Drawing on a widget in gtkmm

abhijangda · **Joined:** Sun Apr 03, 2011 11:12 am **Posts:** 63

I want to draw a line on a gtkmm widget of blue color. That widget has some background color. When I draw over that widget, then the background color and blue color gets mixed to give some other color. How can I draw the line with the blue color. I'm modifying the expose event. This is a custom widget, and m_refGdkWindow represents the GdkWindow of the widget.

Code:

bool gtk_vbox::on_expose_event(GdkEventExpose* event)
{
    //std::cout<<"Widget_expose_event"<<std::endl;
    if(m_refGdkWindow)
    {
        Cairo::RefPtr<Cairo::Context> cr = m_refGdkWindow->create_cairo_context();
        //std::cout<<"X"<<event->area.x<<"Y"<<event->area.y<<std::endl;
        if(event)
        {
            cr->rectangle(event->area.x,event->area.y,event->area.width,event->area.height);
            cr->clip();
            }
        Gdk::Cairo::set_source_color(cr,get_style()->get_bg(Gtk::STATE_NORMAL));
        cr->paint();

        ///Here's the drawing code
        Gdk::Cairo::set_source_color(cr,Gdk::Color("blue"));
        for(int i = 0;i<4;i++)
        {
            if(lines_draw_array[i]&&lines_draw_array[i]->width!=-1)
            {
                cr->set_line_width(lines_draw_array[i]->width);
                cr->move_to(lines_draw_array[i]->x1,lines_draw_array[i]->y1);
                cr->line_to(lines_draw_array[i]->x2,lines_draw_array[i]->y2);            
                cr->stroke();
                }
            }    
         /////////////------------------------------
    
        Gdk::Cairo::set_source_color(cr,Gdk::Color("white"));
        cr->set_line_width(0.5);
        int i=0,j=0;
        int height = get_height();
        int width = get_width();
        for(i=10;i<height;i+=10)
        {
            cr->move_to(0,i);
            cr->line_to(width,i);
            }
         for(j=10;j<width;j+=10)
         {
            cr->move_to(j,0);
            cr->line_to(j,height);
            }       
        cr->stroke();
        cr->set_line_width(1.0);
        Gdk::Cairo::set_source_color(cr,Gdk::Color("black"));
        if(number_of_widgets!=slots)
        {            
            //std::cout<<"NUMBER OF WIDGETS != SLOTS"<<std::endl;
            int height_of_slot = get_height()/slots;
            //std::cout<<"slots"<<slots<<"heightofslot"<<height_of_slot<<std::endl;
            //std::cout<<"spacing"<<spacing<<std::endl;
            for(int i=1;i<slots;i++)
            {
                cr->move_to(0,i*height_of_slot);            
                cr->line_to(get_width(),i*height_of_slot);
                }            
            }
        cr->stroke();
        }
    if(event)
        Gtk::Container::on_expose_event(event);
    return false;
    }

Paul Childs · **Posted:** Sun Nov 04, 2012 9:33 pm

The effect you are getting is called compositing. A tutorial on the different types can be found at http://zetcode.com/tutorials/cairographicstutorial/compositing/.
How come you are using gdk_cairo functions for drawing? You would be better calling the cairo functions directly. You also don't get alpha capability in gdk_cairo which can be set explicitly with cairo_set_source_rgba

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Drawing on a widget in gtkmm

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