Re: Using v4l2 API example as-is for video capture

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On Fri, Jan 8, 2016 at 8:25 AM, Ran Shalit <ranshalit@xxxxxxxxx> wrote:
> Hello,
>
> I am trying to use v4l2 example from API documentation:
> inhttp://linuxtv.org/downloads/v4l-dvb-apis/capture-example.html .
> As a start, I wanted to first try uding it with vivid (virtual device driver).
> I found excelent vivid documentation in the kernel, which is very helpful:
> https://www.kernel.org/doc/Documentation/video4linux/vivid.txt
>
> But on trying to use the example as-is for capturing video frames into
> file, and playing the file, I encounter difficulties.
> I tried the example as-is, and then tried several resolution,
> pixelformat, different inputs, but it always result with video with a
> sync problems ( the test bars of the video are keep moving in the
> horizontal axis, or the text is changing its location for one frame to
> the next).
>
> I compiled the v4l2 API example AS-IS from:
> http://linuxtv.org/downloads/v4l-dvb-apis/capture-example.html with
> minor modification in the --force part of the example (I also tried
> the example as is without modifications but it did not help), so that
> I choose hd input , and 1920x1080 resolution, V4L2_PIX_FMT_YUV420
> (also tried V4L2_PIX_FMT_YUV422P) , progressive.
>
>   if (force_format) {
>             input = 3;  // <<-- HD input device
>             if (-1==xioctl(fd,VIDIOC_S_INPUT,&input))
>             {
>              errno_exit("VIDIOC_S_INPUT");
>             }
>             fmt.fmt.pix.width       = 1920;
>             fmt.fmt.pix.height      = 1080;
>             fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUV420; // <<-
> tried also V4L2_PIX_FMT_YUV422P
>             fmt.fmt.pix.field       = V4L2_FIELD_NONE; // <- trying to
> capture progressive
>
>             if (-1 == xioctl(fd, VIDIOC_S_FMT, &fmt))
>                     errno_exit("VIDIOC_S_FMT");
>
>     } else {
>
> I run the application with (the compiled code using pixelformat =
> V4L2_PIX_FMT_YUV420 trial ):
>
> ./v4l2_example -f -o -c 10  > cap_yuv420p.yuv
>
> And (the compiled code using pixelformat = V4L2_PIX_FMT_YUV422P trial )
>
> ./v4l2_example -f -o -c 10  > cap_yuv422p.yuv
>
> I've tried to play them with:
>
> ffplay -f rawvideo -pixel_format yuv420p -video_size 1920x1080 -i
> cap_yuv420p.yuv
>
> And
>
> ffplay -f rawvideo -pixel_format yuv422p -video_size 1920x1080 -i
> cap_yuv422p.yuv
>
> These are the captured video files from my above trials:
>
> https://drive.google.com/folderview?id=0B22GsWueReZTUS1tSHBraTAyZ00&usp=sharing
>
> I probably am doing something wrong. Is there any idea what's wrong in
> my configurations or how I can debug it better ?
>
> Best Regards,
> Ran


Hi,

I did manage to play the video with the example as-is (no code modification)
But It seems that the captured video is always the same:  422 YUYV.
I tried to change in code to other formats (in the section below force
option as I wrote in the previous post) ,
but although it gives no error (and VIDIOC_G_FMT returns the new
format) ,  and yet - the final video is always in 422, YUYV format.
Is there any hint why we can't change the format to YUV420  ?

Regards,
Ran
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