[cairo] draw lines on only one channel

[Tz-Huan Huang]

I want to draw lines on only one channel (e.g. red channel),
and I don't want to change the contents on other channels.

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.

Is there any way to do this using cairo efficiently?
Thanks a lot.

Tz-Huan