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This paper proposes an algorithm to classify nude pictures. Firstly, it classify pixels in a picture as “skin” and “non-skin”. Then connect the skin pixels into skin regions. For the largest three skin regions, make a bounding polygon (indeed only a quadrilateral as it takes only the leftmost, rightmost, uppermost and lowermost pixels). Base on the skin regions and the polygon, a picture contains nudity unless any of the following is met:

• Total skin pixels <15% pixels in a picture
• Largest skin region has <35% total skin pixels AND 2nd largest has <30% AND 3rd largest has <30%
• Largest skin region has <45% total skin pixels
• Total skin pixels <30% pixels in a picture AND skin pixels in the bounding polygon <55% size of polygon
• Number of skin regions >60 AND average intensity within polygon <0.25

In the last bullet, intensity is the saturation value in HSV space. Converting from RGB to HSV is given by the following formulae:

\begin{aligned} H &= \arccos\frac{\frac{1}{2}[(R-G)+(R-B)]}{\sqrt{(R-G)^2+(R-B)(G-B)}} \\ S &= 1 - 3\frac{\min(R,G,B)}{R+G+B} \\ V &= \frac{1}{3}(R+G+B) \end{aligned}

Extended reading: Finding Naked People by Margaret M. Fleck and David A Forsyth

## Bibliographic data

@inproceedings{
title = "An Algorithm for Nudity Detection",
author = "Rigan Ap-Apid",
booktitle = "Proc. 5th Philippine Computing Science Congress",
page = "201--205",
month = "Mar",
year = "2005",
}