diff --git a/examples/prints/README b/examples/prints/README
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--- a/examples/prints/README
+++ b/examples/prints/README
@@ -1 +1,3 @@
 These are example images from NIST and are in the public domain.
+
+The PNG files have been generated by using the greyscale data as a mask.
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diff --git a/examples/prints/tented_arch.png b/examples/prints/tented_arch.png
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diff --git a/examples/prints/whorl.png b/examples/prints/whorl.png
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