FFmpeg原始碼簡單分析:結構體成員管理系統-AVClass
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FFmpeg的庫函式原始碼分析文章列表:
【架構圖】
【通用】
【解碼】
【編碼】
【其它】
【指令碼】
【H.264】
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打算寫兩篇文章記錄FFmpeg中和AVOption有關的原始碼。AVOption用於在FFmpeg中描述結構體中的成員變數。它最主要的作用可以概括為兩個字:“賦值”。一個AVOption結構體包含了變數名稱,簡短的幫助,取值等等資訊。
所有和AVOption有關的資料都儲存在AVClass結構體中。如果一個結構體(例如AVFormatContext或者AVCodecContext)想要支援AVOption的話,它的第一個成員變數必須是一個指向AVClass結構體的指標。該AVClass中的成員變數option必須指向一個AVOption型別的靜態陣列。
何為AVOption?
AVOption是用來設定FFmpeg中變數的值的結構體。可能說到這個作用有的人會奇怪:設定系統中變數的值,直接使用等於號“=”就可以,為什麼還要專門定義一個結構體呢?其實AVOption的特點就在於它賦值時候的靈活性。AVOption可以使用字串為任何型別的變數賦值。傳統意義上,如果變數型別為int,則需要使用整數來賦值;如果變數為double,則需要使用小數來賦值;如果變數型別為char *,才需要使用字串來賦值。而AVOption將這些賦值“歸一化”了,統一使用字串賦值。例如給int型變數qp設定值為20,通過AVOption需要傳遞進去一個內容為“20”的字串。此外,AVOption中變數的名稱也使用字串來表示。結合上面提到的使用字串賦值的特性,我們可以發現使用AVOption之後,傳遞兩個字串(一個是變數的名稱,一個是變數的值)就可以改變系統中變數的值。
上文提到的這種方法的意義在哪裡?我個人感覺對於直接使用C語言進行開發的人來說,作用不是很明顯:完全可以使用等於號“=”就可以進行各種變數的賦值。但是對於從外部系統中呼叫FFmpeg的人來說,作用就很大了:從外部系統中只可以傳遞字串給內部系統。比如說對於直接呼叫ffmpeg.exe的人來說,他們是無法修改FFmpeg內部各個變數的數值的,這種情況下只能通過輸入“名稱”和“值”這樣的字串,通過AVOption改變FFmpeg內部變數的值。由此可見,使用AVOption可以使FFmpeg更加適應多種多樣的外部系統。
突然想到了JavaEE開發中也有這種類似的機制。網際網路上只可以傳輸字串,即是沒有方法傳輸整形、浮點型這種的資料。而Java系統中卻包含整形、浮點型等各種資料型別。因此開發JSP中的Servlet的時候經常需要將整數字符串手工轉化成一個整型的變數。使用最多的一個函式就是Integer.parseInt()方法。例如下面程式碼可以將字串“123”轉化成整數123。
int a=Integer.parseInt("123");
由此發現了一個結論:程式語言之間真的是相通的!
現在回到AVOption。其實除了可以對FFmpeg常用結構體AVFormatContext,AVCodecContext等進行賦值之外,還可以對它們的私有資料priv_data進行賦值。這個欄位裡通常儲存了各種編碼器特有的結構體。而這些結構體的定義在FFmpeg的SDK中是找不到的。例如使用libx264進行編碼的時候,通過AVCodecContext的priv_data欄位可以對X264Context結構體中的變數進行賦值,設定preset,profile等。使用libx265進行編碼的時候,通過AVCodecContext的priv_data欄位可以對libx265Context結構體中的變數進行賦值,設定preset,tune等。
何為AVClass?
AVClass最主要的作用就是給結構體(例如AVFormatContext等)增加AVOption功能的支援。換句話說AVClass就是AVOption和目標結構體之間的“橋樑”。AVClass要求必須宣告為目標結構體的第一個變數。
AVClass中有一個option陣列用於儲存目標結構體的所有的AVOption。舉個例子,AVFormatContext結構體,AVClass和AVOption之間的關係如下圖所示。
AVOption
下面開始從程式碼的角度記錄AVOption。AVOption結構體的定義如下所示。/**
* AVOption
*/
typedef struct AVOption {
const char *name;
/**
* short English help text
* @todo What about other languages?
*/
const char *help;
/**
* The offset relative to the context structure where the option
* value is stored. It should be 0 for named constants.
*/
int offset;
enum AVOptionType type;
/**
* the default value for scalar options
*/
union {
int64_t i64;
double dbl;
const char *str;
/* TODO those are unused now */
AVRational q;
} default_val;
double min; ///< minimum valid value for the option
double max; ///< maximum valid value for the option
int flags;
#define AV_OPT_FLAG_ENCODING_PARAM 1 ///< a generic parameter which can be set by the user for muxing or encoding
#define AV_OPT_FLAG_DECODING_PARAM 2 ///< a generic parameter which can be set by the user for demuxing or decoding
#if FF_API_OPT_TYPE_METADATA
#define AV_OPT_FLAG_METADATA 4 ///< some data extracted or inserted into the file like title, comment, ...
#endif
#define AV_OPT_FLAG_AUDIO_PARAM 8
#define AV_OPT_FLAG_VIDEO_PARAM 16
#define AV_OPT_FLAG_SUBTITLE_PARAM 32
/**
* The option is inteded for exporting values to the caller.
*/
#define AV_OPT_FLAG_EXPORT 64
/**
* The option may not be set through the AVOptions API, only read.
* This flag only makes sense when AV_OPT_FLAG_EXPORT is also set.
*/
#define AV_OPT_FLAG_READONLY 128
#define AV_OPT_FLAG_FILTERING_PARAM (1<<16) ///< a generic parameter which can be set by the user for filtering
//FIXME think about enc-audio, ... style flags
/**
* The logical unit to which the option belongs. Non-constant
* options and corresponding named constants share the same
* unit. May be NULL.
