Why is PangoCairoRenderer private?

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?

Regards,
Peter

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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


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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
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