[cairo] draw lines on only one channel

[Tz-Huan Huang]

On Wed, Mar 3, 2010 at 22:21, M Joonas Pihlaja <jpihlaja at cc.helsinki.fi> wrote:
>
> Hi,
>
> On Wed, 3 Mar 2010, Tz-Huan Huang wrote:
>
>> For example, when I draw a line on red channel (suppose
>> the color is (1.0, 0.0, 0.0)), then draw a line on blue channel
>> (suppose the color is (0.0, 0.0, 1.0)), I hope the intersect is
>> purple (1.0, 0.0, 1.0) not blue.
>
> It sounds like you want to use CAIRO_OPERATOR_ADD rather than the
> default CAIRO_OPERATOR_OVER.

Oh! That's exactly what I want, thank you very much! :-)

Tz-Huan