For a flash of white across the screen (I use it during sex scenes for when an orgasm happens)
Code:
image whiteflash:
Solid("#fff")
alpha 0.0
linear 0.25 alpha 0.8
linear 0.75 alpha 0.0
linear 1.0 alpha 0.8
linear 0.25 alpha 0.0
And when I want it to happen I use
Code:
show whiteflash zorder 50
I didn't try to make it a transition because I tend to have one running on an image already.