Prevent GStreamer from changing camera format

[Nicolas Dufresne]

Le dim. 18 oct. 2020 17 h 00, Nikola Pantelic <pantelicn63 at gmail.com> a
écrit :

> (This question is also available at
> https://stackoverflow.com/questions/64415461.)
>
>
> Running GStreamer pipeline that feeds from v4l2src element may change
> camera format implicitly.
>
> Here is an example (also note that /dev/video0 is everywhere implied).
>
> One of my cameras supports these formats
>
> $ v4l2-ctl --list-formats-ext
> ioctl: VIDIOC_ENUM_FMT
>     Type: Video Capture
>
>     [0]: 'YUYV' (YUYV 4:2:2)
>         Size: Discrete 640x480
>             Interval: Discrete 0.033s (30.000 fps)
>             Interval: Discrete 0.067s (15.000 fps)
>         Size: Discrete 320x240
>             Interval: Discrete 0.033s (30.000 fps)
>             Interval: Discrete 0.067s (15.000 fps)
>         Size: Discrete 1280x800
>             Interval: Discrete 0.133s (7.500 fps)
>         Size: Discrete 1280x1024
>             Interval: Discrete 0.133s (7.500 fps)
>
> The current format is
>
> $ v4l2-ctl --get-fmt-video
> Format Video Capture:
>     Width/Height      : 640/480
>     Pixel Format      : 'YUYV' (YUYV 4:2:2)
>     Field             : None
>     Bytes per Line    : 1280
>     Size Image        : 614400
>     Colorspace        : Default
>     Transfer Function : Default (maps to Rec. 709)
>     YCbCr/HSV Encoding: Default (maps to ITU-R 601)
>     Quantization      : Default (maps to Limited Range)
>     Flags             :
>
> If I just run this pipeline
>
> $ gst-launch-1.0 -e v4l2src ! videoconvert ! autovideosink
>
> This will change the current format (before the pipeline is run, but
> it will also be left in that state) to
>
> $ v4l2-ctl --get-fmt-video
> Format Video Capture:
>     Width/Height      : 1280/1024
>     Pixel Format      : 'YUYV' (YUYV 4:2:2)
>     Field             : None
>     Bytes per Line    : 2560
>     Size Image        : 2621440
>     Colorspace        : Default
>     Transfer Function : Default (maps to Rec. 709)
>     YCbCr/HSV Encoding: Default (maps to ITU-R 601)
>     Quantization      : Default (maps to Limited Range)
>     Flags             :
>
> And this is not what I want. What I want is for GStreamer to use
> already set format and not change it. How I can accomplish that
> programmatically (gst-launch-1.0 was just an example, I'm primarily
> interested in doing this in code)?
>

When it reach ready state, you can get through properties the device fd,
you can then configure a caps filter to preserve the last format used.

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