map_server provides the map_server ROS Node, which offers map data as a ROS Service. It also provides the map_saver command-line utility, which allows dynamically generated maps to be saved to file.
Map format
Maps manipulated by the tools in this package are stored in a pair of files. The YAML file describes the map meta-data, and names the image file. The image file encodes the occupancy data.
YAML format
The YAML format is best explained with a simple, complete example:
image: testmap.png
resolution: 0.1
origin: [0.0, 0.0, 0.0]
occupied_thresh: 0.65
free_thresh: 0.196
negate: 0
Required fields:
image : Path to the image file containing the occupancy data; can be absolute, or relative to the location of the YAML file
resolution : Resolution of the map, meters / pixel
origin : The 2-D pose of the lower-left pixel in the map, as (x, y, yaw), with yaw as counterclockwise rotation (yaw=0 means no rotation). Many parts of the system currently ignore yaw.
occupied_thresh : Pixels with occupancy probability greater than this threshold are considered completely occupied.
free_thresh : Pixels with occupancy probability less than this threshold are considered completely free.
negate : Whether the white/black free/occupied semantics should be reversed (interpretation of thresholds is unaffected)
Optional parameter:
mode : Can have one of three values: trinary, scale, or raw. Trinary is the default. More information on how this changes the value interpretation is in the next section.
Value Interpretation
Given a pixel that has a COLOR value x in the range [0, 256), how should we interpret this value when put into the ROS message? First we convert integer x to a floating point number p depending on the interpretation of the negate flag from the yaml.
If negate is false, p = (255 - x) / 255.0. This means that black (0) now has the highest value (1.0) and white (255) has the lowest (0.0).
If negate is true, p = x / 255.0. This is the non-standard interpretation of images, which is why it is called negate, even though the math indicates that x is not negated. Nomenclature is hard.
Trinary
The standard interpretation is the trinary interpretation, i.e. interpret all values so that the output ends up being one of three values.
If p > occupied_thresh, output the value 100 to indicate the cell is occupied.
If p < free_thresh, output the value 0 to indicate the cell is free.
Otherwise, output -1 a.k.a. 255 (as an unsigned char), to indicate that the cell is unknown.
Scale
This tweaks the above interpretation to allow for more output values than trinary.
As before if p > occupied_thresh, output the value 100. If p < free_thresh, output the value 0.
Otherwise, output 99 * (p - free_thresh) / (occupied_thresh - free_thresh)
This will allow you to output a full gradient of values ranging from [0, 100]. To output -1, simply use the alpha channel of a png, where any transparency will be interpreted as unknown.
Raw
This mode will output x for each pixel, so output values are [0, 255].
Command-line Tools
map_server
map_server is a ROS node that reads a map from disk and offers it via a ROS service.
The current implementation of the map_server converts color values in the map image data into ternary occupancy values: free (0), occupied (100), and unknown (-1). Future versions of this tool may use the values between 0 and 100 to communicate finer gradations of occupancy.
Usage
map_server <map.yaml>
Example
rosrun map_server map_server mymap.yaml
Note that the map data may be retrieved via either latched topic (meaning that it is sent once to each new subscriber), or via service. The service may eventually be phased out.
Published Topics
map_metadata (nav_msgs/MapMetaData)
Receive the map metadata via this latched topic.
map (nav_msgs/OccupancyGrid)
Receive the map via this latched topic.
Services
static_map (nav_msgs/GetMap)
Retrieve the map via this service.
Parameters
~frame_id (string, default: “map”)
The frame to set in the header of the published map.
map_saver
map_saver saves a map to disk, e.g., from a SLAM mapping service.
Usage
rosrun map_server map_saver [-f mapname]
map_saver retrieves map data and writes it out to map.pgm and map.yaml. Use the -f option to provide a different base name for the output files.
Example
rosrun map_server map_saver -f mymap
Subscribed Topics
map (nav_msgs/OccupancyGrid)
Map will be retrieved via this latched topic.
简单地说，map_server 提供了获取地图数据和保存地图数据的方法；获取地图数据通过map_server节点实现，保存数据通过map_saver节点订阅/map话题实现。
当运行出现下面错误时，可以通过rqt_graph看节点之间是否连通，需要注意的是，map_saver订阅的是/map 话题；
rosrun map_server map_saver -f mymap
[ INFO] [1541980723.161416738]: Waiting for the map