camera.position = Vector3Add(
+            render_state->camera.target,
+            Vector3Scale(new_forward, camera_distance)
+        );
+
+        if (render_state->camera_follow_body) {
+            render_state->camera_offset = Vector3Subtract(
+                render_state->camera.position,
+                render_state->camera.target
+            );
+        }
     }
     if (IsKeyDown(KEY_RIGHT)) {
-        Vector3 pos = render_state->camera.position;
-        float angle = -angle_speed;
-        float x = pos.x * cosf(angle) - pos.z * sinf(angle);
-        float z = pos.x * sinf(angle) + pos.z * cosf(angle);
-        render_state->camera.position.x = x;
-        render_state->camera.position.z = z;
+        Vector3 right = Vector3Normalize(Vector3CrossProduct(up, to_camera));
+        Vector3 forward = Vector3Normalize(to_camera);
+
+        // Rotate forward vector around right axis
+        float cos_a = cosf(-angle_speed);
+        float sin_a = sinf(-angle_speed);
+
+        Vector3 new_forward = Vector3Add(
+            Vector3Scale(forward, cos_a),
+            Vector3Scale(Vector3CrossProduct(right, forward), sin_a)
+        );
+
+        render_state->camera.position = Vector3Add(
+            render_state->camera.target,
+            Vector3Scale(new_forward, camera_distance)
+        );
+
+        if (render_state->camera_follow_body) {
+            render_state->camera_offset = Vector3Subtract(
+                render_state->camera.position,
+                render_state->camera.target
+            );
+        }
     }
 
     // Zoom in/out with up/down keys
     if (IsKeyDown(KEY_UP) && camera_distance > 10.0f) {
-        Vector3 direction = Vector3Subtract(render_state->camera.target, render_state->camera.position);
-        direction = Vector3Normalize(direction);
+        Vector3 direction = Vector3Normalize(Vector3Subtract(render_state->camera.target, render_state->camera.position));
         render_state->camera.position = Vector3Add(render_state->camera.position, Vector3Scale(direction, 2.0f));
+
+        if (render_state->camera_follow_body) {
+            render_state->camera_offset = Vector3Subtract(
+                render_state->camera.position,
+                render_state->camera.target
+            );
+        }
     }
     if (IsKeyDown(KEY_DOWN)) {
-        Vector3 direction = Vector3Subtract(render_state->camera.position, render_state->camera.target);
-        direction = Vector3Normalize(direction);
+        Vector3 direction = Vector3Normalize(Vector3Subtract(render_state->camera.position, render_state->camera.target));
         render_state->camera.position = Vector3Add(render_state->camera.position, Vector3Scale(direction, 2.0f));
+
+        if (render_state->camera_follow_body) {
+            render_state->camera_offset = Vector3Subtract(
+                render_state->camera.position,
+                render_state->camera.target
+            );
+        }
     }
 
     // Toggle info display with I key
@@ -98,6 +159,9 @@ void update_camera(RenderState* render_state, SimulationState* sim) {
             render_state->camera_follow_body = false;
         }
     }
+
+    // Store previous follow state
+    render_state->was_following_body = render_state->camera_follow_body;
 }
 
 // Transform from simulation coordinates (XY plane) to render coordinates (XZ plane)

src/renderer.h

@@ -16,6 +16,8 @@ struct RenderState {
     int body_list_scroll;     // Scroll position for body list
     int body_list_active;     // Active item index in body list
     bool camera_follow_body;  // Whether camera follows selected body
+    Vector3 camera_offset;    // Offset from target when following body
+    bool was_following_body;  // Previous frame follow state
 };
 
 // Renderer initialization and cleanup