Ok, according to the PDFlib docs,
bpc - only for imagetype raw (required in this case). The number of bits per component. Must be 1,2,4 or 8
components - only for imagetype raw (required in this case). The number of image components. Must be 1, 3 or 4.
Read the docs (page 171) for a full list of params I assume are used for the last argument.