How to shuttle GstMeta information between Python and C?

[Arjen Veenhuizen]

I have been having great difficulty grasping the concept of GstMeta and using it from Python. A minimal-not-working-example can be found here.

I created a basic pipeline ([videotestsrc] - [capsfilter] - [xvimagesink[) with a buffer probe on the capsfilter src pad. My goal is to add GstVideoCropMeta to each passing buffer (in python). Xvimagesink already implements a get_meta for this metadata type so that should be easy to debug.

Unfortunately, I am unable to figure out how to actually store x, y, width and height in the GstVideoCropMeta instance and making xvimagesink to pick-up these values.

In C, things look fairly straightforward:

    smeta = (GstVideoCropMeta *) meta;
    dmeta = gst_buffer_add_video_crop_meta (dest);
    if (!dmeta)
      return FALSE;

    GST_DEBUG ("copy crop metadata");
    dmeta->x = smeta->x;
    dmeta->y = smeta->y;
    dmeta->width = smeta->width;
    dmeta->height = smeta->height;

Implying that, after doing the gst_buffer_add_video_crop_meta, I should store the x, y, width and height directly on its return value (a Gst.Meta). But that's doesn't really make sense right?

Setting GST_DEBUG=xvimagesink*:8, I can see that xvimagesink is picking up the metadata, but x, y, width and height are still 0 (or worse, are sometimes random numbers from uninitialized memory locations making things crash and burn).

I have been reading the docs a dozen times but I am still a bit lost.

[Arjen Veenhuizen]

It turns out this is actually quite an old bug which was mentioned in this message on the Gstreamer list in November 2014. In short, we cannot assign values to a GstMeta object in python and recover them in C. The reverse, however, is possible using some tweaks.

I came up with the following work-around.

First of all, use the latest gobject introspection bindings when compiling GStreamer from source. For this, you need to install:

sudo apt-get install gobject-introspection libgirepository1.0-dev

Also, add --enable-introspection when running autogen.sh when compiling the gstreamer packages

autogen.sh  --enable-introspection

and make sure you install the .typelib files in the proper folder.

You can use

python -c 'from gi.repository import Gst; print Gst'

to check the location of the typelib files in use by pygi.

Based on bug #702921 I was able to add a couple of publicly accessible methods (gst_buffer_get_video_meta and gst_buffer_set_video_meta) to gstvideometa.c and gstvideometa.h (gst-plugins-base) which allow me to explicitly set and get the contents of a GstVideoCropMeta metadata instance in the C environment.

In python you can then use:

from gi.repository import Gst, GstVideo
# Add metadata to GstBuffer
gstMeta = buffer.add_meta(GstVideo.VideoCropMeta.get_info(), 0)
# Set metadata values to GstMeta
GstVideo.buffer_set_video_crop_meta(gstMeta, 10, 20, 100, 200)
# To verify whether we really wrote the metadata, we retrieve a fresh reference to the metadata on the GstBuffer
gstVideoVideoCropMeta = GstVideo.buffer_get_video_crop_meta(buffer)
# Should print 10
print gstVideoVideoCropMeta.x

Note, as Stian Stelnes already mentioned in the bugreport above, the publicly accessible methods must be functions, not macros!

[Tim Müller]

On Mon, 2016-04-11 at 07:34 -0700, Arjen Veenhuizen wrote:

Hi Arjen,

So maybe we need some setter functions for meta(s) then?

 Cheers
  -Tim

--
Tim Müller, Centricular Ltd - http://www.centricular.com


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[Arjen Veenhuizen]

Yes, I just filed a patch for setting the MetaStruct's fields to 0 on init, see.

I added a patch for the setter and getter as well (see #764902)