[cairo] Font metrics under font transformations

[Keith Packard]

Around 18 o'clock on Jan 11, Maarten Breddels wrote:

> yup, I vote for vectors solution too, just makes life easier i guess.

I guess I have two questions:

 1)	Where do we expect to use transformed fonts, and how do
 	applications want to set text with transformed fonts?

I see two 'legitimate' cases here --

 a)	Artificial oblique -- shearing text along the X axis.  In this 
	case, the Y values don't change at all, so there's no actual 
	problem

 b)	Inverting the coordinate space.  Right now, cairo uses the upper 
	left corner as 0,0.  I imagine applications like PostScript or PDF
	viewers might want a lower-left origin, in which case they must 
	flip both the graphics coordinate system and the font coordinate
	system.  Do we want to report negative line spacing in this case?
	PostScript does not; to move down the page, you subtract the
	font height.

Other transformations should be considered 'fringe' cases for which "some" 
answer should probably be given, although a strong argument for returning 
an error when asked for font metrics can be made.

 2)	How should an application set text in neat rows when presented
	with line spacing which includes an X component?

For this, I have no good answer -- as we do currently present glyph advance 
values as vectors, it seems logical to do the same with line advances, but 
the inverted coordinate space case makes this less compelling.

-keith


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