How to convert cv::Mat to sensor_msgs::Image::Ptr?

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cv::Mat by itself does not contain all of the information that sensor_msgs::Image does, namely the encoding (e.g. bgr8 vs rgb8) and the header (time stamp and tf frame). So you create a cv_bridge::CvImage object to convert:

#include <cv_bridge/cv_bridge.h>
#include <sensor_msgs/image_encodings.h>

cv_bridge::CvImage out_msg;
out_msg.header   = in_msg->header; // Same timestamp and tf frame as input image
out_msg.encoding = sensor_msgs::image_encodings::TYPE_32FC1; // Or whatever
out_msg.image    = sal_float_image; // Your cv::Mat

saliency_img_pub.publish(out_msg.toImageMsg());

It's a little verbose; in the future we could add constructors to CvImage to streamline this. Note that forgetting to set the header fields is a common bug when using the old CvBridge.