RE: Re: v4l2 + mmap()

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On 02-Jun-01 Gerd Knorr wrote:
> Nemosoft Unv. wrote:
>>  but I was talking about what happens if the buffers
>>  still get full;
> 
> Then your box is to slow for video capture.  Specifying the frame rate
> you want capture might help in this case[1].  For bttv this would'nt
> make a big difference, but with USB hardware which might have to
> post-process video data (uncompress due to the limited bus bandwidth)
> this should help a bit as you avoid processing data which will be
> dropped by the application anyway ...
> 
>>  drop the last or the oldest frame? 
> 
> Whichever is less work for the driver, i.e. probably drop the last one.
> 
>   Gerd
> 
> [1] Hmm, no v4l2 ioctl for this yet.  Add one?


Yes please; this would be great for certain webcams. (though there are
others, like the CPiA cam, who don´t have a fixed framerate at all)

There is mention of an interval setting in VIDIOC_G_PARM/VIDIOC_S_PARM
(which is just the reciproke of a framerate). However, I think that is a
very obscure place to put it. I´d rather suggest the following:

 - add a flag to v4l2_capability.flags: V4L2_FLAG_VARFPS (VARiable Framerate)
 - either add a an ioctl (VIDIOC_S_FPS), or stick it into the v4l2_pix_format
   structure and other ascociated structs.

 - Nemosoft

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