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

[Hans Verkuil <hverkuil@xs4all.nl>]

On 01/08/2016 07:25 AM, Ran Shalit 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

I'm not sure what you're doing, but this just works for me (I use yuv420p to
play this back).

Note: you can find the same code in v4l-utils in contrib/test. That's the code
I used (with you modifications).

Regards,

	Hans