rgbdslam and octomap

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Hi everyone I'm new to ROS and I have the following questions:

how do I get rgbdslam to work with octomap?

I have successfully installed the package and its dependencies to the point that I can launch

roslaunch rgbdslam kinect+rgbdslam.launch

but I don't know how to capture a 3D representation of room. if I press "space" or "enter" I only get one image of the kinect, but what should I do to get the full representation of the room?

also what should I do to get this map saved on octomap?

Also this is the error that I'm receiving:

[FATAL] [1366306714.012014901]: Can't run SiftGPU
[FATAL] [1366306717.289295636]: SiftGPU cannot be used. Detection of keypoints failed

Thanks