GstMeta

This document describes the design for arbitrary per-buffer metadata.

Buffer metadata typically describes the low level properties of the buffer content. These properties are commonly not negotiated with caps but they are negotiated in the bufferpools.

Some examples of metadata:

interlacing information
video alignment, cropping, panning information
extra container information such as granulepos, …
extra global buffer properties

Requirements

It must be fast
- allocation, free, low fragmentation
- access to the metadata fields, preferably not much slower than directly accessing a C structure field
It must be extensible. Elements should be able to add new arbitrary metadata without requiring much effort. Also new metadata fields should not break API or ABI.
It plays nice with subbuffers. When a subbuffer is created, the various buffer metadata should be copied/updated correctly.
We should be able to negotiate metadata between elements

Use cases

Video planes: Video data is sometimes allocated in non-contiguous planes for the Y and the UV data. We need to be able to specify the data on a buffer using multiple pointers in memory. We also need to be able to specify the stride for these planes.
Extra buffer data: Some elements might need to store extra data for a buffer. This is typically done when the resources are allocated from another subsystem such as OMX or X11.
Processing information: Pan and crop information can be added to the buffer data when the downstream element can understand and use this metadata. An imagesink can, for example, use the pan and cropping information when blitting the image on the screen with little overhead.

GstMeta

A GstMeta is a structure as follows:

struct _GstMeta {
  GstMetaFlags       flags;
  const GstMetaInfo *info;    /* tag and info for the meta item */
};

The purpose of this structure is to serve as a common header for all metadata information that we can attach to a buffer. Specific metadata, such as timing metadata, will have this structure as the first field. For example:

struct _GstMetaTiming {
  GstMeta        meta;        /* common meta header */

  GstClockTime   dts;         /* decoding timestamp */
  GstClockTime   pts;         /* presentation timestamp */
  GstClockTime   duration;    /* duration of the data */
  GstClockTime   clock_rate;  /* clock rate for the above values */
};

Or another example for the video memory regions that consists of both fields and methods.

#define GST_VIDEO_MAX_PLANES 4

struct GstVideoMeta {
  GstMeta       meta;

  GstBuffer         *buffer;

  GstVideoFlags      flags;
  GstVideoFormat     format;
  guint              id
  guint              width;
  guint              height;

  guint              n_planes;
  gsize              offset[GST_VIDEO_MAX_PLANES];   /* offset in the buffer memory region of the
                                                    * first pixel. */
  gint               stride[GST_VIDEO_MAX_PLANES];   /* stride of the image lines. Can be negative when
                                                    * the image is upside-down */

  gpointer (*map)    (GstVideoMeta *meta, guint plane, gpointer * data, gint *stride,
                      GstMapFlags flags);
  gboolean (*unmap)  (GstVideoMeta *meta, guint plane, gpointer data);
};

gpointer gst_meta_video_map   (GstVideoMeta *meta, guint plane, gpointer * data,
                               gint *stride, GstMapflags flags);
gboolean gst_meta_video_unmap (GstVideoMeta *meta, guint plane, gpointer data);

GstMeta derived structures define the API of the metadata. The API can consist of fields and/or methods. It is possible to have different implementations for the same GstMeta structure.

The implementation of the GstMeta API would typically add more fields to the public structure that allow it to implement the API.

GstMetaInfo will point to more information about the metadata and looks like this:

struct _GstMetaInfo {
  GType                      api;       /* api type */
  GType                      type;      /* implementation type */
  gsize                      size;      /* size of the structure */

  GstMetaInitFunction        init_func;
  GstMetaFreeFunction        free_func;
  GstMetaTransformFunction   transform_func;
};

The api member will contain a GType of the metadata API. A repository of registered MetaInfo will be maintained by the core. We will register some common metadata structures in core and some media specific info for audio/video/text in -base. Plugins can register additional custom metadata.

For each implementation of api, there will thus be a unique GstMetaInfo. In the case of metadata with a well defined API, the implementation specific init function will setup the methods in the metadata structure. A unique GType will be made for each implementation and stored in the type field.

