Re: setting Gtk2::Gdk::Color pixel

[muppet <scott asofyet org>]

On Nov 15, 2007, at 7:11 PM, Kevin Ryde wrote:

> I had an urge to do some xor-ing of pixels, to flip between colours  
> for
> easy un-drawing of an interactive "lasso" in a widget, so I want to  
> set
> the "foreground" pixel of a GC to a calculated value (as opposed to
> getting it from a colour lookup or alloc).
>
> If I'm not mistaken $gc->set_values and friends take a  
> Gtk2::Gdk::Color
> object in the hash for "GdkGCValues", but I couldn't spot how to set  
> the
> pixel field in such an object.  Is there a secret setter method, or a
> way to create with a given pixel?

The trick is to set the foreground to solid white and set the gdk  
function to XOR.

From some old C code i have sitting around:

        if ( ! r->draginfo.gc ) {
                r->draginfo.gc = gdk_gc_new (GTK_WIDGET (c)->window);
                gdk_gc_set_function (r->draginfo.gc, GDK_XOR);
                gdk_gc_set_foreground (r->draginfo.gc,
                                       &(GTK_WIDGET (c)->style- 
>white));
        }

which translates into perl as

        $gc = Gtk2::Gdk::GC->new ($widget->window);
        $gc->set_function ('xor');
        $gc->set_foreground ($widget->style->white);


> (I see Gtk2::Gdk::Color->new takes the RGB fields, but not a pixel.)
>
> (Of course why GdkGCValues etc uses a GdkColor struct when it only  
> looks
> at the pixel value is anyone's guess.  Must have seemed like a good  
> idea
> at the time.)

My understanding is that you specify the RGB values you want, and the  
GdkColor contains the actual visual-specific pixel value allocated by  
the underlying system.  This was very important back in the colormap  
and low-bitdepth days.


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