Ros stereo image proc with two USB Webcams

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I tried to follow the tutorial: http://wiki.ros.org/stereo_image_proc I am trying to connect two USB Webcams and do stereo image processing aand looking to obtain the point cloud by this tutorial. Each time I do rostopic list, it does NOT show me the following : /stereo/left/image_raw /stereo/left/camera_info /stereo/right/image_raw /stereo/right/camera_info

I was wondering that whether it can be used with two webcams or not or is it not meant for that and only recognizes the stereo cameras.

It would be great if anyone helps, I have been stuck on this for about few weeks.