Just like paintings, even the tattoo was two dimensional for most of its history. The instruments used and the techniques used did not allow the realization in all the three dimensions, but now times have changed and the tendency of a 3d tattoo has become very common.
The 3d can be applied to almost any subject, from the Mayan symbols, animals, portraits, landscapes hyper-realistic, up to the lettering tattoos and is a very effective technique to make the drawing more alive.
