Hi Jia,
rad understands some special variables named after the command they affect to pass such options. There used to be a render= setting in the rif-file, but now you should be able to pass all rpict options by a line such as rpict=-ab 2 -ad 512 -lw 0.002 and so on. Please check the manpage for rad again.
Cheers, Lars.
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Dipl.-Ing. Architect Lars O. Grobe