Help Topic: Reverse Image Search
The Basics: GreyHen has multiple ways to identify images which look alike, which are primarily used to check if an uploaded image is a duplicate. As it turns out, this is the same functionality which is required to perform a reverse image search, like on Google Images (not exactly as potent, but still pretty good). To perform a Reverse Image Search, visit the Reverse Image Search Page.
Performing a Search: Searching using this page is rather simple - enter the URL of an image on the internet that you want to search with, and click "Submit". Two different results lists will display, each with links to possible matching files.
GreyHash-14: GreyHash-14 is GreyHen's own image hashing algorithm, which checks to see if images are similar based on their width:height ratio and dominant colors. It's no shape-matching algorithm, but it's pretty good at identifying multiple sizes of the same image, even after resolution changes and jpeg artifacting. Note that this is a very broad search, and will often match you with multiple versions of the same image.
SHA-1: SHA-1 is a hashing algorithm created by the United States National Security Agency... in 1995. This algorithm may be a few months older than I am, but it runs quicker than MD5, the most common modern hashing algorithm, and takes up less space. If you get a match in this section, then it's almost certainly an exact file match - you got the image from the same source we did, or at least, our sources got it from the same source. Note that, due to compression, some jpeg files that are exactly the same, pixel-for-pixel, will have different SHA-1 hashes - GreyHash-14 is your friend in these situations.
Multiple Files: You may see two results that look very similar, such as "4594" and "4594-1". This is because GreyHen backs up files even when they get overwritten - "4594" is the largest, most accurate and watermark-free version we have, while "4594-1" is likely smaller, or perhaps contains a watermark. Both of these links will bring you to the same file, so don't be worried about which one to click.