[Cairo] Re: [xsvg] cairo_text_extents ?

[Turner David]

Hello,

> 1. The main difference is that in my scheme, while the glyphs 
> are transformed by exactly the concatenation of the matrices, the "advance" 
> values are instead calculated by the font based on the font_matrix, in possibly 
> font-specific ways. In particular it cannot have a non-zero 
> y_advance and thus the advance cannot rotate even though the font transform 
> can. Also if the font defines a "vertical advance" value then when it is 
> rotated 90 degrees the x_advance should change to this value. Also the 
> exact point rotated around should be decided by the font, so that as  you 
> rotate the letters it smoothly transitions from horizontal to vertical 
> layout (this rotation point can be figured out for each letter from a font 
> description that has both horizontal and vertical metrics).
> 
> 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.

Regards,

- David Turner
- The FreeType Project  (www.freetype.org)

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