Re: Why is PangoCairoRenderer private?

[Peter Frentrup <peter_frentrup gmx de>]

Owen Taylor schrieb:
> On Sat, 2007-05-19 at 15:58 +0200, Peter Frentrup wrote:
> > Hello,
> >
> > I am working on a little text editor using Cairo and Pango (but without
> > GTK+/GTKTextView, because it does not have a native look-and-feel on
> > Windows).
> >
> > At first I thought, I could use the pangocairo API, but it does not give
> > me the possibility to draw selected text in a different style without
> > setting the background-attribute (which seems to be slow when changing
> > the selection range).
> > So I read a bit about the PangoRenderer class. I could write my own
> > subclass of PangoRenderer, but this would basically be a copy of the
> > PangoCairoRenderer code plus a few additions.
> >
> > So my suggestion is to make PangoCairoRenderer public to let Pango-users
> > subclass it; or is there another solution for my problem?
>
> See GdkPangoRenderer for an example of building a customized renderer
> that backends to Pango/Cairo. (Effectively it "has a" PangoCairoRenderer
> rather than "is a" PangoCairoRenderer")
>
> But I'm not really sure that it's going to help you with selection 
> at all ... the technique used by the GTK+ widgets for drawing a
> partially selected paragraph is:
>
>  - Draw the paragraph normally
>  - Create a clip region based on pango_layout_line_get_x_ranges()
>  - Draw the paragraph in the selection colors
>
> (Optimization to do this only for partially selected lines is 
> possible, but I wouldn't do it before finding the entire paragraph
> to be a bottleneck..)
>
> The reason that you found setting attributes slow is probably
> because you were causing a re-layout every time the user moved 
> dragged the mouse a character. Setting attributes also just doesn't
> work well for selection ... consider selecting only the 'f' of 
> a 'fi' ligature. The use of clipping avoids that problem.
>
> 					- Owen
Thanks for your quick reply.
I will study the GdkPangoRenderer code.;The idea with clipping sound 
good. I haven't thought of something like this. Maybe I can adopt that 
clipping method for wxWidgets or so.

Regards,
Peter