1/*!\page usage Usage
2
3    The vpx multi-format codec SDK provides a unified interface amongst its
4    supported codecs. This abstraction allows applications using this SDK to
5    easily support multiple video formats with minimal code duplication or
6    "special casing." This section describes the interface common to all codecs.
7    For codec-specific details, see the \ref codecs page.
8
9    The following sections are common to all codecs:
10    - \ref usage_types
11    - \ref usage_features
12    - \ref usage_init
13    - \ref usage_errors
14
15    Fore more information on decoder and encoder specific usage, see the
16    following pages:
17    \if decoder
18    - \subpage usage_decode
19    \endif
20    \if decoder
21    - \subpage usage_encode
22    \endif
23
24    \section usage_types Important Data Types
25    There are two important data structures to consider in this interface.
26
27    \subsection usage_ctxs Contexts
28    A context is a storage area allocated by the calling application that the
29    codec may write into to store details about a single instance of that codec.
30    Most of the context is implementation specific, and thus opaque to the
31    application. The context structure as seen by the application is of fixed
32    size, and thus can be allocated with automatic storage or dynamically
33    on the heap.
34
35    Most operations require an initialized codec context. Codec context
36    instances are codec specific. That is, the codec to be used for the encoded
37    video must be known at initialization time. See #vpx_codec_ctx_t for further
38    information.
39
40    \subsection usage_ifaces Interfaces
41    A codec interface is an opaque structure that controls how function calls
42    into the generic interface are dispatched to their codec-specific
43    implementations. Applications \ref MUSTNOT attempt to examine or override
44    this storage, as it contains internal implementation details likely to
45    change from release to release.
46
47    Each supported codec will expose an interface structure to the application
48    as an <code>extern</code> reference to a structure of the incomplete type
49    #vpx_codec_iface_t.
50
51    \section usage_features Features
52    Several "features" are defined that are optionally implemented by codec
53    algorithms. Indeed, the same algorithm may support different features on
54    different platforms. The purpose of defining these features is that when
55    they are implemented, they conform to a common interface. The features, or
56    capabilities, of an algorithm can be queried from it's interface by using
57    the vpx_codec_get_caps() method. Attempts to invoke features not supported
58    by an algorithm will generally result in #VPX_CODEC_INCAPABLE.
59
60    Currently defined features available in both encoders and decoders include:
61    - \subpage usage_xma
62
63    \if decoder
64    Currently defined decoder features include:
65    - \ref usage_cb
66    - \ref usage_postproc
67    \endif
68
69    \section usage_init Initialization
70    To initialize a codec instance, the address of the codec context
71    and interface structures are passed to an initialization function. Depending
72    on the \ref usage_features that the codec supports, the codec could be
73    initialized in different modes. Most notably, the application may choose to
74    use \ref usage_xma mode to gain fine grained control over how and where
75    memory is allocated for the codec.
76
77    To prevent cases of confusion where the ABI of the library changes,
78    the ABI is versioned. The ABI version number must be passed at
79    initialization time to ensure the application is using a header file that
80    matches the library. The current ABI version number is stored in the
81    preprocessor macros #VPX_CODEC_ABI_VERSION, #VPX_ENCODER_ABI_VERSION, and
82    #VPX_DECODER_ABI_VERSION. For convenience, each initialization function has
83    a wrapper macro that inserts the correct version number. These macros are
84    named like the initialization methods, but without the _ver suffix.
85
86
87    The available initialization methods are:
88    \if encoder - #vpx_codec_enc_init (calls vpx_codec_enc_init_ver()) \endif
89    \if multi-encoder - #vpx_codec_enc_init_multi (calls vpx_codec_enc_init_multi_ver()) \endif
90    \if decoder - #vpx_codec_dec_init (calls vpx_codec_dec_init_ver()) \endif
91
92
93
94    \section usage_errors Error Handling
95    Almost all codec functions return an error status of type #vpx_codec_err_t.
96    The semantics of how each error condition should be processed is clearly
97    defined in the definitions of each enumerated value. Error values can be
98    converted into ASCII strings with the vpx_codec_error() and
99    vpx_codec_err_to_string() methods. The difference between these two methods is
100    that vpx_codec_error() returns the error state from an initialized context,
101    whereas vpx_codec_err_to_string() can be used in cases where an error occurs
102    outside any context. The enumerated value returned from the last call can be
103    retrieved from the <code>err</code> member of the decoder context as well.
104    Finally, more detailed error information may be able to be obtained by using
105    the vpx_codec_error_detail() method. Not all errors produce detailed error
106    information.
107
108    In addition to error information, the codec library's build configuration
109    is available at runtime on some platforms. This information can be returned
110    by calling vpx_codec_build_config(), and is formatted as a base64 coded string
111    (comprised of characters in the set [a-z_a-Z0-9+/]). This information is not
112    useful to an application at runtime, but may be of use to vpx for support.
113
114
115    \section usage_deadline Deadline
116    Both the encoding and decoding functions have a <code>deadline</code>
117    parameter. This parameter indicates the amount of time, in microseconds
118    (us), that the application wants the codec to spend processing before
119    returning. This is a soft deadline -- that is, the semantics of the
120    requested operation take precedence over meeting the deadline. If, for
121    example, an application sets a <code>deadline</code> of 1000us, and the
122    frame takes 2000us to decode, the call to vpx_codec_decode() will return
123    after 2000us. In this case the deadline is not met, but the semantics of the
124    function are preserved. If, for the same frame, an application instead sets
125    a <code>deadline</code> of 5000us, the decoder will see that it has 3000us
126    remaining in its time slice when decoding completes. It could then choose to
127    run a set of \ref usage_postproc filters, and perhaps would return after
128    4000us (instead of the allocated 5000us). In this case the deadline is met,
129    and the semantics of the call are preserved, as before.
130
131    The special value <code>0</code> is reserved to represent an infinite
132    deadline. In this case, the codec will perform as much processing as
133    possible to yield the highest quality frame.
134
135    By convention, the value <code>1</code> is used to mean "return as fast as
136    possible."
137
138*/
139
140
141/*! \page usage_xma External Memory Allocation
142    Applications that wish to have fine grained control over how and where
143    decoders allocate memory \ref MAY make use of the eXternal Memory Allocation
144    (XMA) interface. Not all codecs support the XMA \ref usage_features.
145
146    To use a decoder in XMA mode, the decoder \ref MUST be initialized with the
147    vpx_codec_xma_init_ver() function. The amount of memory a decoder needs to
148    allocate is heavily dependent on the size of the encoded video frames. The
149    size of the video must be known before requesting the decoder's memory map.
150    This stream information can be obtained with the vpx_codec_peek_stream_info()
151    function, which does not require a constructed decoder context. If the exact
152    stream is not known, a stream info structure can be created that reflects
153    the maximum size that the decoder instance is required to support.
154
155    Once the decoder instance has been initialized and the stream information
156    determined, the application calls the vpx_codec_get_mem_map() iterator
157    repeatedly to get a list of the memory segments requested by the decoder.
158    The iterator value should be initialized to NULL to request the first
159    element, and the function will return #VPX_CODEC_LIST_END to signal the end of
160    the list.
161
162    After each segment is identified, it must be passed to the codec through the
163    vpx_codec_set_mem_map() function. Segments \ref MUST be passed in the same
164    order as they are returned from vpx_codec_get_mem_map(), but there is no
165    requirement that vpx_codec_get_mem_map() must finish iterating before
166    vpx_codec_set_mem_map() is called. For instance, some applications may choose
167    to get a list of all requests, construct an optimal heap, and then set all
168    maps at once with one call. Other applications may set one map at a time,
169    allocating it immediately after it is returned from vpx_codec_get_mem_map().
170
171    After all segments have been set using vpx_codec_set_mem_map(), the codec may
172    be used as it would be in normal internal allocation mode.
173
174    \section usage_xma_seg_id Segment Identifiers
175    Each requested segment is identified by an identifier unique to
176    that decoder type. Some of these identifiers are private, while others are
177    enumerated for application use. Identifiers not enumerated publicly are
178    subject to change. Identifiers are non-consecutive.
179
180    \section usage_xma_seg_szalign Segment Size and Alignment
181    The sz (size) and align (alignment) parameters describe the required size
182    and alignment of the requested segment. Alignment will always be a power of
183    two. Applications \ref MUST honor the alignment requested. Failure to do so
184    could result in program crashes or may incur a speed penalty.
185
186    \section usage_xma_seg_flags Segment Flags
187    The flags member of the segment structure indicates any requirements or
188    desires of the codec for the particular segment. The #VPX_CODEC_MEM_ZERO flag
189    indicates that the segment \ref MUST be zeroed by the application prior to
190    passing it to the application. The #VPX_CODEC_MEM_WRONLY flag indicates that
191    the segment will only be written into by the decoder, not read. If this flag
192    is not set, the application \ref MUST insure that the memory segment is
193    readable. On some platforms, framebuffer memory is writable but not
194    readable, for example. The #VPX_CODEC_MEM_FAST flag indicates that the segment
195    will be frequently accessed, and that it should be placed into fast memory,
196    if any is available. The application \ref MAY choose to place other segments
197    in fast memory as well, but the most critical segments will be identified by
198    this flag.
199
200    \section usage_xma_seg_basedtor Segment Base Address and Destructor
201    For each requested memory segment, the application must determine the
202    address of a memory segment that meets the requirements of the codec. This
203    address is set in the <code>base</code> member of the #vpx_codec_mmap
204    structure. If the application requires processing when the segment is no
205    longer used by the codec (for instance to deallocate it or close an
206    associated file descriptor) the <code>dtor</code> and <code>priv</code>
207    members can be set.
208*/
209