[Cairo] Re: [xsvg] cairo_text_extents ?

[Owen Taylor]

> > in the cairo design I have no guaranteed way to make 
> > y_advance be zero. Even if I don't rotate the font transform the interface seems to 
> > allow the font to set it non-zero, perhaps due to internal rounding to pixels 
> > when the main transform is rotated. I want to be able to *guarantee* (by 
> > making it not exist in the interface) that y_advance is zero. If this is 
> > not forced, there are two ways to lay out tilted text (rotate the 
> > text_transform, and rotate the font_transform), where the advance values may be 
> > calculated differently (one by the font and the other by the calling program), and 
> > thus output text will not match between them, which is a bad thing that Cairo 
> > is supposed to fix.
> > 
> 
> 
> Unfortunately, you cannot have a single advance value, since certain scripts
> really require both x and y. For example, have a look at the Nastaliq screenshots
> at the following page:
> 
>   http://www.freetype.org/opentype/index.html
> 
> Not to say that using two matrices isn't a good idea, just that the specific "advance"
> advantage isn't valid.

You don't necessarily need non-zero y-advance to do Nastaliq - in Pango,
for example the y-advance is strictly zero, but for each glyph in a
string of glyphs, the dx/dy from the nominal position is stored.

I think this model is easier to manage, since the baseline of the *line*
in Nastaliq is horizontal, it's just individual words that get slanted.

(Figure 12 in http://people.redhat.com/otaylor/grid-fitting/ shows
Pango laying out Nastaliq text.)

Also, it should be noted that using a y-advance to do Nastaliq won't
work for OpenType Nastaliq fonts ... the positioning methods there are
substantially more complex and generally handle pairs of glyphs, not
isolated glyphs.

Regards,
						Owen