Mauro Carvalho Chehab a écrit :
Hi Mauro and Markus,
Just to summ up what I understood we need:
What do we need in userspace, only for v4l (dvb is not concerned):
- colorspace translations
- filters that be done in hardware if the selected hardware can,
otherwise software plugin
- decompression algorithm like stk11xx or usbvision (the decompression
algorithm is in kernelspace since it is of linear complexity but shall
be moved to userspace)
Yes. The first focus, IMO, should be the last one.
OK, so we must open the V4L2 API to add a custom pixel format.
While brainstorming, I thought it would be funny to create userspace
drivers like FUSE based projects do.
Applications would see nothing but a new device driver to access, all
decompression algorithm would be in userspace.
The path from the application to the v4l2 kernel driver:
[Application]<->[/mnt/video0 (created by FUSE-like userspace
lib)]<->[/dev/video0 (created by kernel v4l2 driver)]
There are limitations that I don't know, but this would be transparent
for the applications.
Using pwlib will not mean that application developers will use pwlib
to decode v4l driver outputs.
C bindings are much more popular than C++ bindings and do not prevent
object oriented design.
IMO, we should implement very simple and efficient C subroutines.
Application developers implement their own codecs.
They can do it if they want. However, if we have a very consistent and
easy to use subroutines for those weird decompress stuff, it is likely
that they will use it.
As an example, every application do deinterlacing internally or not...
Application developers will probably not use pwlib v4l extensions
because they will prefer to write adapted codecs for their framework.
I think we shouldn't deal with deinterlacing. the API should be as
simple as possible, focusing on implementing some stuff highly linked
with the hardware (like specific decompression stuff, proprietary
colospace conversions, etc).
Much more important for me is to see the actual specification of the
needed v4l extensions points, with advice/participation of
application/codec developers.
As an example, we could empacket frames with a header containing
audio/video format as it is done for MPEG streams.
Is it possible without breaking the current ABI?
Yes, it is possible. In fact, some devices currently work by generating
a common audio and video stream. The driver may just send the packet
as-is, leaving to the userspace API the function to de-merge and
synchronize audio and video.
So we have a userspace lib that will demux/uncompress device driver output.
This lib will know each device driver particularities to know how to
demux/uncompress in a a/v format that will be understood by the application.
Do application developers would cope with that?
Maybe.
Cheers,
Mauro
Thanks
Thierry
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