diff --git a/cpdfpng.ml b/cpdfpng.ml
index d15e810..7a02b75 100644
--- a/cpdfpng.ml
+++ b/cpdfpng.ml
@@ -1,4 +1,6 @@
-(* Read a non-interlaced, non-transparent 24 bit PNG for inclusion in a PDF file *)
+(* Read and write non-interlaced, non-transparent 24 bit PNGs. Such a PNG may
+   be loaded into a PDF simply by extracting its width and height from the
+   IHDR, and concatenating all its IDAT data sections together. *)
 open Pdfutil
 open Pdfio