Re: process_item and shaping

On Thu, 2004-08-26 at 08:28, Daniel Glassey wrote:
> Hi,
> I can see from bug #62014 that pango was changed to shape each character
> at most twice in pango-layout.c:process_item/insert_run/shape_run. I'm
> trying to work out why the second one still needs to be done and why
> pango_glyph_item_split isn't used.

There is no guarantee that characters will never interact across a
line break point. This is especially true when character-wrapping is
on ... simple case, consider character wrapping on for English and
breaking in the middle of a 'fi' ligature. 

> There is a line that says:
> 	      /* Shaped items should never be broken */
> 	      g_assert (!shape_set);
> so I'm guessing there may be a good reason but the assert is strange
> because shape_set is always false.

shape_set is for PangoAttrShape, which is used for something like
inserting an image into the text ... when you want to say - leave
20x30 pixels for this character and don't draw it.

> I'm looking at this because shaping is particularly expensive with
> graphite, though even the basic shaper is a big hit. If there isn't a
> reason to not do it I'll investigate using pango_glyph_item_split and
> submit a bug etc later.

We theoretically could add some sort of per-shaper flag which said
"this shaper never will reshape at line break points", though since
the shaper doesn't have the line break point information, it's sort
of a difficult guarantee to make. 

Hmm, I suppose what we could do instead is have a PangoVisAttr flag 
which says "breaking before this glyph won't cause reshaping". If
you want to experiment, that's the approach I'd recommend.


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