feat: add display_amp parameter for visual displacement amplification

Supports display_amp: [ax, ay, az] in input.txt. On rendering, each atom's
displacement from its frame-0 equilibrium is multiplied by the corresponding
factor. Physics is unchanged; only the rendered positions are scaled.

Useful for visualizing small-amplitude waves that would otherwise be invisible.
Example: display_amp: [1.0, 1.0, 5.0] exaggerates z-direction motion 5x.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

draw.py

@@ -67,6 +67,20 @@ N_FRAMES  = DISP_ALL_X.shape[0]
 NT        = int(disp_data["n_total_frames"])
 N_ATOMS   = int(disp_data["n_total_particles"])
 DT        = float(h.get("DT", 0.001))
+
+# 视觉位移放大：display_amp: [ax, ay, az]，对偏离第0帧的位移乘以倍数
+_damp_raw = h.get("display_amp", "")
+if _damp_raw.strip():
+    import ast as _ast
+    _damp_vals = _ast.literal_eval(_damp_raw.strip())
+    _damp = np.array(_damp_vals, dtype=np.float64)
+    if _damp.shape == (3,) and not np.allclose(_damp, 1.0):
+        _eq_x = DISP_ALL_X[0:1, :]   # 第0帧作为平衡位置参考
+        _eq_y = DISP_ALL_Y[0:1, :]
+        _eq_z = DISP_ALL_Z[0:1, :]
+        DISP_ALL_X = _eq_x + (DISP_ALL_X - _eq_x) * _damp[0]
+        DISP_ALL_Y = _eq_y + (DISP_ALL_Y - _eq_y) * _damp[1]
+        DISP_ALL_Z = _eq_z + (DISP_ALL_Z - _eq_z) * _damp[2]
 NSTEP     = int(h.get("NSTEP", 1))
 DISP_STEP = np.arange(N_FRAMES) * NSTEP
 DISP_T    = DISP_STEP * DT
@@ -119,11 +133,31 @@ box_color_b = float(h.get("box_color_b", 0.85))
 info_margin  = 8
 axis_length  = 10.0
 
+import math as _math_cam
+
+_cx = float(h.get("camera_center_x", 0.0))
+_cy = float(h.get("camera_center_y", 0.0))
+_cz = float(h.get("camera_center_z", 0.0))
+
+# 若 input.txt 指定了摄像机自身坐标，则由坐标反推 distance/elevation/azimuth
+if h.get("camera_pos_x") is not None:
+    _px = float(h["camera_pos_x"])
+    _py = float(h["camera_pos_y"])
+    _pz = float(h["camera_pos_z"])
+    _dx, _dy, _dz = _px - _cx, _py - _cy, _pz - _cz
+    _dist = _math_cam.sqrt(_dx*_dx + _dy*_dy + _dz*_dz) or 1.0
+    _elev = _math_cam.degrees(_math_cam.asin(max(-1.0, min(1.0, _dy / _dist))))
+    _azim = _math_cam.degrees(_math_cam.atan2(_dx, _dz))
+else:
+    _dist = float(h.get("camera_distance", 40.0))
+    _elev = float(h.get("camera_elevation", 0))
+    _azim = float(h.get("camera_azimuth", 0))
+
 initial_camera = {
-    "distance": float(h.get("camera_distance", 40.0)),
-    "elevation": float(h.get("camera_elevation", 0)),
-    "azimuth": float(h.get("camera_azimuth", 0)),
-    "center": (0, 0, 0),
+    "distance":  _dist,
+    "elevation": _elev,
+    "azimuth":   _azim,
+    "center":    (_cx, _cy, _cz),
 }
 
 
@@ -595,7 +629,9 @@ def _load_move_camera_txt():
     return segs if segs else None
 
 # 先试 move_camera.txt 直读，没有则用 display.txt 缓存
-_CAM_MOTION = _load_move_camera_txt()
+# header 中 camera_keyframes 为空字符串表示 move_camera=0（开关关闭），跳过文件加载
+_camera_motion_enabled = bool(h.get("camera_keyframes", ""))
+_CAM_MOTION = _load_move_camera_txt() if _camera_motion_enabled else None
 if not _CAM_MOTION:
     _CAM_MOTION = json.loads(h.get("camera_keyframes", "null")) if h.get("camera_keyframes") else None
 if _CAM_MOTION:
@@ -609,14 +645,14 @@ if _CAM_MOTION:
 
 def _update_motion_camera(f_idx):
     """速度段驱动：每帧累加平移/旋转。
-    
+
     时间交叠时所有段同时生效，按文件中出现的顺序依次作用。
-    矩阵操作不具有对易性，排在前面的段优先作用于相机位置。
+    只有当前帧存在活动段时才覆写相机，否则保留用户键盘/鼠标操作的结果。
     """
     if not _CAM_MOTION:
         return
     global _cam_center, _cam_elev, _cam_azim, _cam_dist
-    # 找当前帧所有活动的段（时间交叠=同时作用），按文件顺序依次应用
+    active = False
     for seg in _CAM_MOTION:
         if seg["start"] <= f_idx < seg["end"]:
             _cam_center[0] += seg["v"][0]
@@ -624,12 +660,13 @@ def _update_motion_camera(f_idx):
             _cam_center[2] += seg["v"][2]
             _cam_elev += seg["r"][0]
             _cam_azim += seg["r"][1]
-            # rz 预留
+            active = True
 
-    view.camera.center    = tuple(_cam_center)
-    view.camera.distance  = _cam_dist
-    view.camera.elevation = _cam_elev
-    view.camera.azimuth   = _cam_azim
+    if active:
+        view.camera.center    = tuple(_cam_center)
+        view.camera.distance  = _cam_dist
+        view.camera.elevation = _cam_elev
+        view.camera.azimuth   = _cam_azim
 
 
 def update(event):