there's actually a really easy and effective way to remove logos which are all partially transparent, which a lot of network stations use lately (cbs, fox, nbc, abc, cw, ...) which i figured out a while back and was meaning to put into a web page or something so other people could use it. well i never really got around to it so i thought i would just post it here so at least maybe someone will find it useful.
what you will need is two things:
1. the logo against an all black background. it doesn't matter if the rest of the frame isn't all black, just black the rest out. these sorts of frames are usually pretty common and not too hard to get.
2. the logo against an all white background. this one is usually harder to find, although depending on the logo you may not actually need to have this one - if the logo seems to "disappear" when there's something bright behind it, you don't actually need this, since it would just be an all white frame anyway.
once you have these two things there are a lot of things you can generate, like the logo before they made it transparent. however you don't need to do that to remove it from the source video. you do need the alpha mask though. so
next what you want to do is generate the alpha mask used. i use GIMP for this because it's pretty straightforward. first you make a bottom layer of all white. subtract the frame you captured which is the logo against all white (paste as a new layer in GIMP and change mode to subtract), then add the frame where it's the logo against all black (new layer, mode=add).
this is the alpha mask. but when you remove the logo you actually want the inverse of this, so now flatten the image using gimp so that it's all one layer, and then invert the colors (colors menu -> invert i think?). this is your inverted alpha mask.
now to remove the logo from any other frame all you need is the one where it's the logo against all black and your inverted alpha mask. basically what you're going to do is take each source frame, subtract the black logo, and then divide by the inverted alpha mask. i did this in virtualdub by modifying an open source frame blending plugin i found online, you can find my modification at http://www.megaupload.com/?d=52AJKLW7 (my mod is in the Release folder, the .vdf, the original is located in the root dir). it's a pretty straightforward filter, you just add in one to subtract the black logo image, and then another to divide by the inverted alpha mask image.
in terms of how well this actually removes logos, it can never be perfectly 1:1 since you lose information whenever you composite images like that, but it's pretty close, about as close as you can get to the original image. i tested it with some hdtv caps and the effectiveness varies with how opaque the logo is (more opaque than transparent means more data lost), and there's generally more mpeg artifacts around logos anyway, since that's how mpeg works, but overall the quality of picture recovered was vastly superior to what i was getting out of the virtualdub delogo filter.
if some part of this is unclear let me know. hope this helps someone
plugin also attached.
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screencast to clarify some things attached