Converting float point binary data into int64

I'm in situation that I have a float64 image stored in a binary format. I would like to read the content of the file and store it in int64, so that I can tonemap it.

The question how would I convert float images into int64 in C++.
Here is my approach of reading the file
The file is here
https://www.mediafire.com/file/ffv6qzvpg05xaja/15_init_image.bin/file

#include <iostream>
#include <stdlib.h>
#include <fstream>
#include <vector>
#include "FreeImage.h"

int main()
{

    std::ifstream stream("c:/output/15_init_image.bin", std::ios::in | std::ios::binary);
    std::vector<uint8_t> contents((std::istreambuf_iterator<char>(stream)), std::istreambuf_iterator<char>());

    int width = 2048; // get the width of the image ;
    int height = 3096; // get the height of the image ;
    int bpp = 32;
    FreeImage_Initialise();
    int pitch = 512;

    FIBITMAP* src = FreeImage_ConvertFromRawBitsEx(true, contents.data(), FIT_BITMAP, width, height, pitch, FI_RGBA_RED_MASK,
        FI_RGBA_GREEN_MASK, FI_RGBA_BLUE_MASK, FALSE);
    FreeImage_Save(FIF_JPEG, src, "viewport.jpeg");

    
    return 0;

}

From the comments:
> > Do you want the numeric value of the float64 stored in an int64, or do you want the bit pattern of the float stored as an int64?
>
> the bit pattern actually

Getting the bit pattern with C++20:

    double d   = 1.0;
    
    uint64_t u = std::bit_cast<uint64_t>(d);
 
    u ^= 0x80'00'00'00'00'00'00'00; // toggle sign bit
    
    d = std::bit_cast<double>(u);

    std::cout << d; // prints -1

Getting the bit pattern without C++20:

double d = 0.1;

std::uint64_t u;

std::memcpy(&u, &d, sizeof d);

Once you have the bit pattern, you may also be interested in this post