Re: Using Cairo and getting data back out.

["Chris Wilson" <christopher g wilson gmail com>]

Here is the method I successfully use:

I have a class called daViewMap which is derived from Gtk::DrawingArea
I use a Cairo::ImageSurface to import a 2-D array into an useful image form for Cairo.

class daViewMap : public Gtk::DrawingArea
{
public:
     Cairo::RefPtr<Cairo::ImageSurface> viewSurface;
     Cairo::RefPtr<Cairo::Context> myCr;
};

daViewMap::daViewMap() :
{
    viewSurface = Cairo::ImageSurface::create(&viewMap[0][0], Cairo::FORMAT_A8, VIEW_MAP_X, VIEW_MAP_Y, VIEW_MAP_X );
    myCr = Cairo::Context::create(viewSurface);
}

I perform all the cairo-based drawing using the context daViewMap->myCr
and then extract the image via a button callback:

void exportImage(std::string filename)
{
        Glib::RefPtr<Gdk::Window> win2 = daViewMap->get_window();
        if (win2)
        {
            Glib::RefPtr<Gdk::Drawable> dra2 = Glib::RefPtr<Gdk::Window>::cast_dynamic(win2);
            Glib::RefPtr<Gdk::Pixbuf> img2 = Gdk::Pixbuf::create(dra2 ,0 ,0 ,daViewMap->get_allocation().get_width(), daViewMap->get_allocation().get_height());
            img2->save(filename,Glib::ustring("png"));
        }
}

Note the use of a Pixbuf to extract the image from the drawing area's space, and then we use that to save it out to a file.

Hope that makes sense...
Chris Wilson

On Mon, Aug 11, 2008 at 5:48 PM, John Hobbs <john velvetcache org> wrote:

Hello, I apologize if this is off-topic as it deals a lot with Cairo,
I feel it is a borderline thing.  Please feel free to send me packing
to somewhere else if I'm wrong on that point :-)

I'm trying to load an image into a DrawingArea, then put lines on it
with Cairo and get that back out.  Is this possible or do I need to go
figure out something with GC and Drawable?

Here's the guts of the procedure so far, sorry it's big but I don't
know how to trim it down and keep it understandable.

---------------------------------------------
class ExampleWindow : public Gtk::Window {

       public:
               ExampleWindow (std::string path) {
                       imagePB = Gdk::Pixbuf::create_from_file(path);
                       drawOnMe.signal_expose_event().connect(sigc::mem_fun(*this,&ExampleWindow::on_da_expose_event));
                       add(drawOnMe);
                       show_all();
               }

       private:
               Gtk::DrawingArea drawOnMe;
               Glib::RefPtr<Gdk::Pixbuf> imagePB;

               bool on_da_expose_event (GdkEventExpose * event) {
                       std::cout << "Expose Event" << std::endl;
                       Glib::RefPtr<Gdk::Window> window = drawOnMe.get_window();
                       Cairo::RefPtr<Cairo::Context> context = window->create_cairo_context();
                       Cairo::RefPtr<Cairo::ImageSurface> image_surface =
Cairo::ImageSurface::create (Cairo::FORMAT_RGB24,
imagePB->get_width(), imagePB->get_height());
                       Cairo::RefPtr<Cairo::Context> image_context =
Cairo::Context::create(image_surface);

                       Gdk::Cairo::set_source_pixbuf (image_context, imagePB, 0.0, 0.0);
                       image_context->paint();

                       context->set_source(image_surface, 0.0, 0.0);
                       context->paint();

                       context->set_line_width(5.0);
                       context->set_source_rgb(0.8, 0.0, 0.0);
                       context->move_to(10,20);
                       context->line_to(100, 200);
                       context->stroke();

                       return false;
               }
};
---------------------------------------------

I tried getting a surface from that context after the stroke() call
and then a write to PNG:

Cairo::RefPtr<Cairo::Surface> sfc = context->get_target();
sfc->write_to_png("please_work.png");

That just crashed on an expose event and I didn't bother to debug. I
have the feeling I've gone wrong in my understanding of Cairo.

So I guess what it boils down to is I'm curious if there is an easier
path by using something in Gtkmm (which I chased my tail for a long
time trying to find something) or if there is a way to get that drawn
data back out of there.

Thanks for any and all help :-)

- John Hobbs

john velvetcache org
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