double rendering of borders ...

[Ivan Timofeev]

On 01.08.2012 20:59, Michael Meeks wrote:
>
> On Wed, 2012-08-01 at 19:56 +0400, Ivan Timofeev wrote:
>> I solved the problem by the following change in renderAreaToPix:
>> cairo_data[x*4+0] * alpha
>
> 	That appears to me to break the compositing :-) it might happen to
> improve things for this case of course.

Well, now I have read about alpha compositing and cairo... :)

We use CAIRO_FORMAT_ARGB32 and the cairo manual says[1]:

  Pre-multiplied alpha is used. (That is, 50% transparent red is 
0x80800000, not 0x80ff0000.)

so cairo_data elements already multiplied by alpha.

[1] 
http://www.cairographics.org/manual/cairo-Image-Surfaces.html#cairo-format-t