The vpx multi-format codec SDK provides a unified interface amongst its supported codecs. This abstraction allows applications using this SDK to easily support multiple video formats with minimal code duplication or "special casing." This section describes the interface common to all codecs. For codec-specific details, see the Supported Codecs page.
The following sections are common to all codecs:
Fore more information on decoder and encoder specific usage, see the following pages:
There are two important data structures to consider in this interface.
A context is a storage area allocated by the calling application that the codec may write into to store details about a single instance of that codec. Most of the context is implementation specific, and thus opaque to the application. The context structure as seen by the application is of fixed size, and thus can be allocated with automatic storage or dynamically on the heap.
Most operations require an initialized codec context. Codec context instances are codec specific. That is, the codec to be used for the encoded video must be known at initialization time. See vpx_codec_ctx_t for further information.
A codec interface is an opaque structure that controls how function calls into the generic interface are dispatched to their codec-specific implementations. Applications MUST NOT attempt to examine or override this storage, as it contains internal implementation details likely to change from release to release.
Each supported codec will expose an interface structure to the
application as an
extern reference to a structure of
the incomplete type vpx_codec_iface_t.
Several "features" are defined that are optionally implemented by codec algorithms. Indeed, the same algorithm may support different features on different platforms. The purpose of defining these features is that when they are implemented, they conform to a common interface. The features, or capabilities, of an algorithm can be queried from it's interface by using the vpx_codec_get_caps() method. Attempts to invoke features not supported by an algorithm will generally result in VPX_CODEC_INCAPABLE.
Currently defined features available in both encoders and decoders include:
Currently defined decoder features include:
To initialize a codec instance, the address of the codec context and interface structures are passed to an initialization function. Depending on the Features that the codec supports, the codec could be initialized in different modes. Most notably, the application may choose to use External Memory Allocation mode to gain fine grained control over how and where memory is allocated for the codec.
To prevent cases of confusion where the ABI of the library changes, the ABI is versioned. The ABI version number must be passed at initialization time to ensure the application is using a header file that matches the library. The current ABI version number is stored in the preprocessor macros VPX_CODEC_ABI_VERSION, VPX_ENCODER_ABI_VERSION, and VPX_DECODER_ABI_VERSION. For convenience, each initialization function has a wrapper macro that inserts the correct version number. These macros are named like the initialization methods, but without the _ver suffix.
The available initialization methods are:
Almost all codec functions return an error status of type
vpx_codec_err_t. The semantics of how
each error condition should be processed is clearly defined in the
definitions of each enumerated value. Error values can be converted
into ASCII strings with the vpx_codec_error()
methods. The difference between these two methods is that vpx_codec_error()
returns the error state from an initialized context, whereas
can be used in cases where an error occurs outside any context. The
enumerated value returned from the last call can be retrieved from
err member of the decoder context as well.
Finally, more detailed error information may be able to be obtained
by using the vpx_codec_error_detail()
method. Not all errors produce detailed error information.
In addition to error information, the codec library's build configuration is available at runtime on some platforms. This information can be returned by calling vpx_codec_build_config(), and is formatted as a base64 coded string (comprised of characters in the set [a-z_a-Z0-9+/]). This information is not useful to an application at runtime, but may be of use to vpx for support.
Both the encoding and decoding functions have a
deadline parameter. This parameter indicates the
amount of time, in microseconds (us), that the application wants
the codec to spend processing before returning. This is a soft
deadline -- that is, the semantics of the requested operation take
precedence over meeting the deadline. If, for example, an
application sets a
deadline of 1000us, and the frame
takes 2000us to decode, the call to vpx_codec_decode() will return after 2000us. In
this case the deadline is not met, but the semantics of the
function are preserved. If, for the same frame, an application
instead sets a
deadline of 5000us, the decoder will
see that it has 3000us remaining in its time slice when decoding
completes. It could then choose to run a set of Postprocessing filters, and
perhaps would return after 4000us (instead of the allocated
5000us). In this case the deadline is met, and the semantics of the
call are preserved, as before.
The special value
0 is reserved to represent an
infinite deadline. In this case, the codec will perform as much
processing as possible to yield the highest quality frame.
By convention, the value
1 is used to mean "return
as fast as possible."