Re: Gnome2::Canvas surprising behaviour

[muppet <scott asofyet org>]

On Aug 17, 2005, at 8:31 PM, James Muir wrote:

> I discovered something surprising with the "aliased" version of the  
> Gnome2::Canvas.
>
> If you place a Gnome2::Canvas::Text item on an "anti-aliased"  
> canvas and forget to set the 'fill-color' you see nothing. This  
> seems reasonable. If you place a Gnome2::Canvas::Text item on an  
> "aliased" canvas, the text will be displayed even without setting  
> the 'fill-color'. Nice, I suppose, but is this a bug in  
> Gnome2::Canvas?

Has nothing to do with Gnome2::Canvas --- it's just an implementation  
detail of libgnomecanvas-2.0.

The classic gdk canvas uses gdk to do its drawing, and canvas items  
are drawn with the draw() vfunc.  gnome_canvas_text_draw() (a private  
function) calls gdk_draw_layout() to draw the text with the item's  
gc, which was created in realize() with all the defaults.  The  
defaults, of course, include a text drawing color of black.  When you  
do a set_property on the canvas text item in gdk mode, the rgb value  
you set gets placed directly into the gc.

The antialiased canvas does everything with client-side rendering.   
To achieve this, the canvas items are drawn with the render() vfunc  
rather than the draw() vfunc.  The canvas text item keeps a current  
rgba foreground value, and uses this as the foreground color and  
alpha for blitting the image drawn by pango_ft2_render_layout() into  
the temporary pixel buffer passed in to gnome_canvas_text_render().   
It just so happens that there is no initial value explicitly set in  
the instance initializer, so it takes the value given to the entire  
block by memset() at allocation time -- 0.  Since the rgba value is  
an unsigned 32 bit int, that means the RGB is all zero, or black...   
but the alpha is also 0, and alpha of 0 means "completely transparent".

So, you may want to file a bug against libgnomecanvas in  
bugzilla.gnome.org, pointing out that the gnome_canvas_item_text()  
needs to set text->rgba = 0x000000ff....  but now that gtk+ 2.8 is  
out with cairo support, and havoc and company are talking about a new  
canvas widget, don't be surprised if you get a lukewarm response.

(keep in mind, however, that it will still be many months before any  
Gnome2::Canvas replacement is viable.)


In the meantime, it's best just to be explicit and always set a fill  
color on your Gnome2::Canvas::Text items.  ;-)


--
"the ternary operator makes it a bit less ugly."
    -- kaffee