OpenCV convert CV_8U to CV_64F - c++

Question

I am trying to convert a grayscale image to the type CV64F. From the OpenCv documentation I have understood that a grayscale image is of the type CV_8U. I have also found that imshow plots different types differently, hence I need to divide by 255 before converting. But after converting the image I still get many saturated pixels.
I am using this image, saved as a jpg:
http://www.ele.uri.edu/~hansenj/projects/ele585/lab2/cameraman.gif
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <iostream>
#include <cstring>
#include <cmath>
int main()
{
Mat I, input_image;
string path = "C:/<your_path>/camera_man.jpg";
input_image = imread(path.c_str(), 0); // Read the file as grayscale
imshow("Original", input_image);
// Convert image to CV_64F
input_image *= (double) 1 / 255;
input_image.convertTo(I, CV_64F);
imshow("Converted", I);
}

Answer 1

When you do this:
input_image *= (double) 1 / 255; // (1)
input_image.convertTo(I, CV_64F); // (2)
You are dividing each value in a CV_8UC1 matrix by 255 in (1), so each pixel will be:
new_value = static_cast<uchar>(old_value / 255)
so that new_value can have only values 0 for 0 <= old_value < 255, and 1 for old_value = 255. Then the conversion is applied in (2) on truncated values.
So, you need either to first convert to CV_64FC1 and then divide:
input_image.convertTo(I, CV_64F);
I *= (double)1 / 255;
or apply scaling directly during the conversion:
input_image.convertTo(I, CV_64F, 1.0 / 255.0);