Snapshotting frames from a playbin instance

[James Felix Black]

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.

Cheers!

[1]https://github.com/detrout/qt-gstreamer/tree/master/examples/player
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