diff --git a/cpdfpng.ml b/cpdfpng.ml
index 1b423ae..3d7813d 100644
--- a/cpdfpng.ml
+++ b/cpdfpng.ml
@@ -1,21 +1,59 @@
 (* Read a non-interlaced, non-transparent 24 bit PNG for inclusion in a PDF file *)
+open Pdfutil
+open Pdfio
+
 type t =
   {width : int;
    height : int;
-   idat : Pdfio.bytes}
+   idat : bytes}
 
 exception BadPNG of string
 
+let string_of_tag t =
+  Printf.sprintf "%c%c%c%c"
+    (char_of_int (i32toi (Int32.shift_right t 24)))
+    (char_of_int (i32toi (Int32.logand 0x000000FFl (Int32.shift_right t 16))))
+    (char_of_int (i32toi (Int32.logand 0x000000FFl (Int32.shift_right t 8))))
+    (char_of_int (i32toi (Int32.logand 0x000000FFl t)))
+
+let read_unsigned_4byte i = 0l
+
+let read_data l i = bytes_of_string ""
+
+let read_chunk i =
+  let chunklen = read_unsigned_4byte i in
+  let chunktype = read_unsigned_4byte i in
+  let chunkdata = read_data chunklen i in
+  let _ (* crc *) = read_unsigned_4byte i in
+    (string_of_tag chunktype, chunkdata) 
+
 let read_png i =
-  (* File signature *)
-  (* IHDR *)
-  (* IDAT *)
-  (* IEND *)
-  {width = 0;
-   height = 0;
-   idat = Pdfio.bytes_of_string ""}
+  try
+    i.seek_in 8;
+    let ihdr, ihdrdata = read_chunk i in
+    if ihdr <> "IHDR" then raise (Pdf.PDFError "read_png: first table not IHDR") else
+    let width = 0 in
+    let height = 0 in
+    let bitdepth = 0 in
+    let colortype = 0 in
+    let compressionmethod = 0 in
+    let filtermethod = 0 in
+    let interlacemethod = 0 in
+    let idat = ref None in
+      begin try
+        while true do
+          let chunkname, chunkdata = read_chunk i in
+            if chunkname = "IDAT" then
+              idat := Some chunkdata
+        done
+      with
+        _ -> ()
+      end;
+      {width; height; idat = unopt !idat}
+  with
+    e -> raise (Pdf.PDFError (Printf.sprintf "read_png: failed on %s" (Printexc.to_string e)))
 
 let _ =
   read_png
-    (Pdfio.input_of_string
+    (input_of_string
       (Pdfutil.contents_of_file "/Users/john/Desktop/cpdfdraw/Untitled.png"))