Re: save, restore and *_preserve

[Ian Martin <martin_id vodafone co nz>]

On 24/11/13 07:50, creak ml wrote:

Hi!

I'm using a Gtk::DrawingArea to do some a special profiling drawing tool.

At some point, I want to draw a box, fill it with a color, stroke it in black and write a text on it (and not drawing out of the box). I succeeded in a way, but I'm not sure it's the perfect way of doing it.

Since the drawings in this DrawingArea will be pretty intensive, I'd like to have some advice from you because I'm a bit lost with what save, restore and *_preserve methods do...

Here the (simplified) code of the drawing:

    // In the on_draw() method.
    // cr is a Cairo::RefPtr<Cairo::Context> const&.

    // For each box I've got to draw.

    // Set the rectangle bounds.
    cr->rectangle(rectX, rectY, rectWidth, rectHeight);
   
    // Save it, fill, preserve and restore.
    cr->save();
    cr->set_source_rgb(1.0, 0.8, 0.5);
    cr->fill_preserve();
    cr->restore();

    // Save it again, stroke, preserve and restore.
    cr->save();
    cr->set_source_rgb(0.0, 0.0, 0.0);
    cr->set_line_width(1.0);
    cr->stroke_preserve();
    cr->restore();

    // Initialize pango font.
    Pango::FontDescription font;
    font.set_family("DejaVu Sans");
    font.set_size(12 * PANGO_SCALE);
    font.set_stretch(Pango::STRETCH_CONDENSED);

    // Initialize pango layout.
    Glib::RefPtr<Pango::Layout> layout = create_pango_layout(text);
    layout->set_font_description(font);

    // Get the text dimensions.
    int textWidth;
    int textHeight;
    layout->get_pixel_size(textWidth, textHeight);

    // Position the text in the middle.
    // Save, clip, set color, move to the middle, draw text, reset clip, restore.
    cr->save();
    cr->clip();
    cr->set_source_rgb(0.0, 0.0, 0.0);
    cr->move_to(rectX + (rectWidth - textWidth) / 2, rectY + (rectHeight - textHeight) / 2);
    layout->show_in_cairo_context(cr);
    cr->reset_clip();
    cr->restore();

Trust me, if I remove one save/restore or one preserve, the draw is becoming pretty f*cked up.

Maybe a first thing would be to cache the font.

Do you have other ideas, GTKMM gurus? ;)

Cheers,

Creak

Hi Creak,
I assume you found the manual- https://developer.gnome.org/gtkmm-tutorial/unstable/index.html and have read the chapter on drawing?  It should answer your questions.  The online (or offline, if you've got the docs ) reference manual https://developer.gnome.org/gtkmm/unstable/  lists all the functions including the ones you're asking about; generally the *mm documentation is of a high standard.  Make sure you're using references for the version you're using; there's been a number of changes between version 2 and 3 of gtkmm.

tldr: a Cairo::context has a set of attributes at any point in time, including clip regions, colour for background and foreground, line size, etc.  By encasing all writes to the Context in save and restore, you prevent your code from modifying the base context.  With that understanding, you may want to encase your code slightly differently; for instance, saving before clipping, then saving/ restoring around each paint (the save / restore functions push the context onto a stack, so you can have effectively unlimited depth).

The *_preserve functions write to the context but preserve the path they write on, which is lost with the other (stroke and fill) methods.  Useful if you want to e..g. stroke in one colour and fill in another.  Have a look at the examples in the manual for ideas.


HTH,
Ian.