code: def convert_ppm_and_pgm(self, ppmfile, pgmfile, outfile):
          
          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))
docs: Convert a PPM and PGM file containing raw pixel data into a
          PNG outfile with the parameters set in the writer object.