You 3D-ers might have some info I'm looking for. I'm looking to do some on-the-fly compositing in a web browser. The tried and true method is flash video using the VP6 codec, which supports a transparency channel - you get 8-bits of alpha info, a true RGBA method.

However, the client requirement is Silverlight, no flash allowed. There isn't a direct method, other than a 1-bit approach in which you can designate a transparent color and a tolerance (range) for it. Unfortunately, this method has significant aliasing artifacts, and is unacceptable picture quality.

As it turns out, MPEG-4/h.264 does have RGBA support in its spec, just like VP6. But, I can find no encoders that support it, including Expression, Main Concept, Sorenson Squeeze, Telestream Episode, and Super C (x.264).

I understand from Adam Stanislav's site that he and perhaps some others on this forum are working on multiplexing methods for 3D, using AVISynth. I really don't know much about it - is it possible that I could mux an alpha channel with it? The goal here is to end up with 8bits per pixel of Alpha, not 24... in an h.264 container.

I'm assured by my Silverlight guy that if I can encode it, he can decode it.

Sorry, I don't know anything about Silverlight. I tend to ignore any me-too technology Microsoft pushes just to take the business away from some smaller company (not that Adobe is small).

I'm as ready to bash Adobe, Microsoft, Apple, and Google as the next guy - they all deserve it. But, it isn't Adobe's codec, VP6 belonged to On2, which was bought by Google. And now Apple won't allow Flash on the iPad or iPhone, and it's a crazy mess we have to navigate. Maybe this will all be replaced by html5 in a few years, in the meantime I've got a client who decided on Silverlight for reasons of interoperability with .net.

But the question regards muxing an Alpha channel in h.264, possibly with AVISynth.

Yes, it is a crazy mess, I'll agree about that. Quite frankly the only reason I have not written a complete NLE system from scratch yet is the file format jungle. I can do all the editing (it's just high school math), but I am clueless when it comes to reading many of the different file formats and even more so when it comes to writing them.