How to make existing app windows transparent in smartphones?

I am trying to find out if there is a way to make existing app windows transparent in a smartphone. The platform could be any of iphone/android/WP7. In my own apps, I can make use of the alpha blending features and control transparency, but is there a way to make 开发者_Python百科already existing windows transparent? Thanks!

---

If you would want to modify all applications on a given target, I think you'd have to modify the default themes, as far as Android is concerned at least. But I don't see how that is done easily. I'm guessing that you might need to re-compile parts of the source or, in worst case, even "jailbreak" the phone.

If you only need to alter some specific applications it would perhaps be easier to talk directly to the developers of those apps.

---

On Windows Phone 7 many controls have an opacity member. Also Windows Phone 7 does support alpha channels in images and controls.