Snapshotting frames from a playbin instance

[Thiago Santos]

On 08/19/2015 02:42 PM, James Felix Black wrote:
> Linux ARM (nVidia Jetson TK1), gstreamer-1.5.2, Qt 5.5, qt-streamer 1.2.0
> I have a simple X11 application that I'm working, based for now on the 
> qt-gstreamer demo code [1] that plays a video to the screen, and 
> snapshots "interesting" frames to disk. I'm using Qt and the 
> qt-gstreamer package, and I am able to access sample data directly off 
> the pipeline (using the "sample" property at a given time), and write 
> the raw video to disk as such:
>   QGlib::Value val = pipeline->property("sample");
>   GstSample * v = (GstSample *)g_value_get_boxed(val);
>   // write the contents of v's memory buffer to disk ....
> What I get is a raw file of 4bytes * 1920 * 1080. That's cool! But 
> what I want to do is:
>   GstCaps * jpegCaps = gst_caps_new_simple("image/jpeg", "width", 
> G_TYPE_INT, 640, NULL);
>   GError * err;
>   GstSample * jpeg = gst_video_convert_sample(v, jpegCaps, 100, &err);
> ... and then write the contents of *that* sample to disk, instead. 
> However, if I do that conversion, jpeg is null and the error is:
> "Cannot find any image encoder for caps image/jpeg"
> OK, that seems fair. But my pipeline (again see the example code at 
> [1]) is constructed as such:
>         m_pipeline = 
> QGst::ElementFactory::make("playbin").dynamicCast<QGst::Pipeline>();
> My question then becomes: how should I approach this problem? Should I 
> modify the pipeline? I see that there's a "convert-sample" signal on 
> playbin;  is that what I should be using?
> Any light anyone can shed would be most welcome.

Do you have jpegenc (or any other jpeg encoder) in your system? 
gst-inspect-1.0 jpegenc should show it. Or gst-inspect-1.0 |grep jpeg.



> Cheers!
> [1] https://github.com/detrout/qt-gstreamer/tree/master/examples/player
>
>
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-- 
Thiago Sousa Santos
Senior Multimedia Engineer, Open Source Group
Samsung Research America - Silicon Valley

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