## Friday, October 20, 2006

### Why OCRing spam images is useless

Nick FitzGerald forwards me another animated GIF spam that takes the animation plus transparency trick I outlined in the blog post A spam image that slowly builds to reveal its message to a new level. And it shows why spammers will work around OCR as fast as they can.

Here's what you see in the spam image:

Looks simple enough until you take a look at the GIF file that actually generated what you see. It's animated and it has three frames:

The first image is the GIF's background and is displayed for 10ms then the second image is layered on top with a transparent background so that the two images merge together and the image the spammer wants you to see appears. That image remains on screen for 100,000 ms (or 1 minute 40 seconds). After that the image is completely blanked out by the third frame.

My favourite touch is that it's not the entire image that's transparent, not even the white background, but just those pixels necessary to make the black pixels underneath show through. If you look carefully above you can see that some of pixels appear yellow (which is the background color of this site) indicating where the transparency is.

That is darn clever.

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If you enjoyed this blog post, you might enjoy my travel book for people interested in science and technology: The Geek Atlas. Signed copies of The Geek Atlas are available.

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