[FFmpeg-devel] [RFC] Exporting the alpha mode from decoders
Carl Eugen Hoyos
cehoyos at ag.or.at
Fri Feb 6 21:16:50 CET 2015
wm4 <nfxjfg <at> googlemail.com> writes:
> The pixfmt docs seems to imply that the extra
> component must be set to 0 if a RGB0 format is
> used. Camtasia puts random stuff.
The documentation sounds wrong to me: The pix_fmt
was needed because of hardware providing 32bit rgb,
there is no guarantee what the X bits contain and
it makes no sense for FFmpeg to require a specific
It is good behaviour that libswscale writes a
defined value though (as it does iirc).
(This is unrelated but I don't think "random" is
the right word in above sequence: The fact that
I did not interpret the alpha value as random
was apparently the reason I never changed the
pix_fmt. But you are clearly right that RGB32 is
> What about AV_PIX_FMT_RGB555? It's documented
> as having 1 alpha bit
I believe this is a documentation issue, see
rgb15to32_c() in libswscale.
> though it doesn't have the alpha pixfmt flag set.
> Fix the docs? Or introduce RGB05551?
Atm, I cannot remember a sample that supports
alpha in RGB555 (I do remember that it is
defined in some specs though).
> There is no AV_PIX_FMT_RGB064.
> Is it guaranteed that there is no 64 bit
> format with padding?
FFmpeg "promises" that RGBA64 contains a valid
alpha channel, just as FFmpeg "promises" bit-
exact H.264 decoding.
> If one ever exists, is it guaranteed that a 0
> variant will be added?
I cannot imagine a screen driver that provides
64 bit rgb screen grabs but please prove me wrong!
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