[cairo] Some problem about Draw a line with alpha channel

[Yong.zhang]

Hi,
I want to use cairo lib to write a simple drawing software with alpha
channel,but there are a lot of problem:

1. the mouse button is pressed,we can draw in the window.
when we use alpha channel to draw a semi-transparent line before the
button is released,how to prevent the color of semi-transparent line to
deepen.

2. how to implement a eraser for semi-transparent which the background
is transparent.

thank you!

-- Yong.zhang