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I think we used to have these in old cereal box surprises or kids books for secret decoder games. There would be a message visually encrypted in a book and the

I think we used to have these in old cereal box surprises or kids books for secret decoder games. There would be a message visually encrypted in a book and the book came with a transparent cellophane key sheet you'd place on top of the image to decode the secret message.

BostonEnginerd 11y ago
Those were usually red color filters which filtered out one of the noise channels, leaving the message.