Using luminosity to copy textures

Hey guys, I know of this website that many of you guys might already know (and forgotten).
Here's the website link:

http://www.planetphotoshop.com/

Here you can find a lot of good tutorials, and the guy who gives them is pretty good at explaining stuff. There I found this tutorial that uses the luminosity of objects to copy textures and reflections (for underwater stuff? maybe or fabrics ?) He uses a mug but you could try it with whatever.

http://www.planetphotoshop.com/graphic-on-mug.html

Good Luck!