mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-30 13:41:48 +00:00
147 lines
6 KiB
Text
147 lines
6 KiB
Text
Caps
|
|
----
|
|
|
|
Caps are lightweight refcounted objects describing media types.
|
|
They are composed of an array of GstStructures plus, optionally,
|
|
a GstCapsFeatures set for the GstStructure.
|
|
|
|
Caps are exposed on GstPadTemplates to describe all possible types a
|
|
given pad can handle. They are also stored in the registry along with
|
|
a description of the element.
|
|
|
|
Caps are exposed on the element pads via CAPS and ACCEPT_CAPS queries.
|
|
|
|
This function describes the possible types that the pad can handle or
|
|
produce (see part-pads.txt and part-negotiation.txt).
|
|
|
|
Various methods exist to work with the media types such as subtracting
|
|
or intersecting.
|
|
|
|
Operations
|
|
~~~~~~~~~~
|
|
Fixating
|
|
--------
|
|
Caps are fixed if they only contain a single structure and this
|
|
structure is fixed. A structure is fixed if none of the fields of the
|
|
structure is an unfixed type, for example a range, list or array.
|
|
|
|
For fixating caps only the first structure is kept as the order of
|
|
structures is meant to express the preferences for the different
|
|
structures. Afterwards, each unfixed field of this structure is set
|
|
to the value that makes most sense for the media format by the element
|
|
or pad implementation and then every remaining unfixed field is set to
|
|
an arbitrary value that is a subset of the unfixed field's values.
|
|
|
|
EMPTY caps are fixed caps and ANY caps are not. Caps with ANY caps features
|
|
are not fixed.
|
|
|
|
Subset
|
|
------
|
|
One caps "A" is a subset of another caps "B" if for each structure in
|
|
"A" there exists a structure in "B" that is a superset of the structure
|
|
in "A".
|
|
|
|
A structure "a" is the subset of a structure "b" if it has the same
|
|
structure name, the same caps features and each field in "b" exists
|
|
in "a" and the value of the field in "a" is a subset of the value of
|
|
the field in "b". "a" can have additional fields that are not in "b".
|
|
|
|
EMPTY caps are a subset of every other caps. Every caps are a subset of
|
|
ANY caps.
|
|
|
|
Equality
|
|
--------
|
|
Caps "A" and "B" are equal if "A" is a subset of "B" and "B" is a subset
|
|
of "A". This means that both caps are expressing the same possibilities
|
|
but their structures can still be different if they contain unfixed
|
|
fields.
|
|
|
|
Intersection
|
|
------------
|
|
The intersection of caps "A" and caps "B" are the caps that contain the
|
|
intersection of all their structures with each other.
|
|
|
|
The intersection of structure "a" and structure "b" is empty if their
|
|
structure name or their caps features are not equal, or if "a" and "b"
|
|
contain the same field but the intersection of both field values is empty.
|
|
If one structure contains a field that is not existing in the other
|
|
structure it will be copied over to the intersection with the same
|
|
value.
|
|
|
|
The intersection with ANY caps is always the other caps and the intersection
|
|
with EMPTY caps is always EMPTY.
|
|
|
|
Union
|
|
-----
|
|
The union of caps "A" and caps "B" are the caps that contain the union
|
|
of all their structures with each other.
|
|
|
|
The union of structure "a" and structure "b" are the two structures "a"
|
|
and "b" if the structure names or caps features are not equal. Otherwise,
|
|
the union is the structure that contains the union of each fields value.
|
|
If a field is only in one of the two structures it is not contained in
|
|
the union.
|
|
|
|
The union with ANY caps is always ANY and the union with EMPTY caps is
|
|
always the other caps.
|
|
|
|
Subtraction
|
|
-----------
|
|
The subtraction of caps "A" from caps "B" is the most generic subset
|
|
of "B" that has an empty intersection with "A" but only contains
|
|
structures with names and caps features that are existing in "B".
|
|
|
|
Basic Rules
|
|
~~~~~~~~~~~
|
|
|
|
Semantics of caps and their usage
|
|
---------------------------------
|
|
A caps can contain multiple structures, in which case any of the
|
|
structures would be acceptable. The structures are in the preferred
|
|
order of the creator of the caps, with the preferred structure being
|
|
first and during negotiation of caps this order should be considered to
|
|
select the most optimal structure.
|
|
|
|
Each of these structures has a name that specifies the media type, e.g.
|
|
"video/x-theora" to specify Theora video. Additional fields in the
|
|
structure add additional constraints and/or information about the media
|
|
type, like the width and height of a video frame, or the codec profile
|
|
that is used. These fields can be non-fixed (e.g. ranges) for non-fixed
|
|
caps but must be fixated to a fixed value during negotiation.
|
|
If a field is included in the caps returned by a pad via the CAPS query,
|
|
it imposes an additional constraint during negotiation. The caps in the
|
|
end must have this field with a value that is a subset of the non-fixed
|
|
value. Additional fields that are added in the negotiated caps give
|
|
additional information about the media but are treated as optional.
|
|
Information that can change for every buffer and is not relevant during
|
|
negotiation must not be stored inside the caps.
|
|
|
|
|
|
For each of the structures in caps it is possible to store caps
|
|
features. The caps features are expressing additional requirements
|
|
for a specific structure, and only structures with the same name _and_
|
|
equal caps features are considered compatible.
|
|
Caps features can be used to require a specific memory representation
|
|
or a specific meta to be set on buffers, for example a pad could require
|
|
for a specific structure that it is passed EGLImage memory or buffers with
|
|
the video meta.
|
|
If no caps features are provided for a structure, it is assumed that
|
|
system memory is required unless later negotiation steps (e.g. the
|
|
ALLOCATION query) detect that something else can be used. The special
|
|
ANY caps features can be used to specify that any caps feature would
|
|
be accepted, for example if the buffer memory is not touched at all.
|
|
|
|
|
|
Compatibility of caps
|
|
---------------------
|
|
Pads can be linked when the caps of both pads are compatible. This is
|
|
the case when their intersection is not empty.
|
|
|
|
For checking if a pad actually supports a fixed caps an intersection is
|
|
not enough. Instead the fixed caps must be at least a subset of the
|
|
pad's caps but pads can introduce additional constraints which would be
|
|
checked in the ACCEPT_CAPS query handler.
|
|
|
|
Data flow can only happen after pads have decided on common fixed caps.
|
|
These caps are distributed to both pads with the CAPS event.
|
|
|