*/
const char *unit;
} AVOption;
下面簡單解釋一下AVOption的幾個成員變數:
name:名稱。
help:簡短的幫助。
offset:選項相對結構體首部地址的偏移量(這個很重要)。
type:選項的型別。
default_val:選項的預設值。
min:選項的最小值。
max:選項的最大值。
flags:一些標記。
unit:該選項所屬的邏輯單元,可以為空。
其中,default_val是一個union型別的變數,可以根據選項資料型別的不同,取int,double,char*,AVRational(表示分數)幾種型別。type是一個AVOptionType型別的變數。AVOptionType是一個列舉型別,定義如下。
enum AVOptionType{
AV_OPT_TYPE_FLAGS,
AV_OPT_TYPE_INT,
AV_OPT_TYPE_INT64,
AV_OPT_TYPE_DOUBLE,
AV_OPT_TYPE_FLOAT,
AV_OPT_TYPE_STRING,
AV_OPT_TYPE_RATIONAL,
AV_OPT_TYPE_BINARY, ///< offset must point to a pointer immediately followed by an int for the length
AV_OPT_TYPE_DICT,
AV_OPT_TYPE_CONST = 128,
AV_OPT_TYPE_IMAGE_SIZE = MKBETAG('S','I','Z','E'), ///< offset must point to two consecutive integers
AV_OPT_TYPE_PIXEL_FMT = MKBETAG('P','F','M','T'),
AV_OPT_TYPE_SAMPLE_FMT = MKBETAG('S','F','M','T'),
AV_OPT_TYPE_VIDEO_RATE = MKBETAG('V','R','A','T'), ///< offset must point to AVRational
AV_OPT_TYPE_DURATION = MKBETAG('D','U','R',' '),
AV_OPT_TYPE_COLOR = MKBETAG('C','O','L','R'),
AV_OPT_TYPE_CHANNEL_LAYOUT = MKBETAG('C','H','L','A'),
#if FF_API_OLD_AVOPTIONS
FF_OPT_TYPE_FLAGS = 0,
FF_OPT_TYPE_INT,
FF_OPT_TYPE_INT64,
FF_OPT_TYPE_DOUBLE,
FF_OPT_TYPE_FLOAT,
FF_OPT_TYPE_STRING,
FF_OPT_TYPE_RATIONAL,
FF_OPT_TYPE_BINARY, ///< offset must point to a pointer immediately followed by an int for the length
FF_OPT_TYPE_CONST=128,
#endif
};
AVClass
AVClass中儲存了AVOption型別的陣列option,用於儲存選項資訊。AVClass有一個特點就是它必須位於其支援的結構體的第一個位置。例如,AVFormatContext和AVCodecContext都支援AVClass,觀察它們結構體的定義可以發現他們結構體的第一個變數都是AVClass。擷取一小段AVFormatContext的定義的開頭部分,如下所示。typedef struct AVFormatContext {
/**
* A class for logging and @ref avoptions. Set by avformat_alloc_context().
* Exports (de)muxer private options if they exist.
*/
const AVClass *av_class;
/**
* The input container format.
*
* Demuxing only, set by avformat_open_input().
*/
struct AVInputFormat *iformat;
/**
* The output container format.
*
* Muxing only, must be set by the caller before avformat_write_header().
*/
struct AVOutputFormat *oformat;
//後文略
擷取一小段AVCodecContext的定義的開頭部分,如下所示。
typedef struct AVCodecContext {
/**
* information on struct for av_log
* - set by avcodec_alloc_context3
*/
const AVClass *av_class;
int log_level_offset;
enum AVMediaType codec_type; /* see AVMEDIA_TYPE_xxx */
const struct AVCodec *codec;
//後文略
下面來看一下AVClass的定義,如下所示。
/**
* Describe the class of an AVClass context structure. That is an
* arbitrary struct of which the first field is a pointer to an
* AVClass struct (e.g. AVCodecContext, AVFormatContext etc.).
*/
typedef struct AVClass {
/**
* The name of the class; usually it is the same name as the
* context structure type to which the AVClass is associated.
*/
const char* class_name;
/**
* A pointer to a function which returns the name of a context
* instance ctx associated with the class.
*/
const char* (*item_name)(void* ctx);
/**
* a pointer to the first option specified in the class if any or NULL
*
* @see av_set_default_options()
*/
const struct AVOption *option;
/**
* LIBAVUTIL_VERSION with which this structure was created.
* This is used to allow fields to be added without requiring major
* version bumps everywhere.
*/
int version;
/**
* Offset in the structure where log_level_offset is stored.
* 0 means there is no such variable
*/
int log_level_offset_offset;
/**
* Offset in the structure where a pointer to the parent context for
* logging is stored. For example a decoder could pass its AVCodecContext
* to eval as such a parent context, which an av_log() implementation
* could then leverage to display the parent context.
* The offset can be NULL.
*/
int parent_log_context_offset;
/**
* Return next AVOptions-enabled child or NULL
*/
void* (*child_next)(void *obj, void *prev);
/**
* Return an AVClass corresponding to the next potential
* AVOptions-enabled child.
*
* The difference between child_next and this is that
* child_next iterates over _already existing_ objects, while
* child_class_next iterates over _all possible_ children.
*/
const struct AVClass* (*child_class_next)(const struct AVClass *prev);
/**
* Category used for visualization (like color)
* This is only set if the category is equal for all objects using this class.
* available since version (51 << 16 | 56 << 8 | 100)
*/
AVClassCategory category;
/**
* Callback to return the category.
* available since version (51 << 16 | 59 << 8 | 100)
*/
AVClassCategory (*get_category)(void* ctx);
/**
* Callback to return the supported/allowed ranges.
