def extract_chunk(input, output)
   lenword = input.read(4)
   length = lenword.unpack('N')[0]
   type = input.read(4)
   data = length>0 ? input.read(length) : ""
   crc = input.read(4)
   return nil if length<0 || !(('A'..'z')===type[0,1])
   #return nil if validate_crc(type+data, crc)
   output.write lenword
   output.write type
   output.write data
   output.write crc
   return type
end

def extract_png(input, output)
    hdr = input.read(8)
    raise "Not a PNG File" if hdr[0,4]!= "\211PNG"
    raise "file not in binary mode" if hdr[4,4]!="\r\n\032\n"
    output.write(hdr)
    loop do
      chunk_type = extract_chunk(input,output)
      p chunk_type
      break if  chunk_type.nil? || chunk_type == 'IEND'
    end
end

i=0
fp = File.new("has_embedded_png.dat","rb")
begin
    m= /\211PNG/.match(fp.read)
    raise "no PNG" if !m
    fp.seek(m.begin(0))
    #fp.seek 22946097
    ofp = File.new("out#{i}.png","wb")
    extract_png(fp,ofp)
end while m