| Note: This is based on the image_transport Tutorials Publishing Images and Subscribing to Images. |
Writing a simple image processor
Description: This code snippet shows how to modify and create a sensor_msgs/Image.Keywords: sensor_msgs/Image image shrinker
Tutorial Level: BEGINNER
Contents
Explanation
I nowhere found a help on using sensor_msgs/Image, so this is my example on manipulating an Image in image_transport.
The image_shrinker receives an Image, scales it down in a most primitive way, and publishes it. It's intended for remote video monitoring using less bandwidth.
Code-Snippet
1 #include <ros/ros.h>
2 #include <image_transport/image_transport.h>
3
4 #define DIVISOR 4
5
6 image_transport::Publisher pub;
7
8 void imageCallback(const sensor_msgs::ImageConstPtr& old_image)
9 {
10 // created shared pointer Image
11 sensor_msgs::Image::Ptr small_image =
12 boost::make_shared<sensor_msgs::Image>();
13
14 // copy image properties
15 small_image->header = old_image->header;
16 small_image->height = old_image->height / DIVISOR;
17 small_image->width = old_image->width / DIVISOR;
18 small_image->encoding = old_image->encoding;
19 small_image->is_bigendian = old_image->is_bigendian;
20 small_image->step = old_image->step / DIVISOR;
21
22 small_image->data.resize(small_image->width * small_image->height);
23
24 // copy every DIVISORth byte
25 // subpixels will be merged if multiple bytes per pixel
26 uint new_index = 0;
27 for(uint row = 0; row < small_image->height; row++) {
28 int row_offset = row*old_image->step*DIVISOR;
29 for(uint col = 0; col < small_image->width; col++) {
30 int old_index = row_offset + col*DIVISOR;
31 small_image->data[new_index++] =
32 old_image->data[old_index];
33 }
34 }
35
36 pub.publish(small_image);
37 }
38
39 int main(int argc, char **argv)
40 {
41 ros::init(argc, argv, "image_shrinker");
42 ros::NodeHandle nh;
43 image_transport::ImageTransport it(nh);
44 image_transport::Subscriber sub =
45 it.subscribe("camera/image_raw", 1, imageCallback);
46 pub = it.advertise("camera/image_small", 1);
47
48 ros::spin();
49 printf("Im out! No error.");
50 }
Execution
Save the code to a package, e.g. learning_image_transport from above mentioned tutorials, and add following to the CMakeLists.txt:
rosbuild_add_executable(image_shrinker src/image_shrinker.cpp)