Need libpng v1.6.x.

png_from_to_buffer.cpp

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <iomanip>

#include <cstring>

#include <png.h>  // Need to be libpng v1.6.x

using namespace std;

int read_png(string file_path, unsigned char** buffer)
{
    ifstream is(file_path, ifstream::binary);
    if (is)
    {
        is.seekg (0, is.end);
        int length = is.tellg();
        is.seekg (0, is.beg);

        *buffer = new unsigned char [length];

        cout << "Reading " << length << " characters ... ";
        is.read ((char*)(&(*buffer)[0]),length);

        if (is)
            cout << "all characters read successfully." << endl;
        else
            cout << "error: only " << is.gcount() << " could be read" << endl;
        
        is.close();
        return length;
    }
}

void write_png(string file_path, unsigned char* buffer, int length)
{
    ofstream os(file_path, ofstream::binary);

    cout << "Writing " << length << " characters ...";
    os.write((char*)buffer, length);
    if (os)
        cout << "all characters written successfully." << endl;
    else
        cout << "error" << endl;

    os.close();
}

int main()
{
    const string Input_PNG = "pic/c.png";
    const string Output_PNG = "output/output.png";
    
    cout << "Input file name: " << Input_PNG << endl;
    unsigned char* file_buffer = nullptr;
    int length = read_png(Input_PNG, &file_buffer);

    // print_mem(buffer, length);

    cout << "PNG_LIBPNG_VER_STRING: " << PNG_LIBPNG_VER_STRING << endl;
    bool is_png = 0 == png_sig_cmp(file_buffer, 0, 8);
    cout << "Is PNG: " << is_png << endl;

    // READING......

    png_image image; /* The control structure used by libpng */
    /* Initialize the 'png_image' structure. */
    memset(&image, 0, (sizeof image));
    image.version = PNG_IMAGE_VERSION;
    if (png_image_begin_read_from_memory(&image, file_buffer, length) == 0) 
    {
        return -1;
    }

    png_bytep buffer;
    image.format = PNG_FORMAT_BGR;
    size_t input_data_length = PNG_IMAGE_SIZE(image);
    if (input_data_length != 720 * 720 * 3) {
        printf("input_data_length:%zd\n", input_data_length);
        return -1;
    }
    buffer = (png_bytep)malloc(input_data_length);
    memset(buffer, 0, input_data_length);
    if (png_image_finish_read(&image, NULL /*background*/, buffer,
                                0 /*row_stride*/, NULL /*colormap*/) == 0) 
    {
        return -1;
    }

    // MODIFICATION ......
    for (int i = 0; i < input_data_length; i++)
    {
        buffer[i] = ~buffer[i];
    }

    // WRITING......

    png_image wimage;
    memset(&wimage, 0, (sizeof wimage));
    wimage.version = PNG_IMAGE_VERSION;
    wimage.format = PNG_FORMAT_BGR;
    wimage.height = 720;
    wimage.width = 720;
    size_t wlength = 0;

    // Get memory size
    bool wresult = png_image_write_to_memory(&wimage, nullptr, &wlength, 0, buffer, 0, nullptr);
    if (!wresult)
    {
        cout << "Error: " << image.message << endl;
    }

    // Real write to memory
    unsigned char* wbuffer = (unsigned char*)malloc(wlength);
    wresult = png_image_write_to_memory(&wimage, wbuffer, &wlength, 0, buffer, 0, nullptr);

    write_png(Output_PNG, wbuffer, wlength);

    free(wbuffer);
    free(buffer);
    free(file_buffer);

    return 0;
}