[ros-users] Sync problem with stereo_image_proc

[Gary Bradski]

What level do you want synchronization on?  Within framerate (both frames
happen randomly but within 1/30 of a second of each other) or exact frame
synchronization? The first is achievable, the second needs extra hardware.

At most, ROS can get you timestamps to align to, but to get actual
simultaneous frames, you'll need a triggering signal in hardware such as a
wire between 2 cameras (that allow for this). Some 1394 cameras may allow
triggering over the 1394 bus and then you'll only have the problem of
getting both buses to trigger at the same time ... which may be
approximately possible, most of the time ... on average. Depends on the
driver and the 1394 port.

I've seen an automotive 1394 article (not ROS related) where they have about
10 cameras. The cameras run the cameras at very high framerates and then
they do timestamp synchronization which allows fairly synchronous alignment
of multiple with no extra hardware.

Gary

On Mon, May 17, 2010 at 5:23 AM, Sacha Aury <sacha at shadowrobot.com> wrote:

> Hi,
>
> I am trying to make a stereo camera acquisition using ROS with the
> cameradc1394 driver and the stereo_proc package. I've got a separate
> computer to stream the image_raw from my two cameras. The stream works,
> but when I launch stereo_image_proc, it seems that my two cameras are
> not synchronized :
>
> [ WARN] 1274098675.074074000: [stereo_image_proc] Low number of
> synchronized left/right image pairs received.
> Left images received: 3079
> Right images received: 3076
> Synchronized pairs: 0
> Possible issues:
>        * The cameras are not synchronized.
>        * The network is too slow. For each synchronized image pair, at most
> 1
> is getting through.
>
> Here is my launch file :
>
> <!-- startstream.launch  -->
> <launch>
>        <param name="display" type="int" value="0"/>
>        <param name="framerate" type="double" value="15"/>
>        <param name="mode" value="MODE_320x240_YUV422"/>
>
>  <node name="camera_left" pkg="cameradc1394" type="cameradc1394"
> respawn="true">
>        <param name="prefix" type="string" value="/stereo/left/"/>
>        <param name="cameraindex" type="int" value="0"/>
>  </node>
>
>
>  <node name="camera_right" pkg="cameradc1394" type="cameradc1394"
> respawn="true">
>        <param name="prefix" type="string" value="/stereo/right/"/>
>        <param name="cameraindex" type="int" value="1"/>
>  </node>
>
> </launch>
>
> Is there any specific way to make the two cameras synchronize ?
>
> I am following that tutorial :
>
> http://www.ros.org/wiki/stereo_image_proc
>
> Thank you for any help.
>
> Sacha
>
>
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