Add searching for similar pictures

[qarmin]

This is implemented in dupeguru

[Syfaro]

Some thoughts about doing this, coming from a person that runs a similar image matching service with 10s of millions of images.

• You can use perceptual image hashing to generate hashes to compare
  • There's the img_hash crate to do this in Rust
  • Perceptual hashes are very resistant to compression and resizing, but are completely incapable of handling cropping
• Hashes can be compared with hamming distance
  • If there's few enough images, you could compare each hash against every other hash, but this scales very poorly
  • You can use a bk-tree to index all of the hashes and very quickly find similar images
  • Indexing 23,543,939 images with 64-bit hashes is using 3.4GB of RAM for my service
• I personally find that with 64-bit hashes a hamming distance of up to 3 indicates a high probability of it being the same image

[qarmin]

Implemented by #69, but for me it is still a little too slow