#1...the FCP-X help forum is here:
http://forums.creativecow.net/fcpxtechnique
This is the FCP 7 and down forum.
#2...You can't. Not without some pretty serious VFX work. Changing lighting from what you shot isn't a simple color correction. Daylight coming in from the window will not only be in the square of the window pane, but all over the edges of the window, and if it is bright, those edges will show over-exposure...you can tell that it is a window frame in daylight. Even if you shot out that window at night, and then layered the video to have the night view on that window pane, because of the daylight streaming in hitting the window frame and everything else, it won't be convincing.
I mean, Digital Domain and ILM might be able to do something, but with serious work, and with additional footage to help sell it. Best to re-shoot.
This is something that you need to notice in the shoot, and take care of at that time. Not everything can be "fixed in post." If it needs to be night, you need to cover the windows with either very dark filters (TV shows and films use these to enable proper exposure outside when shooting inside), shoot at night...or cover the windows completely with blankets or something to block out the light.
Shane
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