I realize it will not feet my needs for making a snake skin, even with the abilty of using the alpha. Too bad, because it was theoricaly a good way to avoid streching of the scales when part of the body inflate... As you said, I can't find a way to get the direction for overlapping properly one scale after another. As we can see here:
Also I realize there is probably no way to get the color attribute to change colors of the scales to create this kind of pattern
I will probably do it with a "feather" particle system or may be by textures dependings of the look the director is looking for.
Any ways, I will definitly use it in a way or another for another purpose, the bark Paul Smith did for exemple or may be his gravel simulation.
Many thanks! Cheers.