[cairo] cairo backend for xpdf - first drop

[Keith Packard]

Around 10 o'clock on Dec 22, Carl Worth wrote:

> The question I had is whether xpdf has all the information it needs to
> be able to compute this adjustment. The answer may be "it depends on
> the document", but I would certainly like to see this happen when
> possible.

When I hacked xpdf (badly) to use Xft, I ignored position information 
except at word boundaries.  I thought the results were an improvement, 
although one would see occasional ragged margins resulting from a sequence 
of mis-placed characters.

I know for myself the resulting improvement in readability more than made 
up for the loss in rendering accuracy, but in an ideal world, we'd make 
this a viewer option.

This also brings up which advance values cairo should present back to the
application; we can either return a 'linear' advance scaled from the
abstract outline, or the hinted advance returned by the rasterizer.  The 
former ensures consistent alignment of text on screen and page, the latter 
provides better appearance on screen.  I suspect the right answer is 
'both', as which is used depends entirely on the application, and, as in 
the above example, it might actually be best to 'mix' the two metrics so 
that intra-word spacing is hinted and inter word spacing is linear.

-keith


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