Modify the world given in [http://gazebosim.org/tutorials?tut=camera_save&cat=sensors]

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You can build a movement plugin to attach to your SDF model of the camera. The easier way I think is to change the position of the model in the world. Otherwise, you can set a velocity to your model and move the camera using it. You can start having a look at this Plugin 101 and then have a look at Model plugin.