crash when using .png heightmap and Velodyne 3D LIDARs

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I'm testing Velodyne high definition 3D LIDARs in gazebo:

roslaunch velodyne_gazebo example.launch

It will load a 16 laser model. And I have already prepared a heightmap following this tutorial . After inserting the heightmap(65x65 (pixels), 10mx10m, a larger map also results in crash ) I made, gazebo will not responde. Sim time and real time will stop.

And in rviz, ROS time will stop, but wall time still goes.

Run roswtf, 3 warnings were found:

WARNING The following node subscriptions are unconnected:
 * /gazebo:
   * /gazebo/set_model_state
   * /gazebo/set_link_state

WARNING These nodes have died:
 * spawn_model-4

WARNING No tf messages

Sometimes the real time foctor will be around 0.3 when I use 1 beam LiDAR in a .png heightmap.
I think the reason gazebo will crash is that my laptop‘s hardware performance is not good enough.

Are there any ways to optimize the program to continue the simulation with my laptop? or should I use another computer?? Or maybe there is another reason for the crash.

update: I tried using my PC, there is still the same problem.

Thanks in advance.