Add per_buffer metadata as VideoCaptionMeta for ndisink and ndisrc

[darksidds]

Tested under both Linux and Windows.

[sdroege]

The caption meta is meant for storing CEA608/708 captions, not arbitrary strings. What kind of metadata is it that you're trying to store here?

You probably want to implement your own GstMeta for this.

[darksidds]

I'm trying to store an arbitrary string. Am I supposed to create a new implementation for GstMeta and call it something like NdiPerFrameMeta?

[sdroege]

Yeah, or with GStreamer 1.20 you can also use the stringly-typed "custom meta" API.

[sdroege]

However there's NDI metadata that maps directly to existing GstMetas, for example for closed captions.

[darksidds]

Yeah, or with GStreamer 1.20 you can also use the stringly-typed "custom meta" API.

Unfortunatelly, GStreamer 1.20 is not out there yet. And probably it would be nice to have a dedicated class for ndi per-buffer metadata. How do you think?

However there's NDI metadata that maps directly to existing GstMetas, for example for closed captions.

Could you please clarify me a couple of things. I would appreciate that a lot:

1. If NDI metadata is mapped directly to existing GstMetas, then how do I access it in my c++ pipelines?

2. Let's say I have a GstBuffer object, and I'm trying to iterate all GstMeta objects stored in it. As it's shown below, I use gst_buffer_iterate_meta function and compare gst_meta->info with known gst_meta_get_info("SomeName") or gotten directly from functions like gst_video_caption_meta_get_info(). But if I don't know what implementation of GstMeta to expect, then I can't access it or its fields. So, how do I do that?

while ((gst_meta = gst_buffer_iterate_meta(buffer, &state))) {
    if (gst_meta->info == gst_video_caption_meta_get_info()) {
        auto video_caption = (GstVideoCaptionMeta *) gst_meta;
        // do stuff
    } else if (gst_meta->info == gst_video_time_code_meta_get_info()) {
        auto timecode_meta = (GstVideoTimeCodeMeta *) gst_meta;
        // do stuff
    } else if (gst_meta->info == gst_meta_get_info("GstNdiSrcMeta")) {
        // do stuff
    } 
}

3. Is there a way to list all registered GstMeta implentations?

[sdroege]

1. If NDI metadata is mapped directly to existing GstMetas, then how do I access it in my c++ pipelines?

By using the relevant GStreamer APIs for those metas. I'm not sure I understand the question :)

2\. Let's say I have a `GstBuffer` object, and I'm trying to iterate all `GstMeta` objects stored in it. As it's shown below, I use `gst_buffer_iterate_meta` function and compare `gst_meta->info` with known `gst_meta_get_info("SomeName")` or gotten directly from functions like `gst_video_caption_meta_get_info()`. But if I don't know what implementation of `GstMeta` to expect, then I can't access it or its fields. So, how do I do that?

You have to know the meta for being able to access its fields. You need to call the corresponding C API or e.g. make use of the public struct definition from the C headers.

Is there a way to list all registered GstMeta implentations?

With some effort, yes. But you can't really do anything with them unless you explicitly know how to handle each.

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I think there's some misunderstanding on your side how this API works and is supposed to be used. Maybe I can give some useful suggestion if we take a step back and you explain what exactly you're trying to do.