def convert_ppm_and_pgm(self, ppmfile, pgmfile, outfile):
        """
        Convert a PPM and PGM file containing raw pixel data into a
        PNG outfile with the parameters set in the writer object.
        """
        pixels = array('B')
        pixels.fromfile(ppmfile,
                        (self.bitdepth/8) * self.color_planes *
                        self.width * self.height)
        apixels = array('B')
        apixels.fromfile(pgmfile,
                         (self.bitdepth/8) *
                         self.width * self.height)
        pixels = interleave_planes(pixels, apixels,
                                   (self.bitdepth/8) * self.color_planes,
                                   (self.bitdepth/8))
        if self.interlace:
            self.write_passes(outfile, self.array_scanlines_interlace(pixels))
        else:
            self.write_passes(outfile, self.array_scanlines(pixels))
Convert a PPM and PGM file containing raw pixel data into a
        PNG outfile with the parameters set in the writer object.