Unable to set Custom Metadata in Buffer from Python

Application Development
1

Hi,

I’m unsure if this is a GStreamer issue or a GObject issue.

I’m trying to set custom metadata to buffers so that I can read them down in the pipeline in a different element. I’m using the buffer.add_custom_meta + custom_meta.get_structure construct to get and set my metadata into the structure, but when I get + read the structure in another element the data is not there. I suspect the get_structure call returns a copy of the underlying structure because if I get and set the structure using ctypes then the data is correctly set.

Here is a small example of what I’m doing.

Upstream:

custom_meta = buffer.add_custom_meta("custom_meta_name_here")
meta_structure = custom_meta.get_structure()
meta_structure.set_value("value_name", 0)

Downstream:

custom_meta = buffer.get_custom_meta("custom_meta_name_here")
meta_structure = custom_meta.get_structure()
value = meta_structure.get_value("value_name") # or get_int, get_<type>, doesn't matter, the data isn't there

If in the upstream element I call gst_custom_meta_get_structure, and gst_structure_set_value using ctypes, then the downstream element is able to read the data from the struct.

I’m guessing I’m receiving a copy of the structure in python as hash(meta_structure) and the address of the pointer returned by gst_custom_meta_get_structure are different.

Thanks.

2 Likes
2

Hi, I also tried this approach and I can reproduce this behavior. When writing the metadata I am never able to read them back with the python interface.

This is a BaseTransform implementation to write metadata and a python script with a probe on the fakesink to read the written metadata.

I cannot read the metadata neither from the BaseTransform itself, just after writing them, nor from the probe in a downstream element.

add_custom_meta.py:

import gi
import random

gi.require_version('Gst', '1.0')
gi.require_version('GstBase', '1.0')
gi.require_version('GstVideo', '1.0')

from gi.repository import Gst, GObject, GstBase, GstVideo

import numpy as np

Gst.init(None)
FIXED_CAPS = Gst.Caps.from_string('video/x-raw,format=I420,width=[1,2147483647],height=[1,2147483647]')

class ExampleTransform(GstBase.BaseTransform):
    __gstmetadata__ = ('Example','Transform', 'Example', 'example')
    __gsttemplates__ = (Gst.PadTemplate.new("src", Gst.PadDirection.SRC, Gst.PadPresence.ALWAYS, FIXED_CAPS),
                        Gst.PadTemplate.new("sink", Gst.PadDirection.SINK, Gst.PadPresence.ALWAYS, FIXED_CAPS))

    def do_transform_ip(self, buffer):
        try:
            # Step 1: write metadata
            custom_meta = buffer.add_custom_meta("my_custom_meta")
            meta_structure = custom_meta.get_structure()
            value = random.randint(0, 1000)
            meta_structure.set_value("value_name", value)


            # Step 2: try to read the just written metadata
            custom_meta2 = buffer.get_custom_meta("my_custom_meta")
            meta_structure2 = custom_meta2.get_structure()
            value2 = meta_structure2.get_value("value_name")

            # Structure 2 has a different address, it is a copy. The read value is None
            print(f"add_custom_meta: meta address  {hash(custom_meta)}")
            print(f"add_custom_meta: meta2 address {hash(custom_meta2)}")
            print(f"add_custom_meta: meta struct address  {hash(meta_structure)}")
            print(f"add_custom_meta: meta2 struct address {hash(meta_structure2)}")
            print(f"add_custom_meta: written value: {value} - read value {value2}")

            return Gst.FlowReturn.OK

        except Gst.MapError as e:
            Gst.error("Mapping error: %s" % e)
            return Gst.FlowReturn.ERROR

GObject.type_register(ExampleTransform)
__gstelementfactory__ = ("add_custom_meta", Gst.Rank.NONE, ExampleTransform)

custom_meta_bug_main.py:

import gi

gi.require_version("Gst", "1.0")
gi.require_version("GstApp", "1.0")
gi.require_version("GstVideo", "1.0")
from gi.repository import Gst, GstApp, GObject, GLib, GstVideo

Gst.init(None)

def transform_func(*args):
    print("in transform")
    return True

meta_info = Gst.meta_register_custom("my_custom_meta", ["tag"], transform_func, None)

def read_custom_meta_callback(pad, info):
    buffer = info.get_buffer()

    custom_meta = buffer.get_custom_meta("my_custom_meta")
    meta_structure = custom_meta.get_structure()
    value = meta_structure.get_value("value_name")

    print("--")
    print(f"probe callback: meta address  {hash(custom_meta)}")
    print(f"probe callback: meta struct address  {hash(meta_structure)}")
    print(f"probe callback: read value {value}")

    print("=============================")

    return Gst.PadProbeReturn.OK


def main():
    pipeline = Gst.parse_launch("videotestsrc ! video/x-raw,format=I420,width=1920,height=1080,framerate=30/1 ! add_custom_meta ! fakesink name=meta_reader")

    src_element = pipeline.get_by_name('meta_reader')
    srcpad = src_element.get_static_pad('sink')
    _ = srcpad.add_probe(Gst.PadProbeType.BUFFER, read_custom_meta_callback)

    mainloop = GLib.MainLoop()

    pipeline.set_state(Gst.State.PLAYING)
    mainloop.run()

if __name__ == "__main__":
    main()

And here is a sample of the output for an execution of the main script:

=============================
add_custom_meta: meta address  139861247934640
add_custom_meta: meta2 address 139861247934640
add_custom_meta: meta struct address  139861248072480
add_custom_meta: meta2 struct address 139861248072784
add_custom_meta: written value: 309 - read value None
--
probe callback: meta address  139861247934640
probe callback: meta struct address  139861248072480
probe callback: read value None
=============================
add_custom_meta: meta address  139861247934688
add_custom_meta: meta2 address 139861247934688
add_custom_meta: meta struct address  139861248072480
add_custom_meta: meta2 struct address 139861248072784
add_custom_meta: written value: 940 - read value None
--
probe callback: meta address  139861247934688
probe callback: meta struct address  139861248072480
probe callback: read value None
=============================
add_custom_meta: meta address  139861247934736
add_custom_meta: meta2 address 139861247934736
add_custom_meta: meta struct address  139861248072480
add_custom_meta: meta2 struct address 139861248072784
add_custom_meta: written value: 253 - read value None
--
probe callback: meta address  139861247934736
probe callback: meta struct address  139861248072480
probe callback: read value None
=============================
3

@ivan94fi thanks for your feedback!

Can someone from the GStreamer team point me in the right direction? As I said I suspect it could be a GObject bug but I’d like to exclude the possibility of a GStreamer bug first, an then posting an issue in the GObject repo.

Thanks.