I am trying to write a set of externals based on the opencv library. My main interest is in the motion tracking aspect of opencv. I am new to programing and I am having a tough time converting the opencv IplImage data type to the Pd symbol data type in order to send it out of outlets and in to inlets. Martin suggested I try something like this: char symstr[10]; t_symbol sym; sprintf(symstr, "%p", frame); sym = gensym(symstr); and then send sym through the outlet and convert it back at the receiving end by extracting the sting from the s_name field and passing it to atol(). But I get this error when I use sym = gensym(symstr); no match for 'operator=' in 'sym = gensym(((char*)(& symstr)))'. Does anyone know how to get around this? Thanks, Alain