MPEG-4 and alpha transparency - the ongoing saga

问题:

I'd like to reach some sort of definitive answer for the following questions:

1. Is alpha transparency supported in MPEG-4-based codecs?
2. Is there any way to be reasonably certain that there is no alpha-channel from the output of ffprobe?

Some links I've found on the subject:

• https://stackoverflow.com/a/5057631/476716

Also, transparent alpha layers, an optional part of the H.264 specification, are now supported in H.264-based QuickTime movies.

• http://software.intel.com/en-us/articles/alpha-channel-support-in-the-h264-decoder-in-ipp/

The support for alpha channel in the H.264 decoder is now available in Intel® Integrated Performance Primitives 6.0 version.

• http://forums.creativecow.net/readpost/335/30475

The MPEG4 Simple Studio Profile supports auxiliary channels, which could be used for alpha or depth -- and presumably even displacement, motion vectors, normals, etc.

However, I've never seen a concrete example, or indeed any details on how you use these announced features.

回答1:

The short answer is that h.264 does not support an alpha channel since no decoder/encoder support the functionality. See this creativecow forum post for more info. While an alpha channel does not work in the h.264 video itself, it is possible to use 2 different h.264 videos to get the same effect. See my post to the creativecow forum for more info.