* available since version (52.12)
*/
int (*query_ranges)(struct AVOptionRanges **, void *obj, const char *key, int flags);
} AVClass;
下面簡單解釋一下AVClass的幾個已經理解的成員變數:
class_name:AVClass名稱。
item_name:函式,獲取與AVClass相關聯的結構體例項的名稱。
option:AVOption型別的陣列(最重要)。
version:完成該AVClass的時候的LIBAVUTIL_VERSION。
category:AVClass的型別,是一個型別為AVClassCategory的列舉型變數。
其中AVClassCategory定義如下。
typedef enum {
AV_CLASS_CATEGORY_NA = 0,
AV_CLASS_CATEGORY_INPUT,
AV_CLASS_CATEGORY_OUTPUT,
AV_CLASS_CATEGORY_MUXER,
AV_CLASS_CATEGORY_DEMUXER,
AV_CLASS_CATEGORY_ENCODER,
AV_CLASS_CATEGORY_DECODER,
AV_CLASS_CATEGORY_FILTER,
AV_CLASS_CATEGORY_BITSTREAM_FILTER,
AV_CLASS_CATEGORY_SWSCALER,
AV_CLASS_CATEGORY_SWRESAMPLER,
AV_CLASS_CATEGORY_DEVICE_VIDEO_OUTPUT = 40,
AV_CLASS_CATEGORY_DEVICE_VIDEO_INPUT,
AV_CLASS_CATEGORY_DEVICE_AUDIO_OUTPUT,
AV_CLASS_CATEGORY_DEVICE_AUDIO_INPUT,
AV_CLASS_CATEGORY_DEVICE_OUTPUT,
AV_CLASS_CATEGORY_DEVICE_INPUT,
AV_CLASS_CATEGORY_NB, ///< not part of ABI/API
}AVClassCategory;
上面解釋欄位還是比較抽象的,下面通過具體的例子看一下AVClass這個結構體。我們看幾個具體的例子:
- AVFormatContext中的AVClass
- AVCodecContext中的AVClass
- AVFrame中的AVClass
- 各種元件(libRTMP,libx264,libx265)裡面特有的AVClass。
AVFormatContext
AVFormatContext 中的AVClass定義位於libavformat\options.c中,是一個名稱為av_format_context_class的靜態結構體。如下所示。static const AVClass av_format_context_class = {
.class_name = "AVFormatContext",
.item_name = format_to_name,
.option = avformat_options,
.version = LIBAVUTIL_VERSION_INT,
.child_next = format_child_next,
.child_class_next = format_child_class_next,
.category = AV_CLASS_CATEGORY_MUXER,
.get_category = get_category,
};
從原始碼可以看出以下幾點
(1)class_name
該AVClass名稱是“AVFormatContext”。
(2)item_name
item_name指向一個函式format_to_name(),該函式定義如下所示。
static const char* format_to_name(void* ptr)
{
AVFormatContext* fc = (AVFormatContext*) ptr;
if(fc->iformat) return fc->iformat->name;
else if(fc->oformat) return fc->oformat->name;
else return "NULL";
}
從函式的定義可以看出,如果AVFormatContext結構體中的AVInputFormat結構體不為空,則返回AVInputFormat的name,然後嘗試返回AVOutputFormat的name,如果AVOutputFormat也為空,則返回“NULL”。
(3)optionoption欄位則指向一個元素個數很多的靜態陣列avformat_options。該陣列單獨定義於libavformat\options_table.h中。其中包含了AVFormatContext支援的所有的AVOption,如下所示。
/*
* 雷霄驊
* [email protected]
* 中國傳媒大學/數字電視技術
* http://blog.csdn.net/leixiaohua1020
*
*/
#ifndef AVFORMAT_OPTIONS_TABLE_H
#define AVFORMAT_OPTIONS_TABLE_H
#include <limits.h>
#include "libavutil/opt.h"
#include "avformat.h"
#include "internal.h"
#define OFFSET(x) offsetof(AVFormatContext,x)
#define DEFAULT 0 //should be NAN but it does not work as it is not a constant in glibc as required by ANSI/ISO C
//these names are too long to be readable
#define E AV_OPT_FLAG_ENCODING_PARAM
#define D AV_OPT_FLAG_DECODING_PARAM
static const AVOption avformat_options[] = {
{"avioflags", NULL, OFFSET(avio_flags), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "avioflags"},
{"direct", "reduce buffering", 0, AV_OPT_TYPE_CONST, {.i64 = AVIO_FLAG_DIRECT }, INT_MIN, INT_MAX, D|E, "avioflags"},
{"probesize", "set probing size", OFFSET(probesize2), AV_OPT_TYPE_INT64, {.i64 = 5000000 }, 32, INT64_MAX, D},
{"formatprobesize", "number of bytes to probe file format", OFFSET(format_probesize), AV_OPT_TYPE_INT, {.i64 = PROBE_BUF_MAX}, 0, INT_MAX-1, D},
{"packetsize", "set packet size", OFFSET(packet_size), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX, E},
{"fflags", NULL, OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = AVFMT_FLAG_FLUSH_PACKETS }, INT_MIN, INT_MAX, D|E, "fflags"},
{"flush_packets", "reduce the latency by flushing out packets immediately", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_FLUSH_PACKETS }, INT_MIN, INT_MAX, E, "fflags"},
{"ignidx", "ignore index", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_IGNIDX }, INT_MIN, INT_MAX, D, "fflags"},
{"genpts", "generate pts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_GENPTS }, INT_MIN, INT_MAX, D, "fflags"},
{"nofillin", "do not fill in missing values that can be exactly calculated", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOFILLIN }, INT_MIN, INT_MAX, D, "fflags"},
{"noparse", "disable AVParsers, this needs nofillin too", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOPARSE }, INT_MIN, INT_MAX, D, "fflags"},
{"igndts", "ignore dts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_IGNDTS }, INT_MIN, INT_MAX, D, "fflags"},
{"discardcorrupt", "discard corrupted frames", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_DISCARD_CORRUPT }, INT_MIN, INT_MAX, D, "fflags"},
{"sortdts", "try to interleave outputted packets by dts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_SORT_DTS }, INT_MIN, INT_MAX, D, "fflags"},
{"keepside", "don't merge side data", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_KEEP_SIDE_DATA }, INT_MIN, INT_MAX, D, "fflags"},
{"latm", "enable RTP MP4A-LATM payload", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_MP4A_LATM }, INT_MIN, INT_MAX, E, "fflags"},
{"nobuffer", "reduce the latency introduced by optional buffering", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOBUFFER }, 0, INT_MAX, D, "fflags"},
{"seek2any", "allow seeking to non-keyframes on demuxer level when supported", OFFSET(seek2any), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, 1, D},
{"bitexact", "do not write random/volatile data", 0, AV_OPT_TYPE_CONST, { .i64 = AVFMT_FLAG_BITEXACT }, 0, 0, E, "fflags" },
{"analyzeduration", "specify how many microseconds are analyzed to probe the input", OFFSET(max_analyze_duration2), AV_OPT_TYPE_INT64, {.i64 = 0 }, 0, INT64_MAX, D},
{"cryptokey", "decryption key", OFFSET(key), AV_OPT_TYPE_BINARY, {.dbl = 0}, 0, 0, D},
{"indexmem", "max memory used for timestamp index (per stream)", OFFSET(max_index_size), AV_OPT_TYPE_INT, {.i64 = 1<<20 }, 0, INT_MAX, D},
{"rtbufsize", "max memory used for buffering real-time frames", OFFSET(max_picture_buffer), AV_OPT_TYPE_INT, {.i64 = 3041280 }, 0, INT_MAX, D}, /* defaults to 1s of 15fps 352x288 YUYV422 video */
{"fdebug", "print specific debug info", OFFSET(debug), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, 0, INT_MAX, E|D, "fdebug"},
{"ts", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_FDEBUG_TS }, INT_MIN, INT_MAX, E|D, "fdebug"},
{"max_delay", "maximum muxing or demuxing delay in microseconds", OFFSET(max_delay), AV_OPT_TYPE_INT, {.i64 = -1 }, -1, INT_MAX, E|D},
{"start_time_realtime", "wall-clock time when stream begins (PTS==0)", OFFSET(start_time_realtime), AV_OPT_TYPE_INT64, {.i64 = AV_NOPTS_VALUE}, INT64_MIN, INT64_MAX, E},
{"fpsprobesize", "number of frames used to probe fps", OFFSET(fps_probe_size), AV_OPT_TYPE_INT, {.i64 = -1}, -1, INT_MAX-1, D},
{"audio_preload", "microseconds by which audio packets should be interleaved earlier", OFFSET(audio_preload), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
{"chunk_duration", "microseconds for each chunk", OFFSET(max_chunk_duration), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
{"chunk_size", "size in bytes for each chunk", OFFSET(max_chunk_size), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
/* this is a crutch for avconv, since it cannot deal with identically named options in different contexts.
