Apple just announced that they will use “perceptual hashing” to detect illegal photos on iPhones. I have some experience to share on this technology.
At my company, we use “perceptual hashes” to find copies of an image where each copy has been slightly altered. This is in the context of stock photography, where each stock agency (e.g. Getty Images, Adobe Stock, Shutterstock) adds their own watermark, the image file ID, or sharpens the image or alters the the colours slightly, for example by adding contrast. For our customers, i.e. the photographers who upload their photos to these agencies and sell them through their platform, we need to find all of their photos’ copies on those agencies. We use perceptual hashes to find them.
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