Get the input and output frame, then call
        :py:meth:`transform`.

def process_frame(self):
        """Get the input and output frame, then call
        :py:meth:`transform`.

        """
        input_name = self.inputs[0]
        output_name = self.outputs[0]
        in_frame = self.input_buffer[input_name].get()
        out_frame = self.outframe_pool[output_name].get()
        out_frame.initialise(in_frame)
        if self.transform(in_frame, out_frame):
            self.send(output_name, out_frame)
        else:
            raise StopIteration()