* to be removed when avconv is fixed */
{"f_err_detect", "set error detection flags (deprecated; use err_detect, save via avconv)", OFFSET(error_recognition), AV_OPT_TYPE_FLAGS, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"err_detect", "set error detection flags", OFFSET(error_recognition), AV_OPT_TYPE_FLAGS, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"crccheck", "verify embedded CRCs", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"bitstream", "detect bitstream specification deviations", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_BITSTREAM }, INT_MIN, INT_MAX, D, "err_detect"},
{"buffer", "detect improper bitstream length", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_BUFFER }, INT_MIN, INT_MAX, D, "err_detect"},
{"explode", "abort decoding on minor error detection", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_EXPLODE }, INT_MIN, INT_MAX, D, "err_detect"},
{"ignore_err", "ignore errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_IGNORE_ERR }, INT_MIN, INT_MAX, D, "err_detect"},
{"careful", "consider things that violate the spec, are fast to check and have not been seen in the wild as errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_CAREFUL }, INT_MIN, INT_MAX, D, "err_detect"},
{"compliant", "consider all spec non compliancies as errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_COMPLIANT }, INT_MIN, INT_MAX, D, "err_detect"},
{"aggressive", "consider things that a sane encoder shouldn't do as an error", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_AGGRESSIVE }, INT_MIN, INT_MAX, D, "err_detect"},
{"use_wallclock_as_timestamps", "use wallclock as timestamps", OFFSET(use_wallclock_as_timestamps), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, D},
{"avoid_negative_ts", "shift timestamps so they start at 0", OFFSET(avoid_negative_ts), AV_OPT_TYPE_INT, {.i64 = -1}, -1, 2, E, "avoid_negative_ts"},
{"auto", "enabled when required by target format", 0, AV_OPT_TYPE_CONST, {.i64 = -1 }, INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"disabled", "do not change timestamps", 0, AV_OPT_TYPE_CONST, {.i64 = 0 }, INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"make_zero", "shift timestamps so they start at 0", 0, AV_OPT_TYPE_CONST, {.i64 = 2 }, INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"make_non_negative", "shift timestamps so they are non negative", 0, AV_OPT_TYPE_CONST, {.i64 = 1 }, INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"skip_initial_bytes", "set number of bytes to skip before reading header and frames", OFFSET(skip_initial_bytes), AV_OPT_TYPE_INT64, {.i64 = 0}, 0, INT64_MAX-1, D},
{"correct_ts_overflow", "correct single timestamp overflows", OFFSET(correct_ts_overflow), AV_OPT_TYPE_INT, {.i64 = 1}, 0, 1, D},
{"flush_packets", "enable flushing of the I/O context after each packet", OFFSET(flush_packets), AV_OPT_TYPE_INT, {.i64 = 1}, 0, 1, E},
{"metadata_header_padding", "set number of bytes to be written as padding in a metadata header", OFFSET(metadata_header_padding), AV_OPT_TYPE_INT, {.i64 = -1}, -1, INT_MAX, E},
{"output_ts_offset", "set output timestamp offset", OFFSET(output_ts_offset), AV_OPT_TYPE_DURATION, {.i64 = 0}, -INT64_MAX, INT64_MAX, E},
{"max_interleave_delta", "maximum buffering duration for interleaving", OFFSET(max_interleave_delta), AV_OPT_TYPE_INT64, { .i64 = 10000000 }, 0, INT64_MAX, E },
{"f_strict", "how strictly to follow the standards (deprecated; use strict, save via avconv)", OFFSET(strict_std_compliance), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "strict"},
{"strict", "how strictly to follow the standards", OFFSET(strict_std_compliance), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "strict"},
{"strict", "strictly conform to all the things in the spec no matter what the consequences", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_STRICT }, INT_MIN, INT_MAX, D|E, "strict"},
{"normal", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_NORMAL }, INT_MIN, INT_MAX, D|E, "strict"},
{"experimental", "allow non-standardized experimental variants", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_EXPERIMENTAL }, INT_MIN, INT_MAX, D|E, "strict"},
{"max_ts_probe", "maximum number of packets to read while waiting for the first timestamp", OFFSET(max_ts_probe), AV_OPT_TYPE_INT, { .i64 = 50 }, 0, INT_MAX, D },
{NULL},
};
#undef E
#undef D
#undef DEFAULT
#undef OFFSET
#endif /* AVFORMAT_OPTIONS_TABLE_H */
AVCodecContext
AVFormatContext 中的AVClass定義位於libavcodec\options.c中,是一個名稱為av_codec_context_class的靜態結構體。如下所示。static const AVClass av_codec_context_class = {
.class_name = "AVCodecContext",
.item_name = context_to_name,
.option = avcodec_options,
.version = LIBAVUTIL_VERSION_INT,
.log_level_offset_offset = offsetof(AVCodecContext, log_level_offset),
.child_next = codec_child_next,
.child_class_next = codec_child_class_next,
.category = AV_CLASS_CATEGORY_ENCODER,
.get_category = get_category,
};
從原始碼可以看出:
(1)class_name
該AVClass名稱是“AVCodecContext”。
(2)item_name
item_name指向一個函式context_to_name (),該函式定義如下所示。
static const char* context_to_name(void* ptr) {
AVCodecContext *avc= ptr;
if(avc && avc->codec && avc->codec->name)
return avc->codec->name;
else
return "NULL";
}
從函式的定義可以看出,如果AVCodecContext中的Codec結構體不為空,則返回Codec的name,否則返回“NULL”。
(3)category
