http://art-architecture.diply.com/different-solutions/first-i-thought-he-was-just-putting-paint-on/26836

http://blogs.discovermagazine.com/d-brief/2016/08/31/picture-frame-illusion/#.V8tvqzV9l8F

https://graphicriver.net/item/pencil-art-photoshop-action/19476077?ref=ksioks

http://fc01.deviantart.net/fs71/f/2013/260/1/5/los_santos_and_blaine_county_by_kawaksallas-d6mo0q8.jpg

https://www.artstation.com/artwork/lDEB6k

http://www.imaginistix.com/

Richard Russell  Harvard University, USA

In the Illusion of Sex, two faces are perceived as male and female.

However, both faces are actually versions of the same androgynous face.

One face was created by increasing the contrast of the androgynous face, while the other face was created by decreasing the contrast. The face with more contrast is perceived as female, while the face with less contrast is perceived as male. The Illusion of Sex demonstrates that contrast is an important cue for perceiving the sex of a face, with greater contrast appearing feminine, and lesser contrast appearing masculine.

Russell, R. (2009) A sex difference in facial pigmentation and its exaggeration by cosmetics. Perception, (38)1211-1219.