ggiBltSetSourceIdx and ggiBltGetSourceIdx set and read, respectively, the texture index of a BOB or other LibBlt data source, which is imparted to a blit operation when a BOB is placed into a Blt with one of the ggiBltPut* functions. Data is taken from the managed area with the given index when corresponding precompiled operation is sent to the target.
(Note: an alternate form is planned which will use a linear address into the managed area, as well.)
Once imparted through a ggiBltPut* function, calling these functions again on this BOB with a different value will not alter the the blit's precompiled operation, only the texture to be imparted to future operations.