[vlc-devel] Decoder/Display implementation selection

[Steve Lhomme]

Hi,

While I'm working on the push implementation it has become clear that 
there's a growing issue of how the user is going to select the 
implementation it wants.

Right now the user can select the display module and the avcodec-hw 
module. Now we're adding a "decoder device" to the mix. Once one is 
picked, the avcodec-hw should be the matching one, otherwise extra 
conversions will be needed. But what if the picked one is not possible ? 
Fallback to software ? Meaning the decoder device is forgotten ? Meaning 
the display module won't get it ? When rendering externally that could 
be a problem as the decoder device already picked the information about 
the external renderer, which must be matched when rendering in the 
display module.

There's also the question of the NVDecoder which a GSoC student is 
working on. It's not a avcodec-hw module but should be able to be picked 
by the user in the list of HW decoders. It could be coupled with a 
"decoder device" if that's what the user will select in the future, 
instead of the "avcodec-hw" module. If that's the case we should be able 
to make the switch soon, I have "decoder device" for D3D11 and 
DXVA2/D3D9 ready to be merged, the others already exist in master.

That means it would replace the "avcodec-hw" selection. Should this 
parameter go away and the HW decoder picked is only the one 
corresponding to the "decoder device" ? And fallback to software if none 
is found. But still carrying the original decoder device in the video 
context up to the display ? (see the external rendering coherence issue 
above)

The default behavior should not be an issue. It could select the decoder 
device based on the OS and if the implementation is available. It's bit 
heavier than the current ffmpeg VLD flavor selection as we need to open 
system resources to know if the HW acceleration is available (and yet we 
don't know exactly the codec/profile needed). On the other hand right 
now we open a whole display module and then check the VLD flavor works. 
So it would probably be done faster anyway.

The display selection should not change. If the HW decoder doesn't match 
the display selected by the user (or automatically selected) some 
conversion should be done. If the mix is not possible too bad.