Image overlay with transparency (sensors_msg/Image) on mapviz

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Hi everyone,

I've written a simple node which reads in a png file with transparency and publishes it to a certain topic. The topic is then subscribed by mapviz to display the image as an overlay. While the image is shown, the background isn't shown as transparent despite setting CvImage's encoding to "bgra8" ( http://docs.ros.org/kinetic/api/cv_br... ).

Is there a reason why this is not working as intended? Perhaps, I am missing something? I tried searching for a solution for hours but to no avail. Any help will be greatly appreciated. Thank you!

The test image I used is from here: https://commons.wikimedia.org/wiki/Fi...

The code of the node is as follows:

#include <ros/ros.h>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <cv_bridge/cv_bridge.h>
#include <sensor_msgs/image_encodings.h>
#include <image_transport/image_transport.h>

int main(int argc, char** argv)
{
  ros::init(argc, argv, "image_publisher");
  ros::NodeHandle nh;

  // cv_bridge::CvImage cv_image()
  // image_transport::ImageTransport it(nh);

  cv_bridge::CvImage cv_image;
  cv_image.image = cv::imread(<png file here>,cv::IMREAD_UNCHANGED);
  cv_image.encoding = "bgra8";
  sensor_msgs::Image ros_image;
  sensor_msgs::CompressedImage compressed_ros_image;
  cv_image.toImageMsg(ros_image);
  cv_image.toCompressedImageMsg(compressed_ros_image, cv_bridge::PNG);

  ROS_INFO_STREAM(cv_image.image);

  ros::Publisher pub = nh.advertise<sensor_msgs::Image>("/test_image", 5, true);
  ros::Publisher compressed_pub = nh.advertise<sensor_msgs::CompressedImage>("/test_image/compressed", 5, true);
  // ros::Publisher pub = nh.advertise<sensor_msgs::Image>("/test_image", 5, true);
  ros::Rate loop_rate(5);

  ros::Duration(1.0).sleep();

  pub.publish(ros_image);
  compressed_pub.publish(compressed_ros_image);

  while (nh.ok())
  {
    // pub.publish(ros_image);
    loop_rate.sleep();
  }
}

Updated:

Here is a snippet of rostopic echo /test_image:

header:
  seq: 0
  stamp:
    secs: 0
    nsecs:         0
  frame_id: ''
height: 983
width: 3050
encoding: "bgra8"
is_bigendian: 0
step: 12200
data: [255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, 255, 255, 255, 0, ...

As you can see, it appears that the received image data has some of the first few pixels' alpha channel value set to 0.