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). Regards, Hans -- 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