option欄位則指向一個元素個數極多的靜態陣列avcodec_options。該陣列單獨定義於libavcodec\options_table.h中。其中包含了AVCodecContext支援的所有的AVOption。由於該陣列定義實在是太多了,在這裡僅貼出它前面的一小部分。
/*
* 雷霄驊
* [email protected]
* 中國傳媒大學/數字電視技術
* http://blog.csdn.net/leixiaohua1020
*
*/
#ifndef AVCODEC_OPTIONS_TABLE_H
#define AVCODEC_OPTIONS_TABLE_H
#include <float.h>
#include <limits.h>
#include <stdint.h>
#include "libavutil/opt.h"
#include "avcodec.h"
#include "version.h"
#define OFFSET(x) offsetof(AVCodecContext,x)
#define DEFAULT 0 //should be NAN but it does not work as it is not a constant in glibc as required by ANSI/ISO C
//these names are too long to be readable
#define V AV_OPT_FLAG_VIDEO_PARAM
#define A AV_OPT_FLAG_AUDIO_PARAM
#define S AV_OPT_FLAG_SUBTITLE_PARAM
#define E AV_OPT_FLAG_ENCODING_PARAM
#define D AV_OPT_FLAG_DECODING_PARAM
#define AV_CODEC_DEFAULT_BITRATE 200*1000
static const AVOption avcodec_options[] = {
{"b", "set bitrate (in bits/s)", OFFSET(bit_rate), AV_OPT_TYPE_INT, {.i64 = AV_CODEC_DEFAULT_BITRATE }, 0, INT_MAX, A|V|E},
{"ab", "set bitrate (in bits/s)", OFFSET(bit_rate), AV_OPT_TYPE_INT, {.i64 = 128*1000 }, 0, INT_MAX, A|E},
{"bt", "Set video bitrate tolerance (in bits/s). In 1-pass mode, bitrate tolerance specifies how far "
"ratecontrol is willing to deviate from the target average bitrate value. This is not related "
"to minimum/maximum bitrate. Lowering tolerance too much has an adverse effect on quality.",
OFFSET(bit_rate_tolerance), AV_OPT_TYPE_INT, {.i64 = AV_CODEC_DEFAULT_BITRATE*20 }, 1, INT_MAX, V|E},
{"flags", NULL, OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, 0, UINT_MAX, V|A|S|E|D, "flags"},
{"unaligned", "allow decoders to produce unaligned output", 0, AV_OPT_TYPE_CONST, { .i64 = CODEC_FLAG_UNALIGNED }, INT_MIN, INT_MAX, V | D, "flags" },
{"mv4", "use four motion vectors per macroblock (MPEG-4)", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_4MV }, INT_MIN, INT_MAX, V|E, "flags"},
{"qpel", "use 1/4-pel motion compensation", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_QPEL }, INT_MIN, INT_MAX, V|E, "flags"},
{"loop", "use loop filter", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_LOOP_FILTER }, INT_MIN, INT_MAX, V|E, "flags"},
{"qscale", "use fixed qscale", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_QSCALE }, INT_MIN, INT_MAX, 0, "flags"},
#if FF_API_GMC
{"gmc", "use gmc", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_GMC }, INT_MIN, INT_MAX, V|E, "flags"},
#endif
#if FF_API_MV0
{"mv0", "always try a mb with mv=<0,0>", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_MV0 }, INT_MIN, INT_MAX, V|E, "flags"},
#endif
#if FF_API_INPUT_PRESERVED
{"input_preserved", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_INPUT_PRESERVED }, INT_MIN, INT_MAX, 0, "flags"},
#endif
{"pass1", "use internal 2-pass ratecontrol in first pass mode", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_PASS1 }, INT_MIN, INT_MAX, 0, "flags"},
{"pass2", "use internal 2-pass ratecontrol in second pass mode", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_PASS2 }, INT_MIN, INT_MAX, 0, "flags"},
{"gray", "only decode/encode grayscale", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_GRAY }, INT_MIN, INT_MAX, V|E|D, "flags"},
#if FF_API_EMU_EDGE
{"emu_edge", "do not draw edges", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_EMU_EDGE }, INT_MIN, INT_MAX, 0, "flags"},
#endif
{"psnr", "error[?] variables will be set during encoding", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_PSNR }, INT_MIN, INT_MAX, V|E, "flags"},
{"truncated", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_TRUNCATED }, INT_MIN, INT_MAX, 0, "flags"},
#if FF_API_NORMALIZE_AQP
{"naq", "normalize adaptive quantization", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_NORMALIZE_AQP }, INT_MIN, INT_MAX, V|E, "flags"},
#endif
{"ildct", "use interlaced DCT", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_INTERLACED_DCT }, INT_MIN, INT_MAX, V|E, "flags"},
{"low_delay", "force low delay", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_LOW_DELAY }, INT_MIN, INT_MAX, V|D|E, "flags"},
{"global_header", "place global headers in extradata instead of every keyframe", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_GLOBAL_HEADER }, INT_MIN, INT_MAX, V|A|E, "flags"},
{"bitexact", "use only bitexact functions (except (I)DCT)", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_BITEXACT }, INT_MIN, INT_MAX, A|V|S|D|E, "flags"},
{"aic", "H.263 advanced intra coding / MPEG-4 AC prediction", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_AC_PRED }, INT_MIN, INT_MAX, V|E, "flags"},
{"ilme", "interlaced motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_INTERLACED_ME }, INT_MIN, INT_MAX, V|E, "flags"},
{"cgop", "closed GOP", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_CLOSED_GOP }, INT_MIN, INT_MAX, V|E, "flags"},
{"output_corrupt", "Output even potentially corrupted frames", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG_OUTPUT_CORRUPT }, INT_MIN, INT_MAX, V|D, "flags"},
{"fast", "allow non-spec-compliant speedup tricks", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_FAST }, INT_MIN, INT_MAX, V|E, "flags2"},
{"noout", "skip bitstream encoding", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_NO_OUTPUT }, INT_MIN, INT_MAX, V|E, "flags2"},
