In Rendair's thread about the wave effects for the reflections, somebody mentioned that you can apply overlay filters which affect the image below. So for example if I had a captcha image below, how could I apply a wave effect on the top to skew those letters?

What I'm trying to do is replicate the Gmail captcha image which is very simple, but merely skews the letters:

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The man who comes back through the Door in the Wall will never be quite the same as the man who went out.