Snapshotting frames from a playbin instance

[Thiago Santos]

On 08/19/2015 03:46 PM, James Felix Black wrote:
> On Wed, Aug 19, 2015, at 13:52, Thiago Santos wrote:
>> 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.
> I was missing jpegenc; thanks. Having installed it, I now get an 
> "Internal data flow error".
>
> default convertframe.c:325:gst_video_convert_sample:Could not convert 
> video frame: Internal data flow error.
>
> This is using a one-second timeout on the frame conversion 
> (GST_SECOND). Hmmm. Any pointers here?
I'd recommend using GST_DEBUG to get some log and check what exactly was 
the error.


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-- 
Thiago Sousa Santos
Senior Multimedia Engineer, Open Source Group
Samsung Research America - Silicon Valley

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