How can I change gamma of the screen form bright to dark-transparent (the way shutdown does)

thank you

Maybe it doesn't change the gamma, but instead it paints all over the screen a small grid of grey pixels. I'm just guessing, i always thought it did it that way. (look at the shutdown dialog, it is not grayed)

Anyway, i have no clue on how to do it. :)

Create a chess-board like pattern brush, use SRCAND ROP with it to mask off half of the pixels; use SRCPAINT with a gray color to draw the gray color.