为什么视口会抖动？

这是代码：

region FOLLOW THE VIEW

half_view_width = camera_get_view_width(view_camera[0]) / 2;

half_view_height = camera_get_view_height(view_camera[0]) / 2;

camera_set_view_pos(view_camera[0], x - half_view_width, y - half_view_height);

#endregion

房间尺寸 640x480

视口尺寸 320x240