VP8 + alpha channel

[Andry Gutyria]

I need to play video files with alpha channel. I've built and linked
VP8 decoder. According to variety of guides, it supports alpha
channel.

But in fact, it wasn't possible to convert video to webm format with
alpha channel preserved (using ffmpeg/libvpx). Source files definitely
contain alpha channel. But no matter which format do I choose in
options (yuva420p, rgba and so on), libvpx just forces encoder to pick
yuv without alpha.

May you please provide me a hint on how to encode webm with alpha
preserved, or are there some other way which may do that?

[John Koleszar]

Hi Andry,

Alpha channel is not supported in VP8 video yet. I'm not sure what
guides you're referring to. It does have some support in the WebP
image format, which is VP8 related, but it's not supported in video.

John

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[Andy Shaules]

Ive thought about hiding a luma keyed frame used to subtract alpha from
the source in an alt-ref-frame.

Possibly the normal decoder would not be told to reference the alt frame
in the bytestream, whereas your player could assume alt-ref's we to be
used to subtract alpha.

For instance, the alt ref frame could contain the image of the
bluescreen that was used without any actors, and then subtract the
values in real time.

I did a similar operation using flv feeds that required a 32bit depth
from a 24 bit source. the blue/green screen stage was sent in a jpg and
used to subtract the alpha , converting the 24 bit image to a 32bit
image in the process.

Fun times

Andy

[Andry Gutyria]

Thanks for info.

[John Koleszar]

Alpha was supported with VP6, but there was no defined method for doing it in VP8 when it launched (or as of a year ago, per this thread). Since then, we've worked on defining a way to do alpha channel with VP8, and are working on getting that support into Chrome. See http://wiki.webmproject.org/alpha-channel for a description of the format.

On Thu, Mar 7, 2013 at 2:48 PM, <rubbe...@kabelmail.de> wrote:

When VP still belonged to On2, it DID support alpha channel. Did they remove that feature? That would have been extremely stoopid since there are no good lossy alternatives.

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[Pavel Koshevoy]

On Thursday, March 7, 2013 6:32:49 PM UTC-7, John Koleszar wrote:

Alpha was supported with VP6, but there was no defined method for doing it in VP8 when it launched (or as of a year ago, per this thread). Since then, we've worked on defining a way to do alpha channel with VP8, and are working on getting that support into Chrome. See http://wiki.webmproject.org/alpha-channel for a description of the format.

Is this the final format, or is it going to change still?  I work for Sorenson Media, so if VP8+alpha spec is ready I could implement it in Squeeze.

Pavel.

[Vignesh Venkatasubramanian]

Yes, this is the final format. We have gone with Design 1 mentioned in that document for implementing VP8 alpha.

 

Pavel.

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