Re: How to store Gdk::Pixbuf pixels into std::string objects

[Krzesimir Nowak <qdlacz gmail com>]

2015-02-23 23:05 GMT+01:00 Glus Xof <gtglus gmail com>:
> 2015-02-23 22:21 GMT+01:00 Markus Kolb <markus kolb+gtkmm tower-net de>:
> > Am 2015-02-23 21:40, schrieb Glus Xof:
> > > Hi guys,
> > >
> > > Before anything, I admit that probably my question is not an specifically
> > > Gtkmm question... but nevertheless I think you may know how Gdk::Pixbuf
> > > is
> > > done...
> > >
> > > Having the following,
> > >
> > > Glib::RefPtr<Gdk::Pixbuf> ref_orig = Gdk::Pixbuf::create_from_file
> > > ("./file.png");
> > >
> > > I'm trying to temporary store (because I need so) the image data into a
> > > std::string object. It seems that get_pixels() could solve the problem,
> > > getting a pointer of type guint8.
> >
> >
> > Seems that gdkmm is not up-to-date here.
> > gdk uses a guchar*. But this shouldn't be a problem.
>
>
> > > So, I wrote as follows,
> > >
> > > std::string image_pixels (reinterpret_cast<const char
> > > *>(ref_orig->get_pixels()), ref_orig->get_rowstride());
> >
> >
> > I think this does not what you expect.
> >
> >
> > https://developer.gnome.org/gdk-pixbuf/stable/gdk-pixbuf-The-GdkPixbuf-Structure.html#gdk-pixbuf-get-rowstride
> >
> > See also the part below the description headline.
> >
> > Maybe you should use gsize Gdk::Pixbuf::get_byte_length()
> > because
> > https://developer.gnome.org/gdk-pixbuf/stable/gdk-pixbuf-The-GdkPixbuf-Structure.html#gdk-pixbuf-get-pixels-with-length
> > is missing.
> >
> > I hope it will help you.
> >
> > But think about the std::string. Big ? ;-)
>
> According to Yann & Markus' comments, I remake my code as follows,
>
> Glib::RefPtr<Gdk::Pixbuf> ref_orig = Gdk::Pixbuf::create_from_file
> ("./file.png");
>
> // Glib::ustring::c_str() or data() allows me to get an std::string
> object...
>
> Glib::ustring image_pixels (
>       reinterpret_cast<const char *>(ref_orig->get_pixels()),
>       ref_orig->get_byte_length());

I think you wanted to use std::string here instead of Glib::ustring.
Glib::ustring expects utf8 strings, not some random binary data.
Especially it does not like embedded zeros. The Glib::ustring
constructor calls utf8_byte_offset
(https://git.gnome.org/browse/glibmm/tree/glib/glibmm/ustring.cc#n60)
returns std::string::npos when it encounters embedded zero. And that
thing is passed to std::string constructor which throws the exception,
because npos is a -1 cast to size_type.

Anyway, I'd recommend rather using std::vector instead.

> Glib::RefPtr<Gdk::Pixbuf> ref_dest =
>       Gdk::Pixbuf::create_from_data (
>          reinterpret_cast<const guint8*>(image_pixels.data()),
> Gdk::COLORSPACE_RGB,
>          ref_orig->get_has_alpha(), ref_orig->get_bits_per_sample(),
>          ref_orig->get_width(), ref_orig->get_height(),
> ref_orig->get_rowstride());
>
> But, at runtime I get a lenght error exception,
>
> terminate called after throwing an instance of 'std::length_error'
>   what():  basic_string::_S_create
>
> Could help ?
>
> Glus
>
>
>
>
>
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>
>
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