The ROS2 image transport supports encoding/decoding with the FFMpeg library, for example encoding h264 and h265 or HEVC, using Nvidia or other hardware acceleration when available. This package is a complete rewrite of an older ROS1 ffmpeg_image_transport package.
The publisher plugin of the transport produces ffmpeg image transport messages. These are raw, encoded packets that are then transmitted and decoded by the subscriber plugin of the transport. The transport library contains both the publisher(encoder) and subscriber(decoder) plugin and therefore must be installed on both sides to be useful.
To extract e.g. frames or an mp4 file from a recorded bag, have a look at the ffmpeg_image_transport_tools repository.
Continuous integration is tested under Ubuntu with the following ROS2 distros:
sudo apt-get install ros-${ROS_DISTRO}-ffmpeg-image-transport
Set the following shell variables:
repo=ffmpeg_image_transport
url=https://github.com/ros-misc-utilities/${repo}.git
and follow the instructions here
Make sure to source your workspace's install/setup.bash
afterwards.
If all goes well you should see the transport show up:
ros2 run image_transport list_transports
should give output (among other transport plugins):
"image_transport/ffmpeg"
- Provided by package: ffmpeg_image_transport
- Publisher:
This plugin encodes frames into ffmpeg compressed packets
- Subscriber:
This plugin decodes frames from ffmpeg compressed packets
Remember to install the plugin on both hosts, the one that is encoding and the one that is decoding (viewing).
The plugin has a few parameters that allow for some amount of control.
encoding
: Only ever tested: libx264
, h264_nvenc
, h264
, hevc_nvenc
, h264_vaapi
.
If you have an Nvidia card it most likely supports hevc_nvenc
.
This will dramatically reduce the CPU load compare to libx264
(the default).
You can list all available codecs with ffmpeg -codecs
. In the relevant row,
look for what it says under (encoders)
.preset
: For instance slow
, ll
(low latency) etc.
To find out what presets are available, run e.g.
fmpeg -hide_banner -f lavfi -i nullsrc -c:v libx264 -preset help -f mp4 - 2>&1
profile
: For instance baseline
, main
. See the ffmpeg website.tune
: See the ffmpeg website.gop_size
: The number of frames inbetween keyframes. Default is 15
.
The larger this number the more latency you will have, but also the more efficient
the transmission becomes.bit_rate
: The max bit rate [in bits/s] that the encoding will target. Default is `8242880
.The parameters are under the ffmpeg_image_transport
variable block. So if you launch
your publisher node (camera driver), you can give it a parameter list on the way like so:
parameters=[{'ffmpeg_image_transport.encoding': 'hevc_nvenc',
'ffmpeg_image_transport.profile': 'main',
'ffmpeg_image_transport.preset': 'll',
'ffmpeg_image_transport.gop': 15}]
The subscriber has only one parameter, which is the map between the encoding that was used
to encode the frames, and the decoder to be used for decoding. The mapping is done via parameters.
To tell the subscriber to use the hevc
decoder instead of the default hevc_cuvid
decoder for decoding incoming hevc_nvenc
packets set a parameter like so after you started the viewer:
ros2 param set <name_of_your_viewer_node> ffmpeg_image_transport.map.hevc_nvenc hevc
You also need to refresh the subscription (drop down menu in the viewer) for the parameter to take hold. If anyone ever figures out how to set the parameters when starting the viewer, please report back.
The image_transport
allows you to republish the decoded image locally,
see for instance here.
Here the ROS parameters work as expected to modify the mapping between
encoding and decoder.
The following line shows how to specify the decoder when republishing.
For example to decode incoming hevc_nvenc
packets with the hevc
decoder:
ros2 run image_transport republish ffmpeg in/ffmpeg:=image_raw/ffmpeg raw out:=image_raw/uncompressed --ros-args -p "ffmpeg_image_transport.map.hevc_nvenc:=hevc"
Republishing is generally not necessary so long as publisher and subscriber both properly use an image transport. Some nodes however, notably the rosbag player, do not support a proper transport, rendering republishing necessary.
Suppose you have raw images in a rosbag but want to play them across a network using
the ffmpeg_image_transport
. In this case run a republish node like this
(assuming your rosbag topic is /my_camera/image_raw
):
ros2 run image_transport republish raw in:=/my_camera/image_raw
The republished topic will be under a full transport, meaning you can now view them with e.g. rqt_image_view
under the topic /out/ffmpeg
.
You can record them in ffmpeg
format by e.g ros2 bag record /out/ffmpeg
.
Let's say you have stored images as ffmpeg packets in a rosbag under the topic /my_camera/ffmpeg
. To view them use this line:
ros2 run image_transport republish ffmpeg in/ffmpeg:=/my_camera/ffmpeg raw
This will republish the topic with full image transport support.
The launch
directory contains an example launch file cam.launch.py
that demonstrates
how to set encoding profile and preset for e.g. a usb camera.
Compile and install ffmpeg. Let's say the install directory is
/home/foo/ffmpeg/build
, then for it to be found while building,
run colcon like this:
colcon build --symlink-install --cmake-args --no-warn-unused-cli -DFFMPEG_PKGCONFIG=/home/foo/ffmpeg/build/lib/pkgconfig -DCMAKE_BUILD_TYPE=RelWithDebInfo
This will compile against the right headers, but at runtime it may
still load the system ffmpeg libraries. To avoid that, set
LD_LIBRARY_PATH
at runtime:
export LD_LIBRARY_PATH=/home/foo/ffmpeg/build/lib:${LD_LIBRARY_PATH}
Follow the instructions
here to build a version of
ffmpeg that supports NVMPI. Then follow the section above on how to
actually use that custom ffmpeg library. As always first test on the
CLI that the newly compiled ffmpeg
command now supports
h264_nvmpi
. The transport can now be configured to use
nvmpi like so:
parameters=[{'ffmpeg_image_transport.encoding': 'h264_nvmpi',
'ffmpeg_image_transport.profile': 'main',
'ffmpeg_image_transport.preset': 'll',
'ffmpeg_image_transport.gop': 15}]
This software is issued under the Apache License Version 2.0.