{"ignorecrop", "ignore cropping information from sps", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_IGNORE_CROP }, INT_MIN, INT_MAX, V|D, "flags2"},
{"local_header", "place global headers at every keyframe instead of in extradata", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_LOCAL_HEADER }, INT_MIN, INT_MAX, V|E, "flags2"},
{"chunks", "Frame data might be split into multiple chunks", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_CHUNKS }, INT_MIN, INT_MAX, V|D, "flags2"},
{"showall", "Show all frames before the first keyframe", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_SHOW_ALL }, INT_MIN, INT_MAX, V|D, "flags2"},
{"export_mvs", "export motion vectors through frame side data", 0, AV_OPT_TYPE_CONST, {.i64 = CODEC_FLAG2_EXPORT_MVS}, INT_MIN, INT_MAX, V|D, "flags2"},
{"me_method", "set motion estimation method", OFFSET(me_method), AV_OPT_TYPE_INT, {.i64 = ME_EPZS }, INT_MIN, INT_MAX, V|E, "me_method"},
{"zero", "zero motion estimation (fastest)", 0, AV_OPT_TYPE_CONST, {.i64 = ME_ZERO }, INT_MIN, INT_MAX, V|E, "me_method" },
{"full", "full motion estimation (slowest)", 0, AV_OPT_TYPE_CONST, {.i64 = ME_FULL }, INT_MIN, INT_MAX, V|E, "me_method" },
{"epzs", "EPZS motion estimation (default)", 0, AV_OPT_TYPE_CONST, {.i64 = ME_EPZS }, INT_MIN, INT_MAX, V|E, "me_method" },
{"esa", "esa motion estimation (alias for full)", 0, AV_OPT_TYPE_CONST, {.i64 = ME_FULL }, INT_MIN, INT_MAX, V|E, "me_method" },
{"tesa", "tesa motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_TESA }, INT_MIN, INT_MAX, V|E, "me_method" },
{"dia", "diamond motion estimation (alias for EPZS)", 0, AV_OPT_TYPE_CONST, {.i64 = ME_EPZS }, INT_MIN, INT_MAX, V|E, "me_method" },
{"log", "log motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_LOG }, INT_MIN, INT_MAX, V|E, "me_method" },
{"phods", "phods motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_PHODS }, INT_MIN, INT_MAX, V|E, "me_method" },
{"x1", "X1 motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_X1 }, INT_MIN, INT_MAX, V|E, "me_method" },
{"hex", "hex motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_HEX }, INT_MIN, INT_MAX, V|E, "me_method" },
{"umh", "umh motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_UMH }, INT_MIN, INT_MAX, V|E, "me_method" },
{"iter", "iter motion estimation", 0, AV_OPT_TYPE_CONST, {.i64 = ME_ITER }, INT_MIN, INT_MAX, V|E, "me_method" },
{"extradata_size", NULL, OFFSET(extradata_size), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX},
{"time_base", NULL, OFFSET(time_base), AV_OPT_TYPE_RATIONAL, {.dbl = 0}, INT_MIN, INT_MAX},
{"g", "set the group of picture (GOP) size", OFFSET(gop_size), AV_OPT_TYPE_INT, {.i64 = 12 }, INT_MIN, INT_MAX, V|E},
{"ar", "set audio sampling rate (in Hz)", OFFSET(sample_rate), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, A|D|E},
{"ac", "set number of audio channels", OFFSET(channels), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, A|D|E},
AVFrame
AVFrame 中的AVClass定義位於libavcodec\options.c中,是一個名稱為av_frame_class的靜態結構體。如下所示。static const AVClass av_frame_class = {
.class_name = "AVFrame",
.item_name = NULL,
.option = frame_options,
.version = LIBAVUTIL_VERSION_INT,
};
option欄位則指向一個元素個數極多的靜態陣列frame_options。frame_options定義如下所示。
static const AVOption frame_options[]={
{"best_effort_timestamp", "", FOFFSET(best_effort_timestamp), AV_OPT_TYPE_INT64, {.i64 = AV_NOPTS_VALUE }, INT64_MIN, INT64_MAX, 0},
{"pkt_pos", "", FOFFSET(pkt_pos), AV_OPT_TYPE_INT64, {.i64 = -1 }, INT64_MIN, INT64_MAX, 0},
{"pkt_size", "", FOFFSET(pkt_size), AV_OPT_TYPE_INT64, {.i64 = -1 }, INT64_MIN, INT64_MAX, 0},
{"sample_aspect_ratio", "", FOFFSET(sample_aspect_ratio), AV_OPT_TYPE_RATIONAL, {.dbl = 0 }, 0, INT_MAX, 0},
{"width", "", FOFFSET(width), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, 0},
{"height", "", FOFFSET(height), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, 0},
{"format", "", FOFFSET(format), AV_OPT_TYPE_INT, {.i64 = -1 }, 0, INT_MAX, 0},
{"channel_layout", "", FOFFSET(channel_layout), AV_OPT_TYPE_INT64, {.i64 = 0 }, 0, INT64_MAX, 0},
{"sample_rate", "", FOFFSET(sample_rate), AV_OPT_TYPE_INT, {.i64 = 0 }, 0, INT_MAX, 0},
{NULL},
};
可以看出AVFrame的選項陣列中包含了“width”,“height”這類用於視訊幀的選項,以及“channel_layout”,“sample_rate”這類用於音訊幀的選項。
各種元件特有的AVClass
除了FFmpeg中通用的AVFormatContext,AVCodecContext,AVFrame這類的結構體之外,每種特定的元件也包含自己的AVClass。下面舉例幾個。
LibRTMP
libRTMP中根據協議型別的不同定義了多種的AVClass。由於這些AVClass除了名字不一樣之外,其他的欄位一模一樣,所以AVClass的宣告寫成了一個名稱為RTMP_CLASS的巨集。#define RTMP_CLASS(flavor)\
static const AVClass lib ## flavor ## _class = {\
.class_name = "lib" #flavor " protocol",\
.item_name = av_default_item_name,\
.option = options,\
.version = LIBAVUTIL_VERSION_INT,\
};
而後定義了多種AVCLass:
RTMP_CLASS(rtmp)
RTMP_CLASS(rtmpt)
RTMP_CLASS(rtmpe)
RTMP_CLASS(rtmpte)
RTMP_CLASS(rtmps)
這些AVClass的option欄位指向的陣列是一樣的,如下所示。
static const AVOption options[] = {
{"rtmp_app", "Name of application to connect to on the RTMP server", OFFSET(app), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
{"rtmp_buffer", "Set buffer time in milliseconds. The default is 3000.", OFFSET(client_buffer_time), AV_OPT_TYPE_STRING, {.str = "3000"}, 0, 0, DEC|ENC},
{"rtmp_conn", "Append arbitrary AMF data to the Connect message", OFFSET(conn), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
