рисунок не принимает пиксель

ошибка в строке

img[i].out[y][x] = pixel;

Если писать без img[i] во всем коде, то out[y][x] должно нормально работать. Но мне нужна группа рисунков, поэтому я в структуру запихнул. Может ещё подскажете способ получше, ну как группой рисункой управлять, потому что массив не получиться, так как y и x тоже будут в массиве. Вот код.

#include <png++/png.hpp>
#include <cstdio>
#include <ctype.h>
#include <unistd.h>
#include <ftw.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
struct img {
    png::image < png::rgba_pixel > *out;
};
static void
convert ( const char *infile, const char *outdir, int hor )
{
    png::image< png::rgba_pixel > image ( infile );
    int height = image.get_height ( );
    int width = image.get_width ( );
    struct img img[hor];
    png::image < png::rgba_pixel > out[hor];
    for ( int i = 0; i < hor; i++ ) {
        img[i].out = new png::image< png::rgba_pixel > ( width / hor, height );
    }
    int slice = width / hor;
    int i = 0;
    for ( int y = 0; y < height; y++ ) {
        for ( int x = 0; x < width; x++ ) {
            if ( x % slice == 0 ) i++;
            png::rgba_pixel pixel = image[y][x];
            img[i].out[y][x] = pixel;
        }
        i = 0;
    }
    char buf[255];
    for ( int i = 0; i < hor; i++ ) {
        sprintf ( buf, "%s_%d.png", infile, i );
        img[i].out->write ( buf );
    }
}
// slice gcup.png outdir 4x0
int main ( int argc, char **argv )
{
    if ( argc < 3 ) return -1;
    if ( access ( argv[2], F_OK ) )  mkdir ( argv[2], 0755 ); 
    int hor = 6;
    convert ( argv[1], argv[2], hor );
}

И вот какая ошибка появляется.

c++ main.cpp `pkg-config --libs libpng --cflags libpng` -o slice
main.cpp: In function ‘void convert(const char*, const char*, int)’:
main.cpp:32:23: error: no match for ‘operator=’ (operand types are ‘png::basic_pixel_buffer<png::basic_rgba_pixel<unsigned char>, std::vector<png::basic_rgba_pixel<unsigned char>, std::allocator<png::basic_rgba_pixel<unsigned char> > >, png::row_traits<std::vector<png::basic_rgba_pixel<unsigned char>, std::allocator<png::basic_rgba_pixel<unsigned char> > > > >::row_type {aka std::vector<png::basic_rgba_pixel<unsigned char>, std::allocator<png::basic_rgba_pixel<unsigned char> > >}’ and ‘png::rgba_pixel {aka png::basic_rgba_pixel<unsigned char>}’)
    img[i].out[y][x] = pixel;
                       ^~~~~
In file included from /usr/include/c++/7/vector:69:0,
                 from /usr/include/png++/palette.hpp:34,
                 from /usr/include/png++/png.hpp:40,
                 from main.cpp:1:
/usr/include/c++/7/bits/vector.tcc:179:5: note: candidate: std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(const std::vector<_Tp, _Alloc>&) [with _Tp = png::basic_rgba_pixel<unsigned char>; _Alloc = std::allocator<png::basic_rgba_pixel<unsigned char> >]
     vector<_Tp, _Alloc>::
     ^~~~~~~~~~~~~~~~~~~
/usr/include/c++/7/bits/vector.tcc:179:5: note:   no known conversion for argument 1 from ‘png::rgba_pixel {aka png::basic_rgba_pixel<unsigned char>}’ to ‘const std::vector<png::basic_rgba_pixel<unsigned char>, std::allocator<png::basic_rgba_pixel<unsigned char> > >&’
In file included from /usr/include/c++/7/vector:64:0,
                 from /usr/include/png++/palette.hpp:34,
                 from /usr/include/png++/png.hpp:40,
                 from main.cpp:1:
/usr/include/c++/7/bits/stl_vector.h:461:7: note: candidate: std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::vector<_Tp, _Alloc>&&) [with _Tp = png::basic_rgba_pixel<unsigned char>; _Alloc = std::allocator<png::basic_rgba_pixel<unsigned char> >]
       operator=(vector&& __x) noexcept(_Alloc_traits::_S_nothrow_move())
       ^~~~~~~~
/usr/include/c++/7/bits/stl_vector.h:461:7: note:   no known conversion for argument 1 from ‘png::rgba_pixel {aka png::basic_rgba_pixel<unsigned char>}’ to ‘std::vector<png::basic_rgba_pixel<unsigned char>, std::allocator<png::basic_rgba_pixel<unsigned char> > >&&’
/usr/include/c++/7/bits/stl_vector.h:482:7: note: candidate: std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(std::initializer_list<_Tp>) [with _Tp = png::basic_rgba_pixel<unsigned char>; _Alloc = std::allocator<png::basic_rgba_pixel<unsigned char> >]
       operator=(initializer_list<value_type> __l)
       ^~~~~~~~
/usr/include/c++/7/bits/stl_vector.h:482:7: note:   no known conversion for argument 1 from ‘png::rgba_pixel {aka png::basic_rgba_pixel<unsigned char>}’ to ‘std::initializer_list<png::basic_rgba_pixel<unsigned char> >’
makefile:3: recipe for target 'all' failed
make: *** [all] Error 1