<p>Seminal work in this field was done in <a href="https://courses.cs.washington.edu/courses/cse455/10au/notes/forsyth.pdf" rel="nofollow">Fleck, Margaret M., David A. Forsyth, and Chris Bregler. "Finding naked people." Computer Vision—ECCV'96. Springer Berlin Heidelberg, 1996. 593-602.</a>. The approach detects skin-colored regions and then determines whether or not the regions match predefined human shapes. More on their skin detection algorithm here: <a href="http://www.cs.hmc.edu/~fleck/naked-skin.html" rel="nofollow">http://www.cs.hmc.edu/~fleck/naked-skin.html</a> .</p>

<p>More recent papers with summaries of current methods are available: </p>

<ol>
<li><a href="http://iseclab.org/people/cplatzer/papers/sfcs05-platzer.pdf" rel="nofollow">http://iseclab.org/people/cplatzer/papers/sfcs05-platzer.pdf</a> </li>
<li><a href="http://arxiv.org/abs/1402.5792" rel="nofollow">http://arxiv.org/abs/1402.5792</a></li>
</ol>

<p>You may also take a look at: <a href="https://stackoverflow.com/questions/713247/what-is-the-best-way-to-programatically-detect-porn-images">What is the best way to programatically detect porn images?</a></p>

<p><strong>update 2016:</strong> use a convnet. They are far better at building high resolution filters. I wrote about it in more detail here </p>

<ul>
<li><p><a href="http://blog.clarifai.com/what-convolutional-neural-networks-see-at-when-they-see-nudity/" rel="nofollow">http://blog.clarifai.com/what-convolutional-neural-networks-see-at-when-they-see-nudity/</a></p></li>
<li><p><a href="https://www.slideshare.net/mobile/RyanCompton1/what-convnets-look-at-when-they-look-at-nudity" rel="nofollow">https://www.slideshare.net/mobile/RyanCompton1/what-convnets-look-at-when-they-look-at-nudity</a></p></li>
</ul>
What methods and algorithms now using for Naughty Detection?
<p>Interestingly, what technologies and algorithms the large companies use. I found only that the Microsoft uses the PhotoDNA technologie, but it is responsible only how photos are compared. <strong>It is interesting also as they automatically detecting pornographic images.</strong> </p>

<p>For example, are used any of methods : Skin Detection, ROI Detection, Bag-of-Visual-Words.</p>