Re: Gtkmm draw line with mouse events

[Gurjot Singh <bhattigurjot gmail com>]

On 14 July 2014 23:19, Chris Vine <chris cvine freeserve co uk> wrote:
> > Suppose I have two functions that are called to draw point and line,
> > something like this:
> > http://pastebin.com/Ancx87EF
>
> This example looks fine.
>
> > But if this is it, it'd re-draws the whole drawing area, so I'd be
> > unable to retain any previously drawn entity.
>
> I don't understand what you mean here.

I meant that, taking an example that I showed earlier, when I call the
on_point_cb() it draws the point like defined and when I call the next
function on_line_cb(), the line is drawn but the previously drawn
point is wiped clean.
IDK maybe this is because I am using queue_draw() which re-draws the
entire widget. But if I understand it correctly, like you suggested
that cairo context is already equipped with clip region and I don't
have to explicitly define this clip_rectangle.

<snip>

> > And that is why I was asking if it could be done something like,
> >
> > void DrawingArea :: on_line_cb()
> > {
> >
> > cr->set_source_rgb(0.0, 0.26, 0.26);
> > cr->save();
> > cr->move_to(0,0);
> > cr->line_to(100,100);
> > cr->restore();
> > cr->stroke();
> >
> > queue_draw();
> > std::cout<<"Line created"<<std::endl;
> > }
> >
> > But this doesn't work because it says that 'cr' is not declared in the
> > scope or something related to this.
>
> As a matter of cairo implementation you need a cairo context to work
> with.  That is provided by the on_draw() method (pre-clipped, as I
> said). As a matter of basic C++, the cairo context object needs to be
> visible to the code addressing it, which means it must be in scope.

I've declared the cairo context in the DrawingArea.h file as:
 Cairo::RefPtr<Cairo::Context> cr;

And the 'scope error' is resolved but I get the segmentation error.
And on running backtrace I get the following:

(gdb) backtrace
#0  0x00007ffff6e24c84 in Cairo::Context::set_source_rgb(double,
double, double) () from /usr/lib/x86_64-linux-gnu/libcairomm-1.0.so.1
#1  0x000000000040d4ca DrawingArea::on_point_cb (this=0x7cce00)
    at /home/src/DrawingArea.cpp:78

What I think is that it is not able to identify the context correctly.
Even though I've declared the 'cr' in .h file, the context cr in
on_draw(const Cairo::RefPtr<Cairo::Context>& cr) might be different.
Confused here. :/


-- 
Gurjot Singh
Blog: http://bhattigurjot.wordpress.com

"It takes a person who is wide awake to make his dream come true." ~
Roger Babson