gdkPixbufRenderThresholdAlpha: gdkPixbufRenderThresholdAlpha
In RGtk2: R Bindings for Gtk 2.8.0 and Above

Takes the opacity values in a rectangular portion of a pixbuf and thresholds them to produce a bi-level alpha mask that can be used as a clipping mask for a drawable.

1 2	gdkPixbufRenderThresholdAlpha(object, bitmap, src.x, src.y, dest.x, dest.y, width = -1, height = -1, alpha.threshold)

`object`	A pixbuf.
`bitmap`	Bitmap where the bilevel mask will be painted to.
`src.x`	Source X coordinate.
`src.y`	source Y coordinate.
`dest.x`	Destination X coordinate.
`dest.y`	Destination Y coordinate.
`width`	Width of region to threshold, or -1 to use pixbuf width
`height`	Height of region to threshold, or -1 to use pixbuf height
`alpha.threshold`	Opacity values below this will be painted as zero; all other values will be painted as one.