My preferred method of achieving invisibility, camouflage, has already been discussed. So this post is about the other, 'classic' (or 'obvious') method -- Trasnsparency, as illustrated here by the Stealth Crane.
The two above photos reminded me of a short story by Jack London, called 'The Shadow And The Flash'. It's about two rival geniuses who are in competition to develop a method of making objects invisible. One tackles the problem head-on, by choosing transparency
as his objective. The other goes the less obvious route, by trying to
make objects 'perfectly black' (that is, absorbing all light); I suppose this can be considered an
idealized form of invisibility-through-camouflage.
Eventually they both succeed in reaching their objectives and render themselves invisible, each in his respective manner, but they also run up against ineradicable flaws in their methods; when the transparency man moves, the continuously changing refraction of light causes his body to send forth flashes of rainbow-colored light, alerting others to his presence; the perfectly black man, of course, cannot hide his shadow (London glosses over the other obvious problem -- that, even in the shade where his shadow would not be detectable, objects behind the perfectly black man would be obscured and give away his silhouette). As for what happened to these men, I shall not spoil it for you -- you should read the story.
With a little assist from a disruptive background, the invisibility effect is greatly enhanced. I can hardly see the crane in the below photo, even though I know it's there. I suppose this could be considered a combination of transparency and camouflage, for the best of both worlds.