[cairo] Dia as a new Cairo testbed

[Keith Packard]

Around 0 o'clock on May 25, Hans Breuer wrote:

> To get an educated opinion about current abilities of Cairo I've written a 
> Dia [1] export filter based on Cairo. The source of it is already in gnome 
> cvs, it even is integrated in the *NIX build process : ./configure --with-cairo

Cool.  We'll try to figure out where the remaining problems are. I tested
the .svg file with the cairo-based 'xsvg' program which generated different
output.

	http://keithp.com/~keithp/download/render-test.html

(The images in this version don't appear at all because the svg-cairo
library doesn't handle them yet).

> The arc drawing is completely bogus. [Doing a simple cairo_arc()
> produces arcs mirrored against the horizontal axis and all my tries
> with cairo_arc_negative() and angle juggling have failed, too.]

I think you haven't quite got the arguments for these functions figured 
out; xsvg draws this figure correctly.

> Linestyle in bezier lines are apparently not supported yet

Yup.  It's not a huge amount of work, but it is yet to be done.

> Image rendering is not as good as expected (At least I would
> expect it to reproduce similar quality as in the pixbuf case
> when using CAIRO_FILTER_BEST).

We're just using bilinear interpolation.  Anisotropic filtering would 
generate better results.

> Rendering of alpha images is completely broken. Maybe caused by 
> not properly using the Cairo Operator.

Perhaps this is a confusion with the premultiplied alpha format which 
cairo uses.  libart uses non-premultiplied alpha (I'm pretty sure).  This
may also be causing troubles mentioned above?

> Text rendering is known to be limited in Cairo (the so called 'toy api'

If dia uses pango for text rendering, we should be good to go once there's a 
cairo backend for pango.

-keith


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