VMR9AlphaBitmap colorkey issue

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I'm trying to use a color key to blend a bitmap and video with
VMR9AlphaBitmap. It works pretty well in general. I set:

bmpInfo.clrSrcKey = RGB(255, 255, 255);
bmpInfo.dwFlags |= VMRBITMAP_SRCCOLORKEY;

If I do this, any lines that I'm drawing on the bitmap that are white
become transparent. This is great - I draw white as by background, and
voila, alpha blending.

However what I don't understand is why my grey doesn't show up, either.
Grey being 192,192,192. Neither does off white - 254, 254, 254.

I guess I don't understand what color keying is - I thought the color
had to match *exactly* the color key or else it wouldn't become
transparent. Does this make sense or does it seem like I must be doing
something wrong? All other 'normal' colors - green, red, blue, yellow -
work just fine.

- Ryan

.

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