A bilevel clipping mask (black and white) will be created and used to draw the image. Pixels below 0.5 opacity will be considered fully transparent, and all others will be considered fully opaque.
For now falls back to GDK_PIXBUF_ALPHA_BILEVEL In the future it will do full alpha compositing.
These values can be passed to Pixbuf.xlibRenderToDrawableAlpha to control how the alpha channel of an image should be handled. This function can create a bilevel clipping mask (black and white) and use it while painting the image. In the future, when the X Window System gets an alpha channel extension, it will be possible to do full alpha compositing onto arbitrary drawables. For now both cases fall back to a bilevel clipping mask.