cairo-set-source (cr source)

cr -- a cairo-t context
source -- a cairo-pattern-t instance to be used as the source for subsequent drawing operations

Sets the source pattern within cr to source. This pattern will then be used for any subsequent drawing operation until a new source pattern is set.

The default source pattern is a solid pattern that is opaque black, that is, it is equivalent to

(cairo-set-source-rgb cr 0.0 0.0 0.0)  

The transformation matrix of the pattern will be locked to the user space in effect at the time of the call of the cairo-set-source function. This means that further modifications of the current transformation matrix will not affect the source pattern. See the cairo-pattern-set-matrix function.