[ros-users] Digital Camera 1394 in ROS?

[Patrick Mihelich]

On Wed, Mar 17, 2010 at 8:27 PM, Jack O'Quin <jack.oquin at gmail.com> wrote:

> I'm having trouble figuring out how to set the camera matrix (K). The
> CameraInfo.msg says:
>
> # Intrinsic camera matrix for the raw (distorted) images
> # Projects 3D points in the camera coordinate frame to 2D pixel
> # coordinates using the focal lengths (fx, fy) and principal point
> # (cx, cy):
> #     [fx  0 cx]
> # K = [ 0 fy cy]
> #     [ 0  0  1]
>
> float64[9]  K # 3x3 row-major matrix
>
> I don't understand how to obtain this information from the dc1394
> interface.
>

Probably you can't.

If it's not available, then I suppose fx, fy, cx and cy would have to
> be parameters defined for each camera. How does the user determine
> them?


Every camera needs to be calibrated separately. As Blaise mentioned we have
the camera_calibration <http://www.ros.org/wiki/camera_calibration> package
for doing that with a checkerboard target.

Are there reasonable defaults to use when these data are not
> available?


You should leave the calibration matrices as all zeros, which is what the
CameraInfo constructor sets them to. image_proc and stereo_image_proc
understand that to mean that the camera is uncalibrated. In that case they
will only publish on the debayered image_mono and image_color topics; all
the other topics (rectified images, stereo outputs) require a calibrated
camera.

If you have very high-quality components (precisely manufactured camera
sensor, distortion-free lens) you might get reasonable results just setting
the values from the spec sheets, though I'd still recommend doing a
calibration. In that case you would set fx = fy = focal length in meters,
and (cx, cy) = the central pixel coordinates. E.g. for 640x480 resolution,
cx = 319.5 and cy = 239.5. Distortion (D) you would leave set to zeros. R
and P you fill as I described earlier.

It would be useful to display the images in rviz (which
> actually does display, but with an error and with the camera window
> covering the upper left of the rviz window).
>

I think rviz is correct to complain, as its image viewer wants to display
the image in the context of whatever else rviz knows about the environment.
That requires knowing the frame and calibration of the camera. Probably
rviz's behavior and error message could be improved for this case though.

image_view doesn't care about the camera info.

How do the PR2 cameras solve this problem? Are the lenses just known
> for each model number?
>

We calibrate all of them individually. For the WGE100 and Prosilica cameras
we have, we store the calibration on the camera itself in non-volatile
memory, but that won't be possible for every camera. As Blaise described
you'll need to load the calibration from a file, and have set_camera_info
save to a file. We use the YAML format in
camera_calibration_parsers<http://www.ros.org/wiki/camera_calibration_parsers>for
archiving CameraInfo.

Cheers,
Patrick
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