{"rtmp_flashver", "Version of the Flash plugin used to run the SWF player.", OFFSET(flashver), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
{"rtmp_live", "Specify that the media is a live stream.", OFFSET(live), AV_OPT_TYPE_INT, {.i64 = 0}, INT_MIN, INT_MAX, DEC, "rtmp_live"},
{"any", "both", 0, AV_OPT_TYPE_CONST, {.i64 = -2}, 0, 0, DEC, "rtmp_live"},
{"live", "live stream", 0, AV_OPT_TYPE_CONST, {.i64 = -1}, 0, 0, DEC, "rtmp_live"},
{"recorded", "recorded stream", 0, AV_OPT_TYPE_CONST, {.i64 = 0}, 0, 0, DEC, "rtmp_live"},
{"rtmp_pageurl", "URL of the web page in which the media was embedded. By default no value will be sent.", OFFSET(pageurl), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC},
{"rtmp_playpath", "Stream identifier to play or to publish", OFFSET(playpath), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
{"rtmp_subscribe", "Name of live stream to subscribe to. Defaults to rtmp_playpath.", OFFSET(subscribe), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC},
{"rtmp_swfurl", "URL of the SWF player. By default no value will be sent", OFFSET(swfurl), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
{"rtmp_swfverify", "URL to player swf file, compute hash/size automatically. (unimplemented)", OFFSET(swfverify), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC},
{"rtmp_tcurl", "URL of the target stream. Defaults to proto://host[:port]/app.", OFFSET(tcurl), AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0, DEC|ENC},
{ NULL },
};
Libx264
Libx264的AVClass定義如下所示。static const AVClass x264_class = {
.class_name = "libx264",
.item_name = av_default_item_name,
.option = options,
.version = LIBAVUTIL_VERSION_INT,
};
其中option欄位指向的陣列定義如下所示。這些option的使用頻率還是比較高的。
static const AVOption options[] = {
{ "preset", "Set the encoding preset (cf. x264 --fullhelp)", OFFSET(preset), AV_OPT_TYPE_STRING, { .str = "medium" }, 0, 0, VE},
{ "tune", "Tune the encoding params (cf. x264 --fullhelp)", OFFSET(tune), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
{ "profile", "Set profile restrictions (cf. x264 --fullhelp) ", OFFSET(profile), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
{ "fastfirstpass", "Use fast settings when encoding first pass", OFFSET(fastfirstpass), AV_OPT_TYPE_INT, { .i64 = 1 }, 0, 1, VE},
{"level", "Specify level (as defined by Annex A)", OFFSET(level), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
{"passlogfile", "Filename for 2 pass stats", OFFSET(stats), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
{"wpredp", "Weighted prediction for P-frames", OFFSET(wpredp), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
{"x264opts", "x264 options", OFFSET(x264opts), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, VE},
{ "crf", "Select the quality for constant quality mode", OFFSET(crf), AV_OPT_TYPE_FLOAT, {.dbl = -1 }, -1, FLT_MAX, VE },
{ "crf_max", "In CRF mode, prevents VBV from lowering quality beyond this point.",OFFSET(crf_max), AV_OPT_TYPE_FLOAT, {.dbl = -1 }, -1, FLT_MAX, VE },
{ "qp", "Constant quantization parameter rate control method",OFFSET(cqp), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, VE },
{ "aq-mode", "AQ method", OFFSET(aq_mode), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, VE, "aq_mode"},
{ "none", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_AQ_NONE}, INT_MIN, INT_MAX, VE, "aq_mode" },
{ "variance", "Variance AQ (complexity mask)", 0, AV_OPT_TYPE_CONST, {.i64 = X264_AQ_VARIANCE}, INT_MIN, INT_MAX, VE, "aq_mode" },
{ "autovariance", "Auto-variance AQ (experimental)", 0, AV_OPT_TYPE_CONST, {.i64 = X264_AQ_AUTOVARIANCE}, INT_MIN, INT_MAX, VE, "aq_mode" },
{ "aq-strength", "AQ strength. Reduces blocking and blurring in flat and textured areas.", OFFSET(aq_strength), AV_OPT_TYPE_FLOAT, {.dbl = -1}, -1, FLT_MAX, VE},
{ "psy", "Use psychovisual optimizations.", OFFSET(psy), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 1, VE },
{ "psy-rd", "Strength of psychovisual optimization, in <psy-rd>:<psy-trellis> format.", OFFSET(psy_rd), AV_OPT_TYPE_STRING, {0 }, 0, 0, VE},
{ "rc-lookahead", "Number of frames to look ahead for frametype and ratecontrol", OFFSET(rc_lookahead), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, VE },
{ "weightb", "Weighted prediction for B-frames.", OFFSET(weightb), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 1, VE },
{ "weightp", "Weighted prediction analysis method.", OFFSET(weightp), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, VE, "weightp" },
{ "none", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_WEIGHTP_NONE}, INT_MIN, INT_MAX, VE, "weightp" },
{ "simple", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_WEIGHTP_SIMPLE}, INT_MIN, INT_MAX, VE, "weightp" },
{ "smart", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_WEIGHTP_SMART}, INT_MIN, INT_MAX, VE, "weightp" },
{ "ssim", "Calculate and print SSIM stats.", OFFSET(ssim), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 1, VE },
{ "intra-refresh", "Use Periodic Intra Refresh instead of IDR frames.",OFFSET(intra_refresh),AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 1, VE },
{ "bluray-compat", "Bluray compatibility workarounds.", OFFSET(bluray_compat) ,AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 1, VE },
{ "b-bias", "Influences how often B-frames are used", OFFSET(b_bias), AV_OPT_TYPE_INT, { .i64 = INT_MIN}, INT_MIN, INT_MAX, VE },
