Around 09/17/2012 11:22 AM, Hans Verkuil scribbled: > On Sun September 16 2012 18:49:00 Georgi Chorbadzhiyski wrote: >> On 9/16/12 7:28 PM, Hans Verkuil wrote: >>> On Sun September 16 2012 17:32:52 Georgi Chorbadzhiyski wrote: >>>> Guys I'm adding v4l2 output device support for VLC/ffmpeg/libav (I'm using >>>> v4l2loopback [1] driver for testing) but I have a problem which I can't seem >>>> to find a solution. >>>> >>>> VLC [2] uses VIDIOC_CROPCAP [3] to detect the pixelaspect ratio of the input >>>> it receives from v4l2 device. But I can't seem to find a way to set struct >>>> v4l2_cropcap.pixelaspect when I'm outputting data to the device and the >>>> result is that VLC assumes pixelaspect is 1:1. >>>> >>>> I was hoping that VIDIOC_S_CROP [4] would allow setting pixelaspect but >>>> according to docs that is not case. What am I missing? >>> >>> The pixelaspect ratio returned by CROPCAP depends on the current video standard >>> of the video receiver or transmitter. >>> >>> So for video capture the pixelaspect depends on the standard (50 vs 60 Hz) and >>> the horizontal sampling frequency of the video receiver (hardware specific). >>> >>> For video output the pixelaspect depends also on the standard and on how the >>> transmitter goes from digital to analog pixels (the reverse of what a receiver >>> does). >>> >>> It is *not* the pixelaspect of the video data itself. For output it is the >>> pixel aspect that the transmitter expects. Any difference between the two will >>> need to be resolved somehow, typically by software or hardware scaling. >> >> Since I'm using virtual output v4l2 loopback device this means I have to set the >> standard somehow, right? > > Yes, just call VIDIOC_S_STD. But the loopback device driver needs to be modified to > have cropcap return the aspect ratio belonging to the given standard (or just 1x1 > for non-PAL/NTSC resolutions). > > I wish personally that this driver was being upstreamed to the kernel. I know that > Mauro (subsystem maintainer) isn't too keen on it, but I think we can convince him > that it is really a useful driver to have. And if it is part of distros anyway, > then we should just accept it (after cleanup, of course). The drivers is very useful I can vote for that with two hands. Along with snd-aloop I'm able to create a stable video/audio source which was not possible before. I'll try to add support for S_FMT to v4l2loopback, lets see where this would lead me (for now I just hacked VLC to set the aspect from the command line instead of relying on CROPCAP). I don't know if the author(s) have any plans of upstreaming it (I've added them to cc, guys any opinion on that?). -- Georgi Chorbadzhiyski http://georgi.unixsol.org/ -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
