[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!

Carl Eugen

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