{ "b-pyramid", "Keep some B-frames as references.", OFFSET(b_pyramid), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, VE, "b_pyramid" },
{ "none", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_B_PYRAMID_NONE}, INT_MIN, INT_MAX, VE, "b_pyramid" },
{ "strict", "Strictly hierarchical pyramid", 0, AV_OPT_TYPE_CONST, {.i64 = X264_B_PYRAMID_STRICT}, INT_MIN, INT_MAX, VE, "b_pyramid" },
{ "normal", "Non-strict (not Blu-ray compatible)", 0, AV_OPT_TYPE_CONST, {.i64 = X264_B_PYRAMID_NORMAL}, INT_MIN, INT_MAX, VE, "b_pyramid" },
{ "mixed-refs", "One reference per partition, as opposed to one reference per macroblock", OFFSET(mixed_refs), AV_OPT_TYPE_INT, { .i64 = -1}, -1, 1, VE },
{ "8x8dct", "High profile 8x8 transform.", OFFSET(dct8x8), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 1, VE},
{ "fast-pskip", NULL, OFFSET(fast_pskip), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 1, VE},
{ "aud", "Use access unit delimiters.", OFFSET(aud), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 1, VE},
{ "mbtree", "Use macroblock tree ratecontrol.", OFFSET(mbtree), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 1, VE},
{ "deblock", "Loop filter parameters, in <alpha:beta> form.", OFFSET(deblock), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
{ "cplxblur", "Reduce fluctuations in QP (before curve compression)", OFFSET(cplxblur), AV_OPT_TYPE_FLOAT, {.dbl = -1 }, -1, FLT_MAX, VE},
{ "partitions", "A comma-separated list of partitions to consider. "
"Possible values: p8x8, p4x4, b8x8, i8x8, i4x4, none, all", OFFSET(partitions), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE},
{ "direct-pred", "Direct MV prediction mode", OFFSET(direct_pred), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, VE, "direct-pred" },
{ "none", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_NONE }, 0, 0, VE, "direct-pred" },
{ "spatial", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_SPATIAL }, 0, 0, VE, "direct-pred" },
{ "temporal", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_TEMPORAL }, 0, 0, VE, "direct-pred" },
{ "auto", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = X264_DIRECT_PRED_AUTO }, 0, 0, VE, "direct-pred" },
{ "slice-max-size","Limit the size of each slice in bytes", OFFSET(slice_max_size),AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, VE },
{ "stats", "Filename for 2 pass stats", OFFSET(stats), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
{ "nal-hrd", "Signal HRD information (requires vbv-bufsize; "
"cbr not allowed in .mp4)", OFFSET(nal_hrd), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, VE, "nal-hrd" },
{ "none", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_NAL_HRD_NONE}, INT_MIN, INT_MAX, VE, "nal-hrd" },
{ "vbr", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_NAL_HRD_VBR}, INT_MIN, INT_MAX, VE, "nal-hrd" },
{ "cbr", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = X264_NAL_HRD_CBR}, INT_MIN, INT_MAX, VE, "nal-hrd" },
{ "avcintra-class","AVC-Intra class 50/100/200", OFFSET(avcintra_class),AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 200 , VE},
{ "x264-params", "Override the x264 configuration using a :-separated list of key=value parameters", OFFSET(x264_params), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
{ NULL },
};
Libx265
Libx265的AVClass定義如下所示。static const AVClass class = {
.class_name = "libx265",
.item_name = av_default_item_name,
.option = options,
.version = LIBAVUTIL_VERSION_INT,
};
其中option欄位指向的陣列定義如下所示。
static const AVOption options[] = {
{ "preset", "set the x265 preset", OFFSET(preset), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
{ "tune", "set the x265 tune parameter", OFFSET(tune), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
{ "x265-params", "set the x265 configuration using a :-separated list of key=value parameters", OFFSET(x265_opts), AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
{ NULL }
};
官方程式碼中有關AVClass和AVOption的示例
官方程式碼中給出了一小段示例程式碼,演示瞭如何給一個普通的結構體新增AVOption的支援。如下所示。typedef struct test_struct {
AVClass *class;
int int_opt;
char str_opt;
uint8_t bin_opt;
int bin_len;
} test_struct;
static const AVOption test_options[] = {
{ "test_int", "This is a test option of int type.", offsetof(test_struct, int_opt),
AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX },
{ "test_str", "This is a test option of string type.", offsetof(test_struct, str_opt),
AV_OPT_TYPE_STRING },
{ "test_bin", "This is a test option of binary type.", offsetof(test_struct, bin_opt),
AV_OPT_TYPE_BINARY },
{ NULL },
};
static const AVClass test_class = {
.class_name = "test class",
.item_name = av_default_item_name,
.option = test_options,
.version = LIBAVUTIL_VERSION_INT,
};
AVClass有關的API
與AVClass相關的API很少。AVFormatContext提供了一個獲取當前AVClass的函式avformat_get_class()。它的程式碼很簡單,直接返回全域性靜態變數av_format_context_class。定義如下所示。const AVClass *avformat_get_class(void)
{
return &av_format_context_class;
}
同樣,AVCodecContext也提供了一個獲取當前AVClass的函式avcodec_get_class()。它直接返回靜態變數av_codec_context_class。定義如下所示。
const AVClass *avcodec_get_class(void)
{
return &av_codec_context_class;
}
至此FFmpeg的AVClass就基本上分析完畢了。下篇文章具體分析AVOption。
雷霄驊
[email protected]
http://blog.csdn.net/leixiaohua1020
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