一、原理：
http://www.ruanyifeng.com/blog/2011/07/principle_of_similar_image_search.html
http://www.ruanyifeng.com/blog/2013/03/similar_image_search_part_ii.html
二、实现
https://www.cnblogs.com/blosaa/p/9553078.html
三、第三方API
阿里云API：https://help.aliyun.com/document_detail/124214.html?spm=a2c4g.11186623.6.635.61472e1dBuJR0o