I realize that this is an older thread, but seeing as the Sharps deck was the winner of this award, is there a place where I can find more about its marking system, or someone who can tell me more about it?
I am aware that it uses shades, but needs some help with the specifics.
In most marking systems, they work as the name would imply - there are marks somewhere on the back of the playing card that will indicate what's printed on the front. These might be additive marks, like someone coloring in a blank space or drawing a line or a dot, or they might be subtractive, like someone scratching a line or a dot into a field of color to reveal the white paper underneath. There are also the easy-to-use "reader" decks, so called because they use simple, easy-to-read symbols rather than a concealed code of some kind - for example, in a regular marked deck, a King of Diamonds might have a specific set of dots or scrawls concealed somewhere in the back design, unknowable without knowing what the legend of the deck is, while in a reader deck, a King of Diamonds might have a "K" and either a "D" or a diamond symbol printed somewhere on the back, so the person using the deck only needs to know letters, numbers and symbols and not some arcane code. Readers tend to be more easily detected but are also far easier to use.
The way a shade system typically works, all the cards are still printed "identically" in terms of the pattern on the back - the shapes, lines, etc. The difference is that certain parts of the pattern are altered in terms of slight changes in the shade of a color used to print them. It's meant to be very subtle but still readable to anyone who knows where and how to look. You can't use a shade system to create a "reader" deck, and in order to be read, you have to know the legend for how it's marked, but they're among the hardest marking systems to spot, not counting decks employing things like UV inks or daubs, RFID chips, etc.
In this way, it shares a certain amount in common with modern steganography, a practice of hiding a message in a digital image. They way that works is that you take your message, usually already encrypted for privacy, and determine the exact length of the string of bits (individual ones and zeros) needed to convey that message. You find a digital image, typically a photograph, that has at least the same number of pixels (acronym for "picture element," the smallest part of a digital image), and you change the least-significant bit, typically the last bit, using a particular formula. In a picture with 32-bit pixels, the color change is a difference of ½^32 - you're making that pixel one 4,294,067,266th closer to either brightest red or darkest violet in the color spectrum. To the naked eye, that's truly undetectable. To a computer, it's easily spotted, especially if the person using the computer has a copy of the original photo with which to compare the one concealing the steganographic message.
But back to my point - it's a change not in the overall shape of the back pattern, but in the coloration of the pattern, making it a far more subtle form of marking and tougher to spot. A riffle test may or may not reveal it, depending on just how big a change the shade is.
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