Along with the metadata description we will have functions to initialize/free (and/or refcount) a specific GstMeta instance. We also have the possibility to add a custom transform function that can be used to modify the metadata when a transformation happens.

There are no explicit methods to serialize and deserialize the metadata. Since each type has a GType, we can reuse the GValue transform functions for this.

The purpose of the separate MetaInfo is to not have to carry the free/init functions in each buffer instance but to define them globally. We still want quick access to the info so we need to make the buffer metadata point to the info.

Technically we could also specify the field and types in the MetaInfo and provide a generic API to retrieve the metadata fields without the need for a header file. We will not do this yet.

Allocation of the GstBuffer structure will result in the allocation of a memory region of a customizable size (512 bytes). Only the first sizeof (GstBuffer) bytes of this region will initially be used. The remaining bytes will be part of the free metadata region of the buffer. Different implementations are possible and are invisible in the API or ABI.

The complete buffer with metadata could, for example, look as follows:

                         +----------------------------------+
GstMiniObject            |  GType (GstBuffer)               |
                         |  refcount, flags, copy/disp/free |
                         +----------------------------------+
GstBuffer                |  pool,pts,dts,duration,offsets   |
                         |  <private data>                  |
                         +..................................+
                         |  next                           ---+
                      +- |  info                           ------> GstMetaInfo
GstMetaTiming         |  |                                  | |
                      |  |  dts                             | |
                      |  |  pts                             | |
                      |  |  duration                        | |
                      +- |  clock_rate                      | |
                         +  . . . . . . . . . . . . . . . . + |
                         |  next                           <--+
GstVideoMeta       +- +- |  info                           ------> GstMetaInfo
                   |  |  |                                  | |
                   |  |  |  flags                           | |
                   |  |  |  n_planes                        | |
                   |  |  |  planes[]                        | |
                   |  |  |  map                             | |
                   |  |  |  unmap                           | |
                   +- |  |                                  | |
                      |  |  private fields                  | |
GstVideoMetaImpl      |  |  ...                             | |
                      |  |  ...                             | |
                      +- |                                  | |
                         +  . . . . . . . . . . . . . . . . + .
                         .                                    .

API examples

Buffers are created using the normal gst_buffer_new() functions. The standard fields are initialized as usual. A memory area that is bigger than the structure size is allocated for the buffer metadata.

gst_buffer_new ();

After creating a buffer, the application can set caps and add metadata information.

To add or retrieve metadata, a handle to a GstMetaInfo structure needs to be obtained. This defines the implementation and API of the metadata. Usually, a handle to this info structure can be obtained by calling a public _get_info() method from a shared library (for shared metadata).

The following defines can usually be found in the shared .h file.

GstMetaInfo * gst_meta_timing_get_info();
#define GST_META_TIMING_INFO  (gst_meta_timing_get_info())

Adding metadata to a buffer can be done with the gst_buffer_add_meta() call. This function will create new metadata based on the implementation specified by the GstMetaInfo. It is also possible to pass a generic pointer to the add_meta() function that can contain parameters to initialize the new metadata fields.

Retrieving the metadata on a buffer can be done with the gst_buffer_meta_get() method. This function retrieves an existing metadata conforming to the API specified in the given info. When no such metadata exists, the function will return NULL.

GstMetaTiming *timing;

timing = gst_buffer_get_meta (buffer, GST_META_TIMING_INFO);

Once a reference to the info has been obtained, the associated metadata can be added or modified on a buffer.

timing->timestamp = 0;
timing->duration = 20 * GST_MSECOND;

Other convenience macros can be made to simplify the above code:

#define gst_buffer_get_meta_timing(b) \
   ((GstMetaTiming *) gst_buffer_get_meta ((b), GST_META_TIMING_INFO)

This makes the code look like this:

GstMetaTiming *timing;

timing = gst_buffer_get_meta_timing (buffer);
timing->timestamp = 0;
timing->duration = 20 * GST_MSECOND;

To iterate the different metainfo structures, one can use the gst_buffer_meta_get_next() methods.

GstMeta *current = NULL;

/* passing NULL gives the first entry */
current = gst_buffer_meta_get_next (buffer, current);

/* passing a GstMeta returns the next */
current = gst_buffer_meta_get_next (buffer, current);

Memory management

allocation

We initially allocate a reasonable sized GstBuffer structure (say 512 bytes).

Since the complete buffer structure, including a large area for metadata, is allocated in one go, we can reduce the number of memory allocations while still providing dynamic metadata.

When adding metadata, we need to call the init function of the associated metadata info structure. Since adding the metadata requires the caller to pass a handle to the info, this operation does not require table lookups.

Per-metadata memory initialisation is needed because not all metadata is initialized in the same way. We need to, for example, set the timestamps to NONE in the MetaTiming structures.

The init/free functions can also be used to implement refcounting for a metadata structure. This can be useful when a structure is shared between buffers.

When the free_size of the GstBuffer is exhausted, we will allocate new memory for each newly added Meta and use the next pointers to point to this. It is expected that this does not occur often and we might be able to optimize this transparently in the future.

free

When a GstBuffer is freed, we potentially might have to call a custom free() function on the metadata info. In the case of the Memory metadata, we need to call the associated free() function to free the memory.

When freeing a GstBuffer, the custom buffer free function will iterate all of the metadata in the buffer and call the associated free functions in the MetaInfo associated with the entries. Usually, this function will be NULL.

Serialization

When a buffer should be sent over the wire or be serialized in GDP, we need a way to perform custom serialization and deserialization on the metadata. For this we can use the GValue transform functions.

Transformations

After certain transformations, the metadata on a buffer might not be relevant anymore.

Consider, for example, metadata that lists certain regions of interest on the video data. If the video is scaled or rotated, the coordinates might not make sense anymore. A transform element should be able to adjust or remove the associated metadata when it becomes invalid.

We can make the transform element aware of the metadata so that it can adjust or remove in an intelligent way. Since we allow arbitrary metadata, we can’t do this for all metadata and thus we need some other way.

One proposition is to tag the metadata type with keywords that specify what it functionally refers too. We could, for example, tag the metadata for the regions of interest with a tag that notes that the metadata refers to absolute pixel positions. A transform could then know that the metadata is not valid anymore when the position of the pixels changed (due to rotation, flipping, scaling and so on).

Subbuffers

Subbuffers are implemented with a generic copy. Parameters to the copy are the offset and size. This allows each metadata structure to implement the actions needed to update the metadata of the subbuffer.

It might not make sense for some metadata to work with subbuffers. For example when we take a subbuffer of a buffer with a video frame, the GstVideoMeta simply becomes invalid and is removed from the new subbuffer.

Relationship with GstCaps

The difference between GstCaps, used in negotiation, and the metadata is not clearly defined.

We would like to think of the GstCaps containing the information needed to functionally negotiate the format between two elements. The Metadata should then only contain variables that can change between each buffer.

For example, for video we would have width/height/framerate in the caps but then have the more technical details, such as stride, data pointers, pan/crop/zoom etc in the metadata.

A scheme like this would still allow us to functionally specify the desired video resolution while the implementation details would be inside the metadata.

Relationship with GstMiniObject qdata

qdata on a miniobject is element private and is not visible to other element. Therefore qdata never contains essential information that describes the buffer content.

Compatibility

We need to make sure that elements exchange metadata that they both understand, This is particularly important when the metadata describes the data layout in memory (such as strides).

The ALLOCATION query is used to let upstream know what metadata we can support.

It is also possible to have a bufferpool add certain metadata to the buffers from the pool. This feature is activated by enabling a buffer option when configuring the pool.

Notes

Some structures that we need to be able to add to buffers.

Clean Aperture
Arbitrary Matrix Transform
Aspect ratio
Pan/crop/zoom
Video strides

Some of these overlap, we need to find a minimal set of metadata structures that allows us to define all use cases.

The results of the search are

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