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regular (non-secure) hash functions do two things: they compress (very lossily) and they make things that are near each other in their domain (inputs) map to th

• mholt 4y ago
  > > they make things that are near each other in their domain (inputs) map to things that are far apart in their codomain (outputs). This is describing a specific subset of hash only. *Cryptographic* hash functions map inputs to outputs with high and uniform dispersion. So you are talking about cryptographic hashes, but different hash functions can have different properties. ThumbHash is absolutely a hash function, which is "any function that can be used to map data of arbitrary size to fixed-size values, though there are some hash functions that support variable length output." (https://en.wikipedia.org/wiki/Hash_function https://en.wikipedia.org/wiki/Hash_function)
  • a-dub 4y ago
    sure. but think about it this way: most real world data is actually highly structured. in the space of bits that aren't encrypted, the real stuff lives in a very, very small subspace and good hash functions seek to avoid collisions in their output.
• shakna 4y ago
  Perceptual hashing wouldn't seem to satisfy your second requirement there, either.
  • a-dub 4y ago
    it does, it just works on estimates of percepts rather than bits. you can think of a perceptual hash as two functions. a perceptual function that maps differing collections of bits that appear the same or similar to the same bits, and then a traditional hash function to ensure that these intermediate values get shuffled.