[XviD-devel] Qeustion about encoding rgba images.

[Michael Militzer]

Hi,

right, the MPEG-4 standard allows coding of alpha channels - however not in
all profiles. XviD just supports the MPEG-4 simple and advanced simple 
profiles and does _not_ support proper coding of alpha channels.

You can use input images with a alpha channel (e.g. XVID_CSP_RGBA) but upon
conversion into YV12, XviD's internal colorspace, the alpha channel will be
_omitted_ by XviD. Therefore, the alpha plane also cannot be reconstructed
anymore at the decoder side then...

If you really need alpha channels, you could either try adding support for
them to XviD yourself or you could have a look at the MPEG-4 reference soft-
ware (since I'm not aware of any popular MPEG-4 encoder implementation
offering support for proper alpha channel coding).

bye,
Michael


Quoting MAILER-DAEMON at edu.bnhof.de:

> 
> I noticed that for encoding/decoding color space there were
> XVID_CSP_RGBA. But when I use that option the encoded mpeg
> stream seems to be identical to XVID_CSP_BGR. If I want to
> decoded in RGB space will it always be interleaved BGR?
> 
> Also looking further into the code in image.h it seems image struct does 
> not have an alpha channel. Is there a different way of specifying the 
> alpha channel to encode? If I remember correctly MPEG-4 has allows alpha
> channels. If its possibel can anyone point me to where I might 
> look?
> 
> I'd appreciate any help.
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