Re: writing a text widget with pango

[Peter Zelezny <pzel dodo com au>]

On 23 May 2002 01:37:15 -0400
Havoc Pennington <hp redhat com> wrote:

> > The only reason I went to line-by-line, was so that I could chop the top
> > few lines off, if the scrollbar was in that position. I guess I could
> > render the whole Layout a few pixels off the top of the window? Either
> > that, or do my own CENTER/RIGHT calculations (which are simple enough).
>
> Yes, what I'd do is just draw the layout at negative coordinates, it
> will be clipped to the window, works great.

But you don't know how many pixels to offset it by, unless you look at
the individual lines' extents anyway.


> > I thought keeping PangoLayouts around was the fastest way to go.
>
> It probably is, but the memory footprint is most likely unacceptable.
> As long as you have only tens of layouts onscreen at once, it should
> be fine to create them on-demand. Or at least just keep a limited-size
> cache.

True, it keeps alot of GSList around etc. So I converted it to create a
Layout on-the-fly, but the results are a little sluggish. Is there any
room for improvement in pango's internal: pango_layout_check_lines()? It
seems to be spending most of its time here.

I did a test, measuring the width and lines-taken of each paragraph on a 
large buffer (250,000 lines). My old XLib widget did it in about 2 seconds,
pango version takes about 1 minute.

Here's a small improvement for a special case:

--- pango-layout.c      Fri May 24 21:09:27 2002
+++ pango-layout.c.new  Thu May 30 15:12:12 2002
@@ -939,6 +939,10 @@
 {
   g_return_val_if_fail (layout != NULL, 0);
 
+  /* layouts with a width of -1 have no wraps; always 1 line */
+  if (layout->width == -1)
+    return 1;
+
   pango_layout_check_lines (layout);
   return g_slist_length (layout->lines);
 }


I'll probably delve into pango_layout_check_lines() next, and see what
can be done. Perhaps there should be a special version of
pango_layout_get_line_count that doesn't create the layout->lines list (for
when you're just measuring, but not going to render).


-- 
Peter Zelezny.