save_png() to any buffer, stream or file

[pfeatherstone]

I have a use case where I need to save an image to png in raw memory. I can achieve this using the following:

#include <dlib/media.h>

using namespace dlib;
using namespace dlib::ffmpeg;

template <class image_type, class Buffer>
void save_png (
    const image_type& img,
    Buffer& buf
)
{
    encoder png_writer([&] {
        encoder::args args;
        args.args_codec.codec_name  = "png";
        args.args_image.h           = img.nr();
        args.args_image.w           = img.nc();
        args.args_image.fmt         = pix_traits<rgb_pixel>::fmt;
        args.args_image.framerate   = 1;
        return args;
    }());

    png_writer.push(img, sink(buf));
    png_writer.flush(sink(buf));
}

int main()
{
    matrix<rgb_pixel> img;
    // set img

   std::ofstream out("/tmp/image.png", std::ios::binary);
   std::vector<char> buf;
   std::ostringstream ss;
   
   save_png(img, out); 
   save_png(img, buf); // I can do it this way
   save_png(img, ss);  // Or this way

}

We should maybe also generalize the libpng wrapper in dlib, which currently only supports saving to file, to support arbitrary buffers or stream. It's possible such an implementation would be more optimal than going via libavcodec. However, you can do this now with dlib using the ffmpeg wrappers.

[pfeatherstone]

Aside, if we were to generalise the PNG wrapper in dlib, we could replace png_init_io() with png_set_write_fn() and provide a callback which writes to buffer, stream or file. Maybe we can submit a PR.

[pfeatherstone]

#2873