Why does Overlay work this way?

[Dark Shikari]

Today I was watching a video on Stage6 and alt-tabbed to an OpenOffice Impress presentation I was making. The background of the slide was a dark blue-black gradiant... and interestingly enough, there was a slice of it that was see-through. Of course, I quickly realized that particular color was used as the "overlay color" by the graphics card, and the Stage6 video was playing using the overlay renderer.

Now, why does it work this way? Who decided that 00000F would be the color that is transparent to overlays?

(Of course, I've always loved overlay mode, as it means I can draw on videos as they play...) :D

[check]

The colour chosen for the colour key is actually video card/driver dependant. Some ATI cards use a hot pink, and most (all?) nvidia setups use pure black. As to why, I'm afraid I don't have any idea :D. If nobody else here knows, I would suggest going over to bug phaeron @ virtualdub.org

[foxyshadis]

Drivers have defaults, but most applications override them, it seems. For more than you probably want to know, see: http://www.virtualdub.org/blog/pivot/entry.php?id=91