[linux-dvb] Re: video4linux converging with linux dvb

[Holger Waechtler <holger@qanu.de>]

Gerd Knorr wrote:

> Rob.McConnell@Zarlink.Com writes:
>
>  
> > Now I have only recently taken a peek at the V4L2 APIs and understand that
> > they support video scaling/cropping and some video output capabilities.
> > However, there are requirements for video encoders (DENC) that don't appear
> > in the API.
> >    
> Jep, mainly because there is no driver yet (at least none known to
> me).  Just pulling some API spec out of thin air without at least one
> reference implementation usually doesn't work out that well.  Even one
> implementation doesn't ensures that you get a reasonable API.
>  
There exist several scaler APIs that have been handcrafted for a 
particular STB. Unfortunally the last-generation STBs implemented very 
different scaler and analog encoder approaches, it was not really 
possible to make a good abstraction. v4l 1+2 mirrors only a tiny 
fraction of the possibility these chipsets delivered: video and 
stillpicture layers could get stacked in arbitrary orders, alpha blended 
or not, colorkeyed or not, in some cases even arithmetic functions 
(adding, saturation, ...) between layers were possible. In most cases 
these capabilities are coupled with serious limitations (layers were 
sometimes tied to particular decoders, sometimes limited to particular 
color depths and pixel formats) so that the same code won't work on two 
STBs.

In the past the special scaler drivers only implemented the function 
subset that was mandatory for a specific application. and provided an 
proprietary API that was again wrapped by DirectFB.

Holger