[cairo] [Fwd: CAIRO_OPERATOR_SOURCE issue]

[Jonathan Morton]

On 8 May 2010 13:14, Clemens Eisserer <linuxhippy at gmail.com> wrote:
> Looks like CAIRO_FORMAT_ARGB32 is premultiplied, so the resulting
> color already contains the alpha-value multiplied in.

Yes, Cairo uses premultiplied alpha throughout.  Presumably
cairo_set_source_rgba() is correcting the colour channels of the given
colour so that they are limited by the alpha channel.

This means that (51,51,51,51) is the correct vector for a 20%-opacity white.

 - Jonathan Morton