mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-25 09:40:37 +00:00
388 lines
15 KiB
Text
388 lines
15 KiB
Text
GstMeta
|
|
-------
|
|
|
|
This document describes the design for arbitrary per-buffer metadata.
|
|
|
|
Buffer metadata typically describes the lowlevel properties of the buffer
|
|
content. These properties are typically not negotiated with caps but they are
|
|
negotiated in the bufferpools.
|
|
|
|
Some examples of metadata:
|
|
|
|
- timestamp, duration
|
|
- offset, offset_end
|
|
- 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 formation when it blits the image on the screen
|
|
with little overhead.
|
|
|
|
|
|
GstMeta
|
|
~~~~~~~
|
|
|
|
A GstMeta is a structure as follows:
|
|
|
|
struct _GstMeta {
|
|
const GstMetaInfo *info; /* tag and info for the meta item */
|
|
};
|
|
|
|
The purpose of the 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 GstMetaVideoPlane {
|
|
gsize offset; /* offset in the buffer memory region of the
|
|
* first pixel. */
|
|
gint stride; /* stride of the image lines. Can be negative when
|
|
* the image is upside-down */
|
|
};
|
|
|
|
struct GstMetaVideo {
|
|
GstMeta meta
|
|
|
|
GstMetaVideoFlags flags
|
|
|
|
guint n_planes;
|
|
GstVideoPlane plane[GST_VIDEO_MAX_PLANES];
|
|
|
|
gpointer (*map) (GstMetaVideo *meta, guint plane, gint *stride,
|
|
GstMapflags flags);
|
|
gboolean (*unmap) (GstMetaVideo *meta, guint plane, gpointer data);
|
|
};
|
|
|
|
gpointer gst_meta_video_map (GstMetaVideo *meta, guint plane, gint *stride,
|
|
GstMapflags flags);
|
|
gboolean gst_meta_video_unmap (GstMetaVideo *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 {
|
|
GQuark api; /* api name */
|
|
GType type; /* implementation type */
|
|
gsize size; /* size of the structure */
|
|
|
|
GstMetaInitFunction init_func;
|
|
GstMetaFreeFunction free_func;
|
|
GstMetaCopyFunction copy_func;
|
|
GstMetaTransformFunction transform_func;
|
|
};
|
|
|
|
api will contain a GQuark 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 | caps, parent, pool |
|
|
+.....................................+
|
|
| next ---+
|
|
+- | info ------> GstMetaInfo
|
|
GstMetaTiming | | | |
|
|
| | dts | |
|
|
| | pts | |
|
|
| | duration | |
|
|
+- | clock_rate | |
|
|
+ . . . . . . . . . . . . . . . . . . + |
|
|
| next <--+
|
|
GstMetaVideo +- +- | info ------> GstMetaInfo
|
|
| | | | |
|
|
| | | flags | |
|
|
| | | n_planes | |
|
|
| | | planes[] | |
|
|
| | | map | |
|
|
| | | unmap | |
|
|
+- | | | |
|
|
| | private fields | |
|
|
GstMetaVideoImpl | | ... | |
|
|
| | ... | |
|
|
+- | | |
|
|
+ . . . . . . . . . . . . . . . . . . + .
|
|
. .
|
|
|
|
|
|
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 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 GstMetaVideo
|
|
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.
|
|
|
|
|
|
Compatibility
|
|
~~~~~~~~~~~~~
|
|
|
|
We need to make sure that elements exchange metadata that they both understand,
|
|
This is particulary important when the metadata describes the data layout in
|
|
memory (such as strides).
|
|
|
|
We would like to use the bufferpool negotiation system to negotiate the possible
|
|
metadata that can be exchanged between elements.
|
|
|
|
When deciding the allocation properties, we will also negotiate the buffer
|
|
metadata structures that we can exchange.
|
|
|
|
|
|
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.
|