97 строки
4.2 KiB
Plaintext
97 строки
4.2 KiB
Plaintext
@TEMPLATE decoder_tmpl.c
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Simple Decoder
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==============
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ INTRODUCTION
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This is an example of a simple decoder loop. It takes an input file
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containing the compressed data (in IVF format), passes it through the
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decoder, and writes the decompressed frames to disk. Other decoder
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examples build upon this one.
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The details of the IVF format have been elided from this example for
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simplicity of presentation, as IVF files will not generally be used by
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your application. In general, an IVF file consists of a file header,
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followed by a variable number of frames. Each frame consists of a frame
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header followed by a variable length payload. The length of the payload
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is specified in the first four bytes of the frame header. The payload is
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the raw compressed data.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ INTRODUCTION
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Standard Includes
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-----------------
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For decoders, you only have to include `vpx_decoder.h` and then any
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header files for the specific codecs you use. In this case, we're using
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vp8. The `VPX_CODEC_DISABLE_COMPAT` macro can be defined to ensure
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strict compliance with the latest SDK by disabling some backwards
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compatibility features. Defining this macro is encouraged.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DEC_INCLUDES
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@DEFAULT
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DEC_INCLUDES
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Initializing The Codec
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----------------------
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The decoder is initialized by the following code. This is an example for
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the VP8 decoder, but the code is analogous for all algorithms. Replace
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`vpx_codec_vp8_dx()` with a pointer to the interface exposed by the
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algorithm you want to use. The `cfg` argument is left as NULL in this
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example, because we want the algorithm to determine the stream
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configuration (width/height) and allocate memory automatically. This
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parameter is generally only used if you need to preallocate memory,
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particularly in External Memory Allocation mode.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DEC_INIT
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@DEFAULT
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DEC_INIT
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Decoding A Frame
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----------------
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Once the frame has been read into memory, it is decoded using the
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`vpx_codec_decode` function. The call takes a pointer to the data
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(`frame`) and the length of the data (`frame_sz`). No application data
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is associated with the frame in this example, so the `user_priv`
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parameter is NULL. The `deadline` parameter is left at zero for this
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example. This parameter is generally only used when doing adaptive
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postprocessing.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DECODE
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@DEFAULT
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DECODE
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Codecs may produce a variable number of output frames for every call to
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`vpx_codec_decode`. These frames are retrieved by the
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`vpx_codec_get_frame` iterator function. The iterator variable `iter` is
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initialized to NULL each time `vpx_codec_decode` is called.
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`vpx_codec_get_frame` is called in a loop, returning a pointer to a
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decoded image or NULL to indicate the end of list.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ GET_FRAME
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@DEFAULT
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ GET_FRAME
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Processing The Decoded Data
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---------------------------
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In this example, we simply write the encoded data to disk. It is
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important to honor the image's `stride` values.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PROCESS_DX
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@DEFAULT
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PROCESS_DX
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Cleanup
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-------
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The `vpx_codec_destroy` call frees any memory allocated by the codec.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DESTROY
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@DEFAULT
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DESTROY
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Error Handling
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--------------
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This example does not special case any error return codes. If there was
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an error, a descriptive message is printed and the program exits. With
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few exeptions, vpx_codec functions return an enumerated error status,
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with the value `0` indicating success.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DIE_CODEC
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@DEFAULT
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ DIE_CODEC
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