I invented the translucent blurred window effect. It first shipped in Mac IE 5.0 in April 2000 (the non-Carbon version only), for the autocomplete window, and later that year for the web page context menu (first in the special MacHack version).
The effect doesn't have to be expensive, and my original implementation was fast on millenium era machines.
The goal is preserve the context of the background, while keeping foreground data readable, in a way that is natural to our human visual system. If you are doing it properly you also shift the brightness values into a more limited range to diminish the contrast and keep the tonal values away from the chosen foreground text color (this is also cheap). Done properly it is visually pleasing with virtually no effect on readability.
People have coded some boneheaded imitations along the way though. They don't add the blur, or they don't adjust the brightness curve, or they make the radius much too big, or they compute some over exact Gaussian blur that is too slow.
It's the nature of blur that it doesn't have to be exact to be visually pleasing and convincing.
The context of the background is self-maintaining. Reducing the viz/readability of the overlay text does nobody but the art critics any favours. It was, and remains, a bad idea.
It's shown up in Vista and some other Windows versions since that. I though they used it fairly tastefully, but I'm not a Windows user so haven't spent much time looking at it. Apple currently has the blur radius so high they have to exaggerate the saturation to make the effect noticeable, which looks OK but I prefer a more optically realistic level of blur.
The effect doesn't have to be expensive, and my original implementation was fast on millenium era machines. The goal is preserve the context of the background, while keeping foreground data readable, in a way that is natural to our human visual system. If you are doing it properly you also shift the brightness values into a more limited range to diminish the contrast and keep the tonal values away from the chosen foreground text color (this is also cheap). Done properly it is visually pleasing with virtually no effect on readability.
People have coded some boneheaded imitations along the way though. They don't add the blur, or they don't adjust the brightness curve, or they make the radius much too big, or they compute some over exact Gaussian blur that is too slow.
It's the nature of blur that it doesn't have to be exact to be visually pleasing and convincing.