incorrect 16bit PNG compression

[christian-rauch]

When a 16bit PNG image is compressed via toCompressedImageMsg the resulting data is not in 16bit format anymore. This can be easily verified by decoding the raw data back to a cv Image:

// original image
cv::Mat depth_mm = cv::Mat(..., CV_16UC1);
{
std::cout << "orig type: " << depth_mm.type() << std::endl;
double min, max;
cv::minMaxLoc(depth_mm, &min, &max);
std::cout << "min/max: " << min << "/" << max << std::endl;
}

// encode / decode
sensor_msgs::CompressedImage::Ptr msg_depth = cv_bridge::CvImage(hdr, "mono16", depth_mm).toCompressedImageMsg(cv_bridge::PNG);
cv::Mat img = cv::imdecode(msg_depth->data, cv::IMREAD_UNCHANGED);
{
std::cout << "DEC type: " << img.type() << std::endl;
double min, max;
cv::minMaxLoc(img, &min, &max);
std::cout << "DEC min/max: " << min << "/" << max << std::endl;
}

This will get me something like:

orig type: 2
min/max: 0/2308
DEC type: 16
DEC min/max: 0/9

The datatype do not match anymore and the original 16 bit value cannot be represented anymore.

A workaround is to manually compress the image:

sensor_msgs::CompressedImage msg_depth;
cv::imencode(".png", depth_mm, msg_depth.data);
msg_depth.format = "16UC1; png compressed ";