How to combine two GdkDrawable as two layers?

[Magicloud Magiclouds <magicloud magiclouds gmail com>]

Dear all,
   I have "GdkDrawable* main" as main layer, and "GdkDrawable* layer1".
   Now I draw a jpeg on main (by GdkPixbuf), and clear layer1 with 
alpha channel value 0, draw a line with alpha channel value 1 (by 
Cairo), then gdk_draw_drawable layer1 to main. The result I wanted was a 
jpeg with a line on it, but I got a jpeg with the region of layer1 
transparent and a line on it.
   What is the problem?

Thanks.