image from one color channel
 $data = read_png($filename);            # Reads image data from PNG file.
 write_png($data,$filename);             # Writes image data to PNG file.
 @bit_planes = bit_plane($data,$type);   # Separates bit planes.
 $bit_planes = bit_plane_array($data,$type); # Separates bit planes.
 ($red,$green,$blue) = rgb($data);       # Separates red/green/blue from Color Image.
 ($red,$green,$blue) = rgb_array($data); # Separates red/green/blue from Color Image.
 info_array($array_ref);    