[gst-devel] Re: [gst-cvs] wtay gst-plugins: gst-plugins/ gst-plugins/gst-libs/gst/video/

[Jan Schmidt]

<quote who="Ronald Bultje">
> On Fri, 16 Jul 2004, Benjamin Otte wrote:
> > Quoting Ronald Bultje <rbultje at ronald.bitfreak.net>:
> > > On Fri, 16 Jul 2004, Wim Taymans wrote:
> > > > Log message:
> > > >         * gst-libs/gst/video/video.h:
> > > >         Added 32 bits RGBA. Not sure if we should use another mime-type
> > > >         for alpha rgb. Currently the presence of the alpha_mask property
> > > >         signals an alpha channel. Ronald?
> > >
> > > Yes, that's the current way. The good thing is that RGB readers can still
> > > read RGBA and just discard the alpha-channel, so having the same mimetype
> > > would make sense in a way. But it's suboptimal in another way, too. I
> > > personally didn't really bother thinking too much about this because I
> > > don't see too many RGBA videos nowadays.
> > >
> > The main reason for using a different mimetype is that it might result in
> > issues when you link a sink element that requires an alpha channel with a
> > source element that doesn't know about alpha channels. That way an alpha
> > channel will be negotiated and the sink element will pretend to use it, while
> > the source element fills the alpha bytes with random garbage.
> 
> That can never happen. The sink element has the alpha_mask property in its
> pad template, and thus the source element's source pad is *required* to
> explicitely contain an alpha_mask property in its negotiation caps. If it
> doesn't, caps negotiation fails and that's exactly what *should* happen.
> 
> The other way around works fine too: if the source element does contain an
> alpha_mask but the sink element doesn't use it, it just ignores this
> property and thereby the alpha_mask.

Barring elements which are buggy and simply proxy the caps-with-alpha across
to their src pads and claim to be supplying an alpha channel when they're
really only copying the RGB channels.

J.
-- 
Jan Schmidt                                  thaytan at mad.scientist.com

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