[cairo] draw lines on only one channel

[M Joonas Pihlaja]

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.

Joonas