Identicon Generator (Github-like)

An Identicon is an icon that contains a hash value representing part of your identity. Although it is not known how Github generates each user's Identicon, it's not too hard to make one.

For a 5x5 "Github-like" identicon, observe that all of them are horizontally symmetric, which means that the pattern itself only contains 5*3 = 15 bits of information. Other than that, the color only contains 3 bytes of info, assuming that each of the rgb-channel takes up 8 bits. Since we don't use many bits, there is no need to compute a strong and long hash value. A sha1 hash would do the favor.

The tricky part is: How to design the pattern, so that the chance of any two users having the same identicon is equally likely/unlikely? Maybe one would come up of an idea like this: For each hex-value in the hash string, mark the corresponding table[0:15] until the table has enough dots. Unfortunately, this method would results in all permutations, for example 'a2ce' and 'ac2e', having the same identicon. It's likely not a good idea!

Another point one can consider is that, since identicons are viewed by humans, two color would become indistinguishable if their pixel values are close to each other. Consider a method in generating the dots like this: Starting from the grid at the upper-left corner. Draw a dot if the first hex-value in the hash string is greater than 0x7, otherwise leave a blank. However, if two bytes from different hash strings are having values that are close to each other, not only would their color be alike, the patterns were also more likely to be the same (For example, 'ad15' would have the same pattern as 'cf06' in this scenario). A better design would be: Even though the two bytes' value are close, the patterns